elements, the silicate minerals are Fe the most common. Thus, we will spend some time here discussing Ca

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1 LECTURE NOTES 1ST SEMESTER UNIT 4 Silicate Structures and Structural Formula As we discussed in a previous lecture, the relative abundance of elements in the Earth's crust Element Wt% Atomic% Volume% determines what minerals will O ~94 form and what minerals will be Si ~6 common. Because Oxygen and Silicon are the most abundant Al elements, the silicate minerals are Fe the most common. Thus, we will spend some time here discussing Ca Na the structure, chemistry, and occurrence of silicate minerals. K Our systematic discussion of the Mg common rock forming minerals Total will follow in the lectures throughout the remainder of the course. In order to discuss the silicates and their structures it is first necessary to remember that the way atoms are packed together or coordinated by larger anions, like oxygen depends on the radius ratio of the cation to the anion, Rx/Rz. Rx/Rz C.N. Type Hexagonal or 12 Cubic Closest Packing 8 Cubic 6 Octahedral 4 Tetrahedral

2 < Triangular 2 Linear Since oxygen is the most abundant element in the crust, oxygen will be the major anion that coordinates the other other cations. Thus, for the major ions that occur in the crust, we can make the following table showing the coordination and coordination polyhedra that are expected for each of the common cations. C.N. Coord. Ionic Ion (with Polyhedron Radius, Å Oxygen) cubic to 1.51 (8) K closest 1.64 (12) 1.18 (8) Na (6) cubic to octahedral 1.12 (8) Ca (6) Mn Fe+2 6 Octahedral Mg

3 Fe Ti Al Al Tetrahedral Si C+4 3 Triangular 0.08 The radius ratio of Si+4 to O-2 requires that Si+4 be coordinated by 4 O-2 ions in tetrahedral coordination. In order to neutralize the +4 charge on the Si cation, one negative charge from each of the Oxygen ions will reach the Si cation. Thus, each Oxygen will be left with a net charge of -1, resulting in a SiO4-4 tetrahedral group that can be bonded to other cations. It is this SiO4-4 tetrahedron that forms the basis of the silicate minerals. Since Si+4 is a highly charged cation, Pauling's rules state that it should be separated a far as possible from other Si+4 ions. Thus, when these SiO4-4 tetrahedrons are linked together, only corner oxygens will be shared with other SiO4-4 groups. Several possibilities exist and give rise to the different silicate groups. Nesosilicates (Island Silicates) If the corner oxygens are not shared with other SiO4-4 tetrahedrons, each tetrahedron will be isolated. Thus, this group is often referred to as the island silicate group. The basic structural unit is then SiO4-4. In this group the oxygens are shared with octahedral groups that contain other cations like Mg+2, Fe+2, or Ca+2. Olivine is a good example:

4 (Mg,Fe)2SiO4. Sorosilicates (Double Island Silicates) If one of the corner oxygens is shared with another tetrahedron, this gives rise to the sorosilicate group. It is often referred to as the double island group because there are two linked tetrahedrons isolated from all other tetrahedrons. In this case, the basic structural unit is Si2O7-6. A good example of a sorosilicate is the mineral hemimorphite Zn4Si2O7(OH).H2O. Some sorosilicates are a combination of single and double islands, like in epidote Ca2(Fe+3,Al)Al2(SiO4)(Si2O7)(OH). Cyclosilicates (Ring Silicates) If two of the oxygens are shared and the structure is arranged in a ring, such as that shown here, we get the basic structural unit of the cyclosilcates or ring silicates. Shown here is a six membered ring forming the structural group Si6O Three membered rings, Si3O9-6, four membered rings, Si4O12-8, and five membered rings Si5O15-10 are also possible. A good example of a cyclosilicate is the mineral Beryl - Be3Al2Si6O18. Inosilicates (Single Chain Silicates) If two of the oxygens are shared in a way to make long single chains of linked SiO4 tetrahedra, we get the single chain silicates or inosilicates. In this case the basic structural unit is Si2O6-4 or SiO3-2. This group is the basis for the pyroxene group of minerals, like the orthopyroxenes (Mg,Fe)SiO3 or the clinopyroxenes

5 Ca(Mg,Fe)Si2O6. Inosilicates (Double Chain Silicates) If two chains are linked together so that each tetrahedral group shares 3 of its oxygens, we can from double chains, with the basic structural group being Si4O11-6. The amphibole group of minerals are double chain silicates, for example the tremolite - ferroactinolite series - Ca2(Mg,Fe)5Si8O22(OH)2. Phyllosilicates (Sheet Silicates) If 3 of the oxygens from each tetrahedral group are shared such that an infinite sheet of SiO4 tetrahedra are shared we get the basis for the phyllosilicates or sheet silicates. In this case the basic structural group is Si2O5-2. The micas, clay minerals, chlorite, talc, and serpentine minerals are all based on this structure. A good example is biotite - K(Mg,Fe)3(AlSi3)O10(OH)2. Note that in this structure, Al is substituting for Si in one of the tetrahedral groups.

6 Tectosilicates (Framework Silicates) If all of the corner oxygens are shared with another SiO4 tetrahedron, then a framework structure develops. The basic structural group then becomes SiO2. The minerals quartz, cristobalite, and tridymite all are based on this structure. If some of the Si+4 ions are replaced by Al+3 then this produces a charge imbalance and allows for other ions to be found coordinated in different arrangements within the framework structure. Thus, the feldspar and feldspathoid minerals are also based on the tectosilicate framework. General Formula for Silicates Based on these basic structural units, we can construct a general structural chemical formula for the silicates. But one substitution in particular tends to mess things up a bit. This is Al+3, the third most abundant element in the Earth's crust. Al+3 has an ionic radius that varies between 0.54 and 0.39 depending on the coordination number. Thus, it could either fit in 6-fold coordination with oxygen or 4-fold coordination with oxygen. Because Al+3 will go into 4-fold coordination with oxygen, it sometimes substitutes for Si+4. If such a substitution takes place, it creates a charge imbalance that must be made up elsewhere in the silicate structure. The other common elements in the Earth's crust that enter the silicates do so in other types of coordination. Ions like Al+3, Mg+2, Fe+2, Fe+3, Mn+2, and Ti+4 enter into 6-fold or octahedral sites. Larger ions like Ca+2, and Na+1, are found in octahedral coordination or 8-fold, cubic coordination sites. Very large cations like K+1, Ba+2, and sometimes Na+1 are coordinated by 12 oxygens in 12-fold coordination sites. We can thus write a general structural formula for the silicates as

7 follows: XmYn(ZpOq)Wr where X represents an 8 to 12 fold coordination site for large cations like K+, Rb+, Ba+2, Na+, and Ca+2. Y represents a 6-fold (octahedral) site for intermediate sized cations like Al+3, Mg+2, Fe+2, Fe+3, Mn+2, and Ti+4. Z represents the tetrahedral site containing Si+4, and Al+3. the ratio p:q depends on the degree of polymerization of the silica (or alumina) tetrahedrons, or the silicate structural type Site C.N. as discussed above. O is oxygen, and W is a hyrdoxyl (OH-1) site into which can substitute large anions like F-1 or Cl-1. Z Y The subscripts m, n, and r depend on the ratio of p to q and are chosen to maintain charge balance. This is summarized in the table shown here. In this table note that there is very little substitution that takes place between ions that X enter the X, Y, and Z sites. The exceptions are mainly substitution of Al+3 for Si+4, which is noted in the Table, and whether the X site is large enough to accept the largest cations like K+1, Ba+2, or Rb+1. Nesosilicates (Island Silicates) Ion Si+4 4 Al+3 Al+3 Fe+3 Fe+2 6 Mg+2 Mn+2 Ti+4 Na+1 8 Ca+2 K Ba+2 Rb+1

8 We now turn our discussion to a systematic look at the most common rock forming minerals, starting with the common nesosilicates. Among these are the olivines, garnets, Al2SiO5 minerals, staurolite, and sphene (the latter two will be discussed in the last lecture on accessory minerals). As discussed above, the nesosilicates or island silicates are based on the isolated SiO4-4 tetrahedral groups. In the olivines, the remaining corner oxygens form octahedral groups that coordinate Mg+2 and Fe+2 ions. Olivines The olivines consist of a complete solid solution between Mg2SiO4 (forsterite, Fo) and Fe2SiO4 (fayalite, Fa). There is limited substitution of the following end members: Ca2SiO4 - larnite Mn2SiO4 - tephroite CaMgSiO4 - monticellite (which is commonly found in metamorphosed dolomites) Also found substituting in octahedral sites are Ni+2 and Cr+3, particularly in Mg-rich olivines.

9 The phase diagram for the common end members of the olivine solid solution series shows that pure forsterite melts at 1890oC and pure fayalite melts at 1205oC. Thus, the olivines are sometimes seen be be zoned from Mg-rich cores to more Fe-rich rims, although such zoning is usually limited to 5 to 10% difference between the cores and the rims. Occurrence Pure forsterite is limited to metamorphosed Mg-rich limestones and dolomitic metamorphic rocks. Fo90-95 is found in ultrabasic igneous rocks, particularly dunites (>90% by volume olivine), and peridotites (Olivine + Cpx + Opx). Fo60-90 is found in basic igneous rocks likes basalts and gabbros, and sometimes in andesites, where it occurs with plagioclase and pyroxene. Fa is found in Fe-rich siliceous igneous rocks like rhyolites and granites. Mg-rich olivines rarely occur in quartz bearing rocks and quartz rarely occurs with Mg-rich olivine because the reaction shown below runs to the right for most pressures and temperatures. Mg2SiO4 + SiO2 <=> 2MgSiO3

10 Fo Qtz En Note however, that Fe-rich olivines can occur with quartz. Structure The structure of the olivines is illustrated on page 439 of Klein and Dutrow. Note that 2 different kinds of octahedral sites occur. One is a regular octahedron, labeled M2, and the other is a distorted octahedron, labeled M1. Fe+2 and Mg+2 have no particular preference for either site, but if Ca+2 is present it prefers the M2 site. Identifying Properties The olivines are orthorhombic (2/m2/m2/m) and usually green colored in hand specimen. The most characteristic property in thin section is their surface texture that kind of looks like a piece of sandpaper (see photo on the back wall of the Mineralogy lab). Because of their good {010} cleavage and common {100} parting, they show parallel extinction relative to the cleavage or parting. Maximum birefringence as seen in the interference colors in thin section varies between 3rd order blue (for Fo rich varieties) and 3rd order yellow (for Fa-rich varieties), but remember that this is the maximum birefringence that will only be seen for grains with and parallel to the microscope stage. Fo-rich olivines are usually clear in thin section, but Fa-rich olivines show pale yellow, greenish yellow, or yellow amber

11 absorption colors and sometimes show pleochroism with = = pale yellow, = orange, yellow, or reddish brown. Because optical properties vary with composition of the olivine, 2V is useful in distinguishing olivine compositions. Look at the graph on page 11 of Deer, Howie, and Zussman. From the graph you can see that very Fo-rich olivines(>fo90) are optically positive with a 2V between 82 and 90o. Between Fo90 and Fa100 the olivine is optically negative with 2V between 90 and 130 (2V between 90o and 50o. Thus, by estimating the 2V, you should be able to estimate the composition of the olivine. Olivines are distinguished from orthorhombic pyroxenes (opx) easily because olivines show higher maximum birefringence and do not show the characteristic {110} cleavage of the pyroxenes. They are distinguished from the clinopyroxenes (Cpx) which show inclined extinction relative their {110} cleavage and show a biaxial positive character with a 2V of 50 to 60o. Garnets Garnets are isometric minerals and thus isotropic in thin section, although sometimes they are seen to be weakly birefringent (slightly anisotropic). They are also nesosilicates, and therefore based on the SiO4 structural unit. The general formula for garnets is: A3B2(Si3O12) where the A sites are cubic sites containing large divalent cations, usually Ca, Fe, Mg, or Mn, and the B sites are octahedral sites occupied by smaller trivalent cations, like Al and Fe+3. Garnets with no Ca in the A site and Al in the B site are called the

12 pyralspite series. These consist of the end members: Pyrope - Mg3Al2Si3O12 Almandine - Fe3Al2Si3O12 Spessartine - Mn3Al2Si3O12 Garnets with Ca in the A site are called the ugrandite series and consist of the end members: Uvarovite - Ca3Cr2Si3O12 Grossularite - Ca3Al2Si3O12 Andradite - Ca3Fe+32Si3O12 Limited solid solution exists between end members of each series. Occurrence The garnets occur mostly in metamorphic rocks where they are often seen to form euhedral (well-formed) crystals. The Mg-rich garnet, pyrope, is found in metamorphic rocks formed at high pressure and in eclogites (basalts metamorphosed at high pressure) and peridotites (ultrabasic rocks containing olivine, Opx, Cpx, and garnet). The Fe-rich garnet, almandine, is the most common garnet and is found in metamorphic aluminous schists. The Mn-rich variety, spessartine, is limited to Mn-rich metamorphic rocks like meta-cherts. Identifying Properties

13 Garnets are generally isotropic although some may be weakly birefringent. In hand specimen they exhibit a wide range of colors and these are sometimes seen in thin section. Color is controlled by the amounts of Fe+2, Fe+3, Mg+2, and Cr+3 present. Pyrope is usually pinkish red to purplish in hand specimen and is usually clear in thin section. Almandine is usually deep red to brownish black in hand specimen and pink in thin section. Spessartine ranges from black to red to brown and orange and is usually pink in thin section. Grossularite has a color in hand specimen that reflects the amount of Fe and Mn present and thus ranges from brown to yellow to pink. If Cr is present, the color is usually green. In thin section grossularite varies in color from clear to brown or green in Crrich varieties. Uvarovite, with high Cr concentration is usually deep green in hand specimen and green in thin section. Andradite ranges from yellow to dark brown, but if appreciable amounts of Ti are present, the color could be black in hand specimen and brown in thin section. The composition and identity of the garnets is best determined either by association with other minerals or by more sophisticated techniques such as electron microprobe or XRD. Garnets are easily distinguished from other minerals by their high relief, isotropic character, and common euhedral habit.

14 Al2SiO5 Minerals The Al2SiO5 minerals are common in aluminous metamorphic rocks (meta-shales and meta-mudstones) and sometimes found in aluminous igneous rocks. In metamorphic rocks the Al2SiO5 polymorphs provide rather general estimates of the pressure and temperature of metamorphism, with Kyanite indicating relatively high pressure, andalusite indicating low temperature and pressure, and sillimanite indicating high temperature. Better estimates of pressure and temperature are provided if two of the minerals are present in the same rock. Sillimanite Sillimanite is orthorhombic with a good {010} cleavage. It generally occurs in long fibrous crystals that are length slow, with extinction parallel to the {010} cleavage. In sections lying on {001}that show well-developed {110} forms, the cleavage is usually seen to cut across the crystal as shown here. Maximum birefringence is generally seen to be between 2o yellow to 2o red. Sillimanite is biaxial positive with a 2V of 21-31o. Andalusite

15 Andalusite is also orthorhombic, but shows a length fast character. It generally tends to occur as euhedral blocky crystals with a maximum birefringence in thin section between 1o yellow and 1o red. It sometimes shows weak pleochroism with = rosepink, = = greenish yellow. Some varieties show a cross, termed the chiastolite cross, which is made up of tiny carbonaceous inclusions oriented along crystallographic directions (see illustration on page 492 of Klein & Dutrow). Andalusite generally occurs as euhedral crystals with an almost square prism. It is biaxial negative with 2V = 73-86o. Kyanite Kyanite is triclinic and thus shows inclined extinction relative to its good {100}and {010}cleavages and {001} parting. In hand specimen kyanite is commonly pale blue in color, but is clear to pale blue in thin section. Because of its good cleavages and parting, two cleavages or partings are seen in any orientation of the crystal in thin section. These cleavages intersect at angles other than 90o and thus look like parallelograms in two dimensions. Because Kyanite has high relief relative to other minerals with which it commonly occurs, it stands out in thin section and sometimes appears to have a brownish color. This color is more due to its high relief and numerous cleavages rather than due to selective absorption. Kyanite is biaxial negative with 2V = 78-83o Staurolite (Mg,Fe)2Al9Si4O22(OH)2 Staurolite is a common mineral in medium grade metamorphic rocks, usually metamorphosed shales. In hand specimen and in thin section it characteristically is seen to show

16 staurolite twinning, either the rightangle cross, twinned on {031} or the oblique cross, twinned on {231} It is monoclinic, but its optical properties are those of an orthorhombic mineral. It has moderate {010} cleavage, which if present, will cause parallel extinction. It's most distinguishing property is its pleochroism, with = colorless, = pale yellow, and = golden yellow. Less distinctive are its positive optic sign and 2V = 82-90o. In many rocks Staurolite shows twinning, and commonly forms euhedral crystals with well developed {100} and {010} crystal faces. In thin section Staurolite is commonly seen to contain tiny inclusions of other minerals, usually quartz. There are very few minerals which can be confused with Staurolite. Sorosilicates Sorosilicates are the double island silicates. Only one important mineral group, the epidote group, has this structure. Epidote, Clinozoisite, Zoisite The important minerals in the epidote group are epidote, clinozoisite, and zoisite. Since the sorosilicates are based on the Si2O7-6 group, the structural formula can be written as: Ca2(Al,Fe+3)Al2O(SiO4)(Si2O7)(OH) Thus, the epidote group contains both the double tetrahedra and the single tetrahedron, separated by groups of AlO6 octahedra and Ca in nine to 10 fold coordination with Oxygen or OH. The formula can be rewritten as: Ca2(Al,Fe+3)Al2Si3O12(OH) Epidote is the Fe-rich variety and has the above general formula.

17 Clinozoisite is the Fe-free variety with the chemical formula: Ca2Al3Si3O12(OH) Both clinozoisite and epidote are monoclinic (2/m). Zoisite has the same chemical formula as clinozoisite, but is orthorhombic. Epidote is usually pistachio green in color with perfect {001} cleavage and imperfect {100} cleavage. It is optically negative with a 2V of 64 90o. It usually shows pleochroism with - colorless to pale yellow, greenish yellow, and - yellowish green, and shows high relief relative to feldspars and quartz. It's birefringence is high enough to show 3rd order interference colors. It usually shows an anomalous blue extinction. Clinozoisite shows similar relief and cleavage to epidote, but it is optically negative with a 2V of 14 to 90o, shows no pleochroism, and lower birefringence (1st to 2nd order interference colors). Zoisite is similar to clinozoisite, except it will show parallel extinction relative to faces parallel to the crystallographic axes. Epidote is a common mineral in low grade metamorphic rocks, particularly metamorphosed volcanic rocks and Fe-Al rich meta shales. Both Clinozoisite and epidote occur as alteration products of plagioclase and as veins in granitic rocks. Cyclosilicates The cyclosilicates are based on rings of SiO4 tetrahedra, with a Si:O ratio of 1:3 The most common minerals based on this structure are Beryl, Cordierite, and Tourmaline. Beryl

18 Be3Al2Si6O18 is hexagonal (6/m2/m2/m) with a strong prismatic habit with the form {10 0} usually the only form present. It is usually deep green to yellowish green in color. Beryl forms different gemstones depending on color - Aquamarine when it is pale greenish-blue, Morganite if pink, and emerald if deep green and transparent. Beryl is a common constituent of coarse grained granitic rocks and pegmatites and is found in aluminous mica schists. In thin section, Beryl shows higher relief than quartz, and is distinguished from quartz by its negative optic sign and length-fast character. The only other mineral that it can be confused with is apatite, but apatite shows even higher relief than Beryl. Cordierite Cordierite is (Mg,Fe)2Al4Si5O18.nH2O. It is orthorhombic (2/m2/m2/m), but shows a pseudohexagonal character due to its common cyclical twinning on {110}. In thin section it may show a twinning that looks like albite twinning, which makes it hard to distinguish from plagioclase. But, cordierite is usually dusted with tiny opaque inclusions. In thick sections it shows a pale -yellow, violet, pale blue pleochroism. It can be distinguished from quartz by its biaxial character. Cordierite is a common constituent of aluminous metamorphic rocks. It is common in contact metamorphic rocks where it is commonly associated with sillimanite or andalusite, feldspars and micas. Tourmaline Tourmaline - Na(Mg,Fe,Mn,Li,Al)3Al6Si6O18(BO3)3(OH)4 is hexagonal (3m) and is commonly found as well-formed prismatic crystals, with a rounded triangular cross section perpendicular to the c crystallographic axis.

19 Tourmaline is a common mineral in pegmatites (SiO2 - rich igneous rocks with large grain size), where it is associated with quartz and alkali feldspar. It is also found in metasomatized rocks of all types, where it is precipitated from a Boron and Silica - rich fluid phase. It's most distinguishing properties are its uniaxial negative optical character and its pleochroism with = dark green or dark blue and = yellow or violet. Tourmaline usually forms in euhedral crystals with well developed prism faces and extinction parallel to the prism faces. COMPILED BY GDC HANDWARA

20 LECTURE NOTES 1ST SEMESTER UNIT 4 Inosilicates (Single Chain Silicates) The single chain silicates have a basic structural unit consisting of linked SiO4 tetrahedra that each share 2 of their oxygens in such a way as to build long chains of SiO4. The basic structural group is thus Si2O6 with an Si:O ratio of 1:3. The most important inosilicates are the pyroxenes. These have a general structural formula of: XYZ2O6 where X = Na+, Ca+2, Mn+2, Fe+2, or Mg+2 filling octahedral sites called M2 Y = Mn+2, Fe+2, Mg+2, Al+3, Cr+3, or Ti+4 filling smaller octahedral sites called M1 Z = Si+4 or Al+3 in tetrahedral coordination. The pyroxenes can be divided into several groups based on chemistry and crystallography: Orthorhombic Pyroxenes (Orthopyroxenes - Opx) These consist of a range of compositions between enstatite - MgSiO3 and ferrosilite - FeSiO3 Monoclinic Pyroxenes (Clinopyroxenes - Cpx) The Diopside- Hedenbergite series - Diopside (CaMgSi2O6) Ferrohedenbergite (CaFeSi2O6) The Sodic Pyroxenes - Jadeite (NaAlSi2O6) and Aegerine (NaFe+3Si2O6) Augite is closely related to the diopside - Hedenbergite series with addition of Al and minor Na substitution - (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6 Pigeonite is also a monoclinic pyroxene with a composition similar to the

21 orthopyroxenes with more Ca substituting for Fe, and Mg. The compositional range of the Carich, Al-free pyroxenes in shown in the triangular composition diagram here. Note that there is complete Mg-Fe substitution and small amounts of Ca substitution into the Orthopyroxene solid solution series. Mg-rich varieties of orthopyroxene are called hypersthene, whereas Ferich varieties are called Ferrosilite. There is also complete Mg-Fe solid solution between Diopside and Ferrohedenbergite, with some depletion in Ca. CaSiO3 is the chemical formula for wollastonite, but wollastonite does not have a pyroxene structure. There is complete Mg-Fe solid solution between the pyroxenes, and as with most Mg-Fe solid solutions, the Mg-rich end members crystallize at higher temperatures than the Fe-rich end members.

22 Solid immiscibility is present between the Diopside - Hedenbergite series and the Orthopyroxene series. This is seen in the phase diagram below which shows a hypothetical phase diagram running from the orthopyroxenes to the clinopyroxenes. Note the solvi. Pigeonite is only stable at higher temperatures and inverts to orthopyroxene if cooled slowly to lower temperatures. Thus, pigeonite is only found in volcanic and shallow intrusive igneous rocks, or as exsolution lamellae in a host augite or opx (more commonly in augite). When pigeonite or augite exsolve they may form exsolution lamellae that form parallel to the (001) plane. At lower temperature the exsolution of Opx or augite result in exsolution lamellae that are parallel to the (100) plane.

23 All pyroxenes show perfect {110} cleavage. When viewed looking down the ccrystallographic axis, the cleavages intersect at near 90o angles (the angles are actually 92-93o and ). This 90 degree cleavage angle is most useful in distinguishing pyroxenes from amphiboles (in amphiboles the cleavages are at 56o and 124o. Distinguishing Opx from Cpx in thin section is accomplished by noting that in all orthorhombic pyroxenes the prismatic {110} cleavage will show parallel extinction. If looking down the caxis the extinction will be symmetrical relative to the two cleavage traces.

24 In Cpx, however, one would see inclined extinction on all faces except {100}. Thus, one should check several grains for extinction before concluding that the mineral is Opx, since there is always a slight chance that one is looking at a {100} face. Note that in Cpx, the maximum extinction angle will only be observed if one is looking at a {010} face. Occurrence and Distinction of the Pyroxenes Augite - is commonly found in both plutonic and volcanic igneous rocks, as well as high grade meta-igneous rocks like gneisses and granulites. It is easily distinguished from amphiboles by the nearly 90ocleavage angles, and is distinguished from Opx by inclined extinction relative to the {110} cleavage, as discussed above. Augite also has higher maximum birefringence than Opx, and shows 2nd to 3rd order interference colors. Augite is optically positive with a 2V of about 60o. It shows high relief, relative to quartz and feldspars and is commonly colorless to brown or green in thin section, showing no pleochroism. Hypersthene - is commonly found in both plutonic and volcanic igneous rocks and in meta-igneous rocks as well. It is distinguished from augite by its lower interference colors and lack of inclined extinction relative to {110}. Hypersthene is sometimes pleochroic, showing light pink to light green colors. The chemical composition of hypersthene can be estimated using 2V (see p. 163 of DHZ). Compositions close to Enstatite are optically positive with a 2V of 60 to 90o, whereas intermediate compositions are optically negative with a 2V of 50 to 90o. Pigeonite - is generally only found in volcanic igneous rocks, although, as mentioned above, it can occur as exsolution lamellae in augites of more slowly cooled igneous rocks. Pigeonite is distinguished from augite by its

25 lower 2V of 0 to 30o, and is distinguished from hypersthene by its lack of pleochroism, lower 2V and inclined extinction relative to the {110} cleavage. Aegerine (acmite) - Aegerine Augite - are sodic pyroxenes and thus are found in alkalic igneous rocks associated with sodic amphiboles, alkali feldspars, and nepheline. The mineral is common in alkali granites, quartz syenites, and nepheline syenites (all alkalic plutonic rocks), and are also found in sodic volcanic rocks like peralkaline rhyolites. Aegerine is distinguished from other clinopyroxenes by a low extinction angle relative to the {110} cleavage (0-10o, with augite having an extinction angle of 35-48o), and by the green brown pleochroism present in aegerine. Aegerine is also optically negative with a 2V of 60 to 70o, whereas Aegerine-augite has a higher 2V and can be optically positive or negative. It is distinguished from the pleochroic sodic amphiboles by its nearly 90o pyroxene cleavage angle. Jadeite - is a sodium aluminum pyroxene that is characterized by its presence in metamorphic rocks formed at relatively high pressure. It can form by a reaction of Albite to produce : NaAlSi3O8 = NaAlSi2O6 + SiO2 Albite Jadeite Quartz Jadeite has a lower refractive index than all other pyroxenes, and has low birefringence, showing low order 1st and 2nd order interference colors. It is monoclinic with an extinction angle of 33 to 40o, and can thus be easily distinguished form hypersthene. It is usually colorless in thin section, helping to distinguish it from augite and aegerine, and has lower birefringence than augite and aegerine. Inosilicates (Double Chain Silicates) - The Amphiboles The amphibole group of minerals is based on the double-chain silicate structure as shown here. The basic structural unit is (Si4O11)-6. The structural formula can

26 be written as: W0-1X2Y5Z8022(OH,F)2 where W = Na+1 or K+1 in the A site with 10 to 12 fold coordination. X = Ca+2, Na+1, Mn+2, Fe+2, Mg+2, Fe+3, in an M4 site with 6 to 8 fold coordination. Y = Mn+2, Fe+2, Mg+2, Fe+3, Al+3. or Ti+4 in an M1 octahedral coordination site. Z = Si+4 and Al+3 in the tetrahedral site. There is complete solid solution between Na and Ca end members and among Mg and Fe end members, with partial substitution of Al+3 for Si+4 in the tetrahedral site, and partial substitution of F for OH in the hydroxyl site. The composition of the common (non-sodic) amphiboles are shown in the diagram here. Note the similarity to the pyroxene compositional diagram, above. Actinolite is the solid solution between Tremolite [Ca2Mg5Si8O22(OH)2] and Ferroactinolite [Ca2Fe5Si8O22(OH)2.] Cummingtonite - Grunerite is a solid solution between Anthophyllite [Mg7Si8O22(OH)2] and Grunerite [Fe7Si8O22(OH)2]. Hornblende is the most common amphibole and has more in common with the Tremolite - Ferroactinolite series, with Al substituting into the Y sites and the tetrahedral site. It thus has the complicated formula:

27 (Ca,Na)2-3(Mg,Fe,Al)5Si6(Si,Al)2O22(OH,F)2 The sodic amphiboles have the following formulae: Glaucophane - Na2Mg3Al2Si8O22(OH)2 Riebeckite - Na2Fe3+2Fe2+3Si8O22(OH)2 Arfvedsonite - NaNa2Fe4+2Fe+3Si8O22(OH)2 All of the amphiboles except Anthophyllite are monoclinic, and all show the excellent prismatic cleavage on {110}. The angles between the cleavages, however are 56o and 124o making all amphiboles easy to distinguish from the pyroxenes. Looking at faces that show only a single cleavage trace would show inclined extinction, except in Anthophyllite. Occurrence and Distinction of the Amphiboles Tremolite - Occurs almost exclusively in low grade metamorphic rocks, particularly those with a high Ca concentration, such as meta-dolomites, meta-ultrabasic rocks. Tremolite in hand specimen is white in color and shows a fibrous habit and the characteristic amphibole cleavage. In thin section it is distinguished from wollastonite and diopside by its amphibole cleavage. In thin section it is clear with no pleochroism, which distinguishes it from other amphiboles. It shows high relief, inclined extinction, and is optically negative with a 2V of about 85o. Actinolite - Also occurs almost exclusively in low grade metamorphic rocks, particularly in meta-basalts and meta-gabbros where it is commonly associated with chlorite. It is green in hand specimen and shows the characteristic amphibole cleavage, usually showing an elongated habit. In thin section it shows a characteristic pale yellow to green pleochroism, has high relief, and is optically negative with a 2V of 60 to 85o. Hornblende - is a common mineral in both igneous and metamorphic rocks. In igneous rocks it is found in andesites, dacites, and rhyolites, as well as in gabbros, diorites, and granites. In metamorphic rocks it is a common constituent of meta-basalts that have been metamorphosed to

28 intermediate grades of regional metamorphism (amphibolites). It is also found in some ultrabasic rocks. In hand specimen it is dark brown to black in color and shows the characteristic amphibole cleavage. In thin section, it shows high relief with a characteristic green - brown - yellow pleochroism. Optic sign and 2V angle cover a wide range and not very useful in the distinction of hornblende. Basaltic Hornblende (also called Oxy-hornblende)- is a dark brown to reddish brown variety of hornblende that results from oxidation during crystallization of basalts, andesites, dacites, and rhyolites. It usually has a dark reaction rim that consists of opaque oxide, and is characteristically pleochroic in yellow to brown to reddish brown colors. Anthophyllite - does not occur in igneous rocks, but is a constituent of metamorphic rocks. It is the only orthorhombic amphibole so it is easily characterized by its parallel extinction relative to the {110} cleavage. Cummingtonite - Grunerite - is more common in metamorphosed igneous rocks where members of the series occur with hornblende. It has been found in siliceous volcanic rocks as well. Cummingtonite is optically positive, while grunerite is optically negative. Members of this series can be distinguished from orthorhombic Anthophyllite by the inclined extinction of the monoclinic Cummingtonite-Grunerite series, and can be distinguished from tremolite and actinolite by the higher refractive indices and higher birefringence of the Cummingtonite Grunerite series. Glaucophane - Riebeckite - Glaucophane is a common mineral in blueschist facies metamorphic rocks that result from low temperature, high pressure metamorphism along ancient subduction zones. Riebeckite is found in alkali granites, syenites, and peralkaline rhyolites. Glaucophane is easily distinguished from the other amphiboles by its characteristic bluelavender pleochroism. Glaucophane is length slow, whereas Riebeckite is length fast. Arfvedsonite - occurs most commonly in peralkaline volcanic rocks and alkaline plutonic igneous rocks, where it typically occurs with the sodic pyroxene aegerine. Its blue green to yellow green pleochroism distinguish it from the other amphiboles. The chart below, also found in your lab assignments, summarizes the properties

29 used to distinguish the amphiboles. COMPILED BY GDC HANDWARA

30 LECTURE NOTES 1ST SEMESTER UNIT 4 Phyllosilicates (Sheet Silicates) The phyllosilicates, or sheet silicates, are an important group of minerals that includes the micas, chlorite, serpentine, talc, and the clay minerals. Because of the special importance of the clay minerals as one of the primary products of chemical weathering and one of the more abundant constituents of sedimentary rocks, they will be discussed in more detail in the next lecture. The basic structure of the phyllosilicates is based on interconnected six member rings of SiO4-4 tetrahedra that extend outward in infinite sheets. Three out of the 4 oxygens from each tetrahedra are shared with other tetrahedra. This leads to a basic structural unit of Si2O5-2. Most phyllosilicates contain hydroxyl ion, OH-, with the OH located at the center of the 6 membered rings, as shown here. Thus, the group becomes Si2O5(OH)-3. When other cations are bonded to the SiO4 sheets, they share the apical oxygens and the (OH) ions which bond to the other cations in octahedral coordination. This forms a layer of cations, usually Fe+2, Mg+2, or Al+3, that occur in octahedral coordination with the O and OH ions of the tetrahedral layer. As shown, here, the triangles become the faces of the octahedral groups that can bind to the tetrahedral layers. The octahedral layers take on the structure of either Brucite [Mg(OH)3], if the cations are +2 ions like Mg+2 or Fe+2, or Gibbsite [Al(OH)3], if the cations are +3 like Al+3. In the brucite structure, all octahedral sites are occupied and all anions are OH-1. In the Gibbsite structure every 3rd cation site is unoccupied and all anions are OH-1. This gives rise to 2 groups of sheet silicates:

31 1. The trioctahedral sheet silicates where each O or OH ion is surrounded by 3 divalent cations, like Mg+2 or Fe The dioctahedral sheet silicates where each O or OH ion is surrounded by 2 trivalent cations, usually Al+3. We can build the structures of the various sheet silicates by starting with the octahedral layers similar to the structures of brucite or gibbsite, as shown below. The trioctahedral phyllosilicates are based on the structure where the octahedral layers are similar to brucite, where Mg+2 occupies the cation position. The dioctahedral phyllosilicates are based on the structure where the octahedral layers are similar to gibbsite, where Al+3 occupies the cation position. The octahedral sheets in both cases are held together by weak Van der Waals bonds. If we start with the brucite and gibbsite structures shown above, and replace 2 of the OH ions with O, where the Oxygens are now the apical Oxygens of the tetrahedral sheets, then we get the structure of the serpentine mineral, Lizardite, if the octahedral layer is trioctahedral, containing Mg+2. If the octahedral layer is dioctahedral, containing Al+3, the structure of the clay mineral Kaolinite, is obtained. This leads to a tetrahedral - octahedral (T-O) structure, where each T-O layer is bonded to the top (or bottom) of another T-O layer by Van der Waals bonds. If 2 more of the OH ions in the octahedral layer are replaced by O, and these O become the apical Oxygens for another tetrahedral layer, the this builds the trioctahedral phyllosilicate talc or the dioctahedral pyrophyllite. This becomes a T-O-T layer that can bond to other T-O-T layers by weak Van der Waals bonds.

32 If an Al+3 is substituted for every 4th Si+4 in the tetrahedral layer, this causes an excess -1 charge in each T-O-T layer. To satisfy the charge, K+1 or Na+1 can be bonded between 2 T-O-T sheets in 12-fold coordination. For the trioctahedral sheet silicates this becomes Phlogopite (Mg-biotite), and for the dioctahedral sheet silicates this becomes Muscovite. This makes a T-O-T - T-O-T layer that, again can bind to another T-O-T - T-O-T layer by weak Van der Waals bonds. It is along these layers of weak bonding that the prominent {001} cleavage in the sheet silicates occurs. Replacing 2 more Si+4 ions with Al+3 ions in the tetrahedral layer results in an excess -2 charge on a T-O-T layer, which is satisfied by replacing the K+1 with Ca+2. This results in the trioctahedral sheet silicate - Clintonite and the dioctahedral sheet silicate Margarite. Because of the differences in charge balance between the trioctahedral and dioctahedral sheet silicates, there is little solid solution between the two groups. However, within the trioctahedral sheet silicates there is complete substitution of Fe+2 for Mg+2 and limited substitution of Mn+2 into the octahedral sites. Within the dioctahedral sheet silicates there is limited substitution of Fe+3 for Al+3 in octahedral sites. In addition, F- or Cl- can substitute for (OH)- in the hydroxyl site. As previously discussed, substitution of F-1 stabilizes the mineral to

33 higher pressures and temperatures. Another group of phyllosilicates that is more of mixture of structural types is the chlorite group. Although chlorite is complex in that the amount of Al that can substitute Mg and Si is variable, one way of looking at the chlorite structure is shown below. Here, the chlorite structure is depicted as consisting of a brucite-like layer (with some Al) sandwiched between tetrahedral layers that are similar to phlogopite. Another important sheet silicate structure is that of vermiculite. This is similar to the talc structure, discussed above, with layers of water molecules occurring between each T-O-T layer. Similarly, insertion of layers of water molecules between the T-O-T sheets of pyrophyllite produces the structure of smectite clays. The vermiculite and smectite groups are therefore expanding type sheet silicates and as the water is incorporated into the structure the mineral increases its volume. Although we have shown that the octahedral layers fit perfectly between the tetrahedral layers, this is an oversimplification. If the tetrahedral layers were stacked perfectly so that apical oxygens were to occur vertically aligned, then the structure would have hexagonal symmetry. But, because this is not the case, most of the phyllosilicates are monoclinic. Serpentine Group The serpentine group of minerals has the formula - Mg3Si2O5(OH)4. Three varieties of serpentine are known. Antigorite and Lizardite are usually massive and fine grained, while Chrisotile is fibrous. As discussed above, the imperfect fit of the octahedral layers and the tetrahedral layers causes the crystal structure to have to bend.

34 In Antigorite the bending of the sheets is not continuous, but occurs in sets, similar to corrugations, as shown here. In Chrisotile, the bending of the sheets is more continuous, resulting in continuous tubes that give the mineral it's fibrous habit. The Chrisotile variety is commonly referred to as asbestos. Occurrence - Serpentine is found as an alteration product of Mg-rich silicates like pyroxene and olivine. It results due to hydration. For example: 2Mg2SiO4 + 3H2O <=> Mg3Si2O5(OH)4 + Mg(OH)2 Olivine water Serpentine Brucite Thus, serpentine is commonly found pseudomorphed after olivines and pyroxenes in altered basic and ultrabasic igneous rocks, like altered peridotites, dunites, and sometimes basalts and gabbros. It is commonly associated with minerals like magnesite (MgCO3), chromite, and magnetite. If the rock is made up almost entirely of serpentine, it is called a serpentinite. Properties - Because the serpentines usually occur either as fine-grained aggregates or fibrous crystals, optical properties are difficult to determine. Most of the time, serpentine can be distinguished by its characteristic pseudomorphing of other crystals like olivines and pyroxenes. In hand specimen it generally tends to have a dark green color with a greasy luster. In thin section it is clear to pale green to pale yellow, but does not show pleochroism, shows a generally low relief compared to minerals like olivine and pyroxene with which it is associated, and show very low order interference colors due to its low birefringence. Talc Talc has the chemical formula - Mg3Si4O10(OH)2. It is probably best know for its low hardness. Although it has a micaceous structure, it is so easily deformed, that crystals are rarely seen. Occurrence - Like serpentine, talc requires an environment rich in Mg. It is therefore found in

35 low grade metamorphic rocks that originated as ultrabasic to basic igneous rocks. Rocks composed almost entirely of talc have a greasy feel and are referred to as soapstone. Properties - Talc is most easily distinguished in hand specimen by its low hardness, greasy feel, and association with other Mg-bearing minerals. When crystals are present they show the characteristic micaceous cleavage on {001}. In thin section, talc is colorless, biaxial negative with a 2V of 0 to 30o. Like other sheet silicates, it shows the well developed {001} cleavage. Maximum interference colors, consistent with a birefringence of 0.05 is 3o yellow. Muscovite has a higher birefringence and higher 2V, properties which easily distinguish the 2 minerals. Mica Group The micas can be divided into the dioctahedral micas and the trioctahedral micas, as discussed above. Muscovite, Paragonite, and Margarite are the white micas, and represent the dioctahedral group, and Biotite and Clintonite (Xanthophyllite) the black or brown mica, represents the trioctahedral group. Muscovite and Biotite are the most common micas, but the Lithium- rich, pink mica, Lepidolite, K(Li,Al)2AlSi3O10(OH)2 is also common, being found mostly in pegmatites. Muscovite Muscovite, KAl3Si3O10(OH)2, and Paragonite, NaAl3Si3O10(OH)2, are two potential end members of the solid solution series involving K and Na. But, there is a large miscibility gap between the two end members with Muscovite being between 65% and 100% of K-rich end member, and Paragonite showing compositions between about 80% and 100% of the Na-rich end member. Occurrence - Muscovite is common constituent of Al-rich medium grade metamorphic rocks where is found in Al-rich schists and contributes to the schistose foliation found in these rocks. Muscovite is also found in siliceous, Al-rich plutonic igneous rocks (muscovite granites), but has not been found as a constituent of volcanic rocks. In these rocks it is commonly found in association with alkali feldspar, quartz, and sometimes biotite, garnet, andalusite, sillimanite, or kyanite. Properties - Muscovite is easily identified in hand specimen by its white to sometimes light brownish color and its perfect {001} cleavage. In thin section, the {001} cleavage is easily seen and it's high birefringence is exhibited by the large change in relief on rotation of the stage and it's 2nd to 4th order interference colors. It is clear and shows no pleochroism (which distinguishes it from Biotite), and it is biaxial negative with a 2V between 28 and 50o. One of the most diagnostic properties of the micas, including muscovite, is the mottled or birds-eye extinction exhibited by these minerals. Biotite

36 Biotite is a solid solution between the end members Phlogopite KMg3AlSi3O10(OH)2 and Annite KFe3AlSi3O10(OH)2, although pure Annite does not occur in nature. In addition, small amounts of Na, Rb, Cs, and Ba may substitute for K, and like in other minerals, F can substitute for OH and increase the stability of Biotite to higher temperatures and pressures. Occurrence - Nearly pure phlogopite is found in hydrous ultrabasic rocks like kimberlite, and is also found in metamorphosed dolomites. Biotite, with more Fe-rich compositions is common in dacitic, rhyolitic, and trachytic volcanic rocks, granitic plutonic rocks, and a wide variety of metamorphic rocks. In metamorphic rocks, biotite usually shows a preferred orientation with its {001} forms parallel to the schistose foliation. Properties - In hand specimen, Biotite is brown to black and shows the perfect {001} micaceous cleavage. In thin section, it shows the perfect cleavage and mottled extinction typical of all micas. It's most characteristic property is its pleochroism, showing yellow to brown to green colors. Hornblende shows similar pleochroic colors, but is distinguished from biotite by the differences in cleavage of the 2 minerals. Biotite is biaxial negative with a low 2V of 0o to 25o. Chlorite Group As discussed above, the Chlorite group has a structure that consists of phlogopite T-O-T layers sandwiching brucite-like octahedral layer. There is substantial substitution of Mg for Fe, and Al can substitute for (Mg, Fe) in both the octahedral sites, as well as for Si in the tetrahedral sites. Thus, chlorite can have a rather complicated formula - (Mg,Fe,Al)3(Si,Al)4O10(OH)6. Occurrence- Chlorite is a common mineral in low grade metamorphic rocks, where it occurs in association with minerals like actinolite, epidote, and biotite. It also forms as an alteration product of pyroxenes, amphiboles, biotite, and garnet in igneous as well a metamorphic rocks. Properties - In hand specimen, chlorite is recognized by its green color, micaceous habit and cleavage, and association with other minerals like actinolite and epidote. In thin section, Chlorite shows low relief and low birefringence, with a characteristic midnight blue to black anomalous interference color. It shows some pleochroism in the range of green to pale yellow. It is easily distinguished from biotite by its lower relief and anomalous interference color. COMPILED BY GDC HANDWARA

37 LECTURE NOTES 1ST SEMESTER UNIT 4 Tectosilicates (Framework Silicates) The tectosilicates or framework silicates have a structure wherein all of the 4 oxygens of SiO4-4 tetrahedra are shared with other tetrahedra. The ratios of Si to O is thus 1:2. Since the Si - O bonds are strong covalent bonds and since the structure is interlocking, the tectosilicate minerals tend to have a high hardness. SiO2 Minerals There are nine known polymorphs of SiO2, one of which does not occur naturally. These are: Crystal System Density (g/cm3) Refractive Index (mean) Stishovite Tetragonal Coesite Monoclinic Low ( ) Quartz Hexagonal High ( ) Quartz Hexagonal Kaetite (synthetic) Tetragonal Low ( ) Tridymite Monoclinic or Orthorhombic High ( ) Tridymite Hexagonal Low ( ) Cristobalite Tetragonal High ( ) Cristobalite Isometric Name Stishovite and Coesite are high pressure forms of SiO2, and thus have much higher densities and refractive indices than the other polymorphs. Stishovite is the only polymorph where the Si occurs in 6 fold (octahedral) coordination with Oxygen, and this occurs due to the high pressure under which the mineral forms. Both Stishovite and Coesite have been found associated with meteorite impact structures.

38 At low pressure with decreasing temperature, SiO2 polymorphs change from high Cristobalite Low Cristobalite - High Tridymite - Low Tridymite - High Quartz - Low Quartz. The high to low transformations are all displacive transformations. Since displacive transformations require little rearrangement of the crystal structure and no change in energy, the high ( ) polymorphs do not exist at the surface of the earth, as they will invert to the low ( ) polymorphs as temperature is lowered. Transformations between Cristobalite, Tridymite, and Quartz, however, as well as between the high pressure polymorphs and Quartz, are reconstructive transformations. Since reconstructive transformations require significant structural rearrangement and significant changes in energy, they occur slowly, and the high temperature and high pressure polymorphs can occur as metastable minerals at the Earth's surface. Quartz Quartz is hexagonal and commonly occurs as crystals ranging in size form microscopic to crystals weighing several tons. Where it crystallizes unhindered by other crystals, such as in cavities in rock or in a liquid containing few other crystals, it shows well-developed hexagonal prisms and sometimes showing apparent hexagonal pyramids or dipyramid. When it crystallizes in an environment where growth is inhibited by the surroundings, it rarely show crystal faces. It is also found as microcrystalline masses, such as in the rock chert, and as fibrous masses, such as in chalcedony. As visible crystals, Quartz is one of the more common rock forming minerals. It occurs in siliceous igneous rocks such as volcanic rhyolite and plutonic granitic rocks. It is common in metamorphic rocks at all grades of metamorphism, and is the chief constituent of sand. Because it is highly resistant to chemical weathering, it is found in a wide variety of sedimentary rocks. Several varieties of Quartz can be found, but these are usually only distinguishable in hand specimen. Rock Crystal - clear Quartz in distinct crystals - usually found growing in open cavities in rock. Amethyst - violet colored Quartz, with the color resulting from trace amounts of Fe in the crystal. Rose Quartz - a pink colored variety, that usually does not show crystal faces, the color resulting from trace amounts of Ti+4. Smokey Quartz - a dark colored variety that may be almost black, usually forming wellformed crystals. The color appears to result from trace amounts of Al+3 in the structure.

39 Citrine - a yellow colored variety. Milky Quartz - a white colored variety with the color being due to fluid inclusions. Milky Quartz is common in hydrothermal veins and pegmatites. A fibrous variety of Quartz is called Chalcedony. It is usually brown to gray to translucent with a waxy luster. It is found lining or filling cavities in rock where it was apparently precipitated from an aqueous solution. When it shows bands of color, it is commonly called by the following names: Carnelian - red colored Chalcedony Chrysoprase - apple-green colored as a result of coloration from NiO. Agate - alternating curving layers of Chalcedony with different colors or different porosities. Onyx - alternating layers of Chalcedony of different colors or porosities arranged in parallel planes. Bloodstone - green Chalcedony containing red spots of jasper (see below) Very fined grained aggregates of cryptocrystalline quartz makes up rock like Flint and Chert. Flint occurs as nodules in limestone, whereas chert is a layered rock deposited on the ocean floor. The red variety of flint is called Jasper, where the color results from inclusions of hematite. Optical Properties Quartz is uniaxial positive with a low relief and low birefringence, thus exhibited only 1o gray to 1o white interference colors. In thin section it is almost always colorless when viewed without the analyzer inserted. One of its most distinguishing properties in thin section is that it usually has a smooth, almost polished-like surface texture. Quartz is easily distinguished from the Feldspars by the biaxial nature of feldspars, and from Nepheline which is uniaxial negative. Apatite, has similar birefringence to quartz, but is uniaxial negative and has a very high relief. In Chalcedony, the fibers are usually elongated perpendicular to the c-crystallographic axis and thus are length fast. Normal quartz, when it show an elongated habit, is elongated parallel to the c axis, and is thus length slow. Tridymite Tridymite is the high temperature polymorph of SiO2. Thus, it is only commonly found in

40 igneous rocks that have been cooled rapidly to surface temperatures, preventing the slow transformation to quartz, the stable form of SiO2 at surface temperatures. Because of this, we only expect to find Tridymite in siliceous volcanic rocks like rhyolites, where it commonly occurs as wedge shaped crystals in cavities in the rock. In volcanic rocks, Tridymite is commonly associated with Cristobalite and Sanidine. Optical Properties Tridymite usually occurs as orthorhombic or monoclinic wedge shaped crystals with a positive 2V between 40 and 90o. The wedge shape of the crystals is the result of twinning on {110}, and usually as 2 to 3 twinned individuals. Although it has similar birefringence to quartz and feldspar, it has lower refractive indices, and thus shows negative relief compared to quartz and feldspars. Cristobalite Cristobalite is also a high temperature SiO2 polymorph, and thus has a similar occurrence to Tridymite. It also occurs in thermally metamorphosed sandstones. In volcanic rocks it can occur both as a lining in open cavities, and as fine grained crystals in the groundmass of the rock. Optical Properties Cristobalite is tetragonal and thus uniaxial. It has a negative optic sign and shows lower relief than quartz, but has similar birefringence. Opal Opal is amorphous, and thus a mineraloid, with a formula - SiO2.nH2O. Feldspars The feldspars are the most common minerals in the Earth's crust. They consist of three endmembers: KAlSi3O8 - Orthoclase (or), NaAlSi3O8 - Albite (ab), and CaAl2Si2O8 -Anorthite (an) KAlSi3O8 and NaAlSi3O8 form a complete solid solution series, known as the alkali feldspars and NaAlSi3O8 and CaAl2Si2O8 form a complete solid solution series known as the plagioclase feldspars. The feldspars have a framework structure, consisting of SiO4 tetrahedra sharing all of the

41 corner oxygens. However, in the alkali feldspars 1/4 of the Si+4 ions are replaced by Al+3 and in the plagioclase feldspars 1/4 to 1/2 of the Si+4 ions are replaced by Al+3. This allows for the cations K+, Na+, and Ca+2 to be substituted into void spaces to maintain charge balance. Compositions of natural feldspars are shown in the diagram below based on the 3 components NaAlSi3O8, - Albite (ab), KAlSi3O8 - Orthoclase (or) and CaAl2Si2O8. The Alkali Feldspars form a complete solid solution between ab and or, with up to 5% of the an component. The high temperature more K-rich variety is called Sanidine and the more Na-rich variety is called anorthoclase. The plagioclase feldspars are a complete solid solution series between ab and an, and can contain small amounts of the or component. Names are given to the various ranges of composition, as shown here in the diagram are: Albite - ab90 to ab100 Oligoclase - ab70 to ab90 Andesine - ab50 to ab70 Labradorite - ab30 to ab50 Bytownite - ab10 - ab30 Anorthite - ab0 to an10 Plagioclase Feldspars Plagioclase is the most common feldspar. It forms initially by crystallization from magma. The plagioclase solid solution series is coupled solid solution where the substitution is: Na+1Si+4 <=> Ca+2Al+3

42 Thus, the general chemical formula for plagioclase can be written as: CaxNa1-xAl1+xSi3-xO8 where x is between 0 and 1. The phase diagram for the plagiocalse series is shown here, and shows that the Anorthite component has a higher melting temperature the than the Albite component. Thus, on crystallization, higher temperatures will favor more An-rich plagioclase which will react with the liquid to produce more Ab-rich plagioclase on cooling. Plagioclase occurs in basalts, andesites, dacites, rhyolites, gabbros, diorites, granodiorites, and granites. In most of these igneous rocks, it always shows the characteristic albite twinning. Plagioclase also occurs in a wide variety of metamorphic rocks, where it is usually not twinned. In such rocks where the plagioclase is not twinned, it is difficult to distinguish from the alkali feldspars. Plagioclase can be a component of clastic sedimentary rocks, although it is less stable near the Earth's surface than alkali feldspar and quartz, and usually breaks down to clay minerals during weathering. Properties In hand specimen, plagioclase is most commonly white colored and shows perfect {100} and good {010} cleavage. It is most easily identified and distinguished from quartz, sanidine, orthoclase, and microcline, by its common polysynthetic twinning on {010}. If this twinning is not present, plagioclase can still be distinguished from quartz by its cleavage, but cannot easily be distinguished from the alkali feldspars. If both plagioclase and alkali feldspar occur in the same rock, the two can usually be distinguished by differences in color or differences in the extent of weathering.

43 In thin section, plagioclase commonly shows the characteristic albite polysynthetic twinning. This twinning is the most characteristic identifying feature of plagioclase, and makes its identification easy when present. Although some cross-hatched twinning may also occur in plagioclase, it is always very simple with only one or two cross twins per grain. Thus, be careful not to identify plagioclase as microcline. The cross-hatched twinning in microcline is always much more complex. Plagioclase often shows zoning. This is exhibited by the extinction position changing from the rim to the core of the crystal. Remember that zoning is caused by incomplete reaction of crystals with liquid during cooling of a solid solution. Often the zoning is very complex, and is sometimes oscillatory. Normal zoning would show Ca - rich cores and Na - rich rims, but reverse zoning is possible under certain conditions. In metamorphic rocks plagioclase may not show twinning making it difficult to distinguish from orthoclase. The two can be distinguished by staining the thin section with stains that make the K-feldspars one color and the more Ca-rich feldspars another color. In this class, we will not have time to look at these staining techniques. You should, however, be aware, that such staining techniques exist, so that if you need them in the future, you can use them. The optical properties of the plagioclase series vary widely as a function of composition of the plagioclase. In general, all plagioclases show low order interference colors, and thus, low birefringence. Optic sign and 2V vary widely, and are thus, not very distinguishing features of plagioclase. Although, as you have seen in lab, it is possible to estimate the composition of plagioclase from a combination of extinction angle and twinning. Alkali Feldspars (K,Na)AlSi3O8 As an alkali feldspar cools from high temperature to lower temperature, the crystal structure changes from that of sanidine, which is monoclinic, through orthoclase, also monoclinic, but with a different crystal structure than sanidine, to microcline, which is triclinic. These transformations are order-disorder transformations, and thus require large amounts of time. Furthermore, if the feldspar is allowed to cool very slowly, then exsolution will occur, and the solid solution will separate into a Na-rich phase and a K-rich phase. Thus, one expects to find sanidine in rocks that were cooled very rapidly from high temperature, i.e. volcanic rocks. Orthoclase and microcline will be found in plutonic igneous rocks (cooled slowly at depth in the earth) and in metamorphic rocks. In addition, in the plutonic rock types if the cooling takes place slowly enough, then perthitic exsolution lamellae may also form. All of the alkali feldspars have low relief and low birefringence. Thus the interference colors may range up to 1o white. Since this is the same interference color we expect for quartz, care

44 must be taken to avoid confusing feldspars and quartz. Sanidine Sanidine generally occurs with an equant habit (almost square) and shows perfect {001} and {010} cleavages, which readily distinguish it from quartz. Rarely does sanidine show twinning, but when it does, it is usually simple twinning. Optic axis figures will only be found on sections showing both cleavages. Sanidine is optically negative with a 2V of 20-50o. This distinguishes it from quartz, which is uniaxial positive, and from the other alkali feldspars which show larger values of 2V. Orthoclase Orthoclase is a common alkali feldspar in granitic rocks and K - Al rich metamorphic rocks. It often shows perfect {001} and {010} cleavages which will distinguish it from quartz. Also, quartz usually shows a smooth surface texture, while orthoclase appears much rougher. Orthoclase is also biaxial, which further distinguishes it from quartz. The 2V of orthoclase varies from 60 to 105o, and thus it may be either positive or negative. The 2V angle distinguishes orthoclase from sanidine, but is otherwise not very useful because of the its wide range. Microcline Microcline is the lowest temperature form of alkali feldspar. Upon cooling, orthoclase must rearrange its structure from monoclinic to triclinic. When this happens, twinning usually results. The twinning characteristic of microcline is a combination of albite twinning and pericline twinning. This results in a cross-hatched pattern (often called tartan twinning) that is the most distinguishing characteristic of microcline. Anorthoclase Anorthoclase is a Na - rich feldspar with approximately equal amounts of the Anorthite (Ca) and orthoclase (K) components. Generally anorthoclase occurs in Na - rich volcanic rocks. Like the other alkali feldspars, it has perfect {001} and {010} cleavages. Sections showing both of the cleavages are best for determining the optic sign and 2V. Anorthoclase sometimes shows twinning, but generally not the multiple twinning seen in the plagioclase feldspars, but a crosshatched twinning similar to that seen in microcline, but on a very fine scale. Anorthoclase, like sanidine shows a low 2V of 5 to 20o, and is optically negative. Anorthoclase can sometimes be distinguished from sanidine by the fact that anorthoclase usually forms crystals with a tabular, elongated habit, while sanidine forms crystals with a more equant habit. Feldspathoids The feldspathoid group of minerals are SiO2 poor, alkali rich minerals that occur in low SiO2, high Na2O - K2O igneous rocks. In general, these minerals are not compatible with quartz, and therefore, are rarely, if ever, seen in rocks that contain quartz. They do, however, often occur with feldspars. Because of the alkalic nature of the rocks that contain feldspathoids, associated

45 pyroxenes and amphiboles are of the sodic variety, i.e. aegerine or riebeckite. The main feldspathoids are Nepheline (Na,K)AlSiO4, Kalsilite KAlSi2O6, and Leucite KAlSi2O6. At high temperature there is complete solid solution between Nepheline and Kalsilite, but at low temperature Nepheline can contain only about 12 wt% K2O. Other similar members of the feldspathoid group are: Sodalite 3NaAlSiO4.NaCl Nosean 3NaAlSiO4.NaSO4 Haüyne 3NaAlSiO4.Ca(Cl,SO4) Nepheline Nepheline occurs in both volcanic and plutonic alkaline igneous rocks. In hand specimen, Nepheline is difficult to distinguish from the feldspars, and thus must usually be identified by its association with other alkalic minerals. Nepheline has a yellowish colored alteration product, called cancrinite. Nepheline is hexagonal, and thus uniaxial, making it easy to distinguish from the feldspars. Furthermore, it is optically negative, making it distinguishable from quartz. It usually shows no cleavage, has low birefringence, and low relief (refractive indices are smaller than the feldspars). The only other common mineral with which nepheline could be confused is apatite, which is also uniaxial negative. Apatite, however, shows much higher relief than does nepheline. Sodalite Sodalite occurs predominantly in alkali-rich plutonic igneous rocks, like syenites, but can also be found in volcanic rocks. It is essentially 3 nepheline molecules with an added NaCl molecule. It is a clear colored isometric mineral with low relief. Thus, the only thing sodalite might be confused with is a hole in the thin section. The blue color of sodalite in hand specimen and its association with other alkali-rich minerals is usually necessary to detect its presence in a rock. Leucite Leucite is found in alkalic volcanic rocks, and is rarely found in plutonic rocks. It is a tetragonal mineral, however, its refractive indices and are so close together that it almost always appears isometric. It usually occurs as small, slightly rounded, low relief grains that go extinct upon insertion of the analyzer. Commonly, leucite contains tiny inclusions within the mineral, and sometimes shows a slight twinning, barely visible with the analyzer inserted. Oxides The oxide minerals are very common and usually occur as accessory minerals in all kinds of rocks. The most common oxide minerals are the following: Corundum - Al2O3 Corundum is hexagonal and optically negative. It occurs in Al-rich igneous and metamorphic rocks. If transparent blue, it is the gemstone sapphire, if transparent red, it is the gemstone

46 ruby. When it occurs as an accessory mineral it usually shows its hexagon shaped outline when looking down the c-axis. It has high refractive indices, thus shows very high relief in thin section. But it has low birefringence and commonly shows lamellar twinning. Spinel - MgAl2O4 Spinel is an isometric mineral that occurs ultrabasic rocks like peridotite, and in many low silica ignoeous rocks like basalts, where it contains high concentrations of Cr. It is also found in Al-rich contact metamorphic rocks. It shows a wide variety of colors depending on trace amounts of other ions substituting for both Mg and Al. Because of the isometric nature, Spinel is difficult to distinguish from garnet, although spinel tends to occur as much smaller crystals. Chromite - Fe+2Cr2O4 Chromite is a major ore of Cr. It is found in in low silica, Mg-rich igneous rocks, usually associated with Olivine. Often it is seen as small inclusions in Olivine, indicating that it is an early crystallizing phase in basaltic and gabbroic magmas. Chromite is isometric, and usually opaque in thin section. Electron Microprobe analysis is usually necessary to distinguish it from other opaque oxide minerals. Magnetite - Fe3O4 Magnetite is one of the most common oxide minerals. It is a major ore of Fe, and is found as an accessory mineral in all rock types. It is isometric and commonly crystallizes with an octahedral habit. In hand specimen it is most easily identified by its strongly magnetic nature, black color, and hardness of 6. In thin section it is opaque and thus difficult to distinguish from the other opaque oxide minerals. As discussed below, it forms a solid solution with Ulvospinel - Fe2TiO4. Ilmenite - FeTiO3 Ilmenite is a major ore of Ti. It is found as a common accessory mineral in a wide range of igneous volcanic and plutonic rocks, as well as metamorphic and clastic sedimentary rocks. It forms a solid solution series with Hematite, as will be discussed below, and commonly occurs along with Magnetite. Ilmenite is hexagonal, but is usually opaque which makes its distinction from other oxide minerals difficult. Ilmenite, however, often shows an elongated or acicular habit, whereas Magnetite usually crystallizes as more equant crystals with an octahedral habit. Hematite - Fe2O3 Hematite is one of the most important ores of Fe. It is more oxidized than Magnetite, and thus forms as an alteration product of magnetite as well as other Fe bearing minerals. In most unaltered igneous rocks, hematite occurs as a component of Ilmenite in solid solution. Hematite is hexagonal, but rarely occurs in crystals where its symmetry can be determined. It is found in a variety of forms, ranging from oolitic spherules, to massive fine grained aggregates, to botryoidal masses. It is most easily distinguished by its black to dark red color and reddish brown streak. In thin section it is not easily distinguished from other opaque oxide minerals. Iron-Titanium Oxide Geothermometer

47 Under magmatic conditions, Ilmenite and Hematite form a complete solid solution series, often called the rhombohedral series since both minerals crystallize in the hexagonal system. Similarly Magnetite and Ulvospinel form a complete solid solution series, called the spinel series. The possible ranges of solid solution are shown in the diagram to the right. Coexisting compositions (as illustrated by the tie line) depend on temperature and the fugacity (similar to partial pressure) of Oxygen. If magma is rapidly cooled so as to preserve the compositions of the high temperature solid solutions, it is possible to calculate the temperature and fugacity of Oxygen that were present just before eruption of the magma. Minerals that allow for determination of the temperature of formation of minerals are referred to as a geothermometer. The example illustrated here is an important one, called the Iron-Titanium Oxide Geothermometer. Carbonates The carbonates are an important group of minerals near the Earth's surface. Carbonate minerals make up the bulk of limestones and dolostones. Are found as cementing agents in clastic sedimentary rocks, and make up the shells of many organisms. The carbonates are based on the CO3-2 structural unit, which has carbon surrounded by 3 oxygens in triangular coordination. Thus each Oxygen has a residual charge of -2/3. In the carbonate structure, no two triangles share the corner oxygens and the C-O bonds are highly covalent. There are three structural types of carbonates: Calcite Group Aragonite Group Dolomite Group Calcite CaCO3 Aragonite CaCO3 Dolomite CaMg(CO3)2 Magnesite MgCO3 Witherite BaCO3 Ankerite CaFe(CO3)2 Siderite FeCO3 Strontianite SrCO3 Rhodochrosite MnCO3 Cerussite PbCO3 Smithsonite ZnCO3 In addition, there are the hydroxyl Cu carbonates - Malachite, Cu2CO3(OH)2 and Azurite Cu3(CO3)2(OH)2.

48 The Calcite Group The calcite group minerals are all hexagonal. They have Ca, Mg, Fe, Mn, or Zn divalent cations in 6-fold coordination with the CO3-2 groups, in a structure that is similar to that of NaCl. All members of this group show rhombohedral cleavage {01 2}, thus breaking into rhomb-shaped cleavage blocks. Calcite CaCO3- The most common carbonate mineral is calcite. It is the principal constituent of limestone and its metamorphic equivalent - marble. Deposits of fine grained calcite in powder form are referred to as chalk. It forms the cementing agent in many sandstones, and is one of the more common minerals precipitated by living organisms to form their skeletal structures. Calcite is also precipitated from groundwater where it form veins, or in open cavities like caves and caverns can form the cave decorations - like stalactites and stalagmites, and encrustations. It is also precipitated from hot springs where it is called travertine. Calcite does occur in rare igneous rocks called carbonatites. These form from carbonate magmas. Calcite is also precipitated from hydrothermal fluids to form veins associated with sulfide bearing ores. Properties In hand specimen, calcite is distinguished by its rhombohedral cleavage, its hardness of 3, and by its effervescence in dilute HCl. It can range in color from white, to slightly pink, to clear, but dark colored crystals can also occur. In thin section it is most readily distinguished by its high birefringence, showing high order white interference colors, by its rhombohedral cleavage and its uniaxial negative character. Because of its high birefringence, it shows a large change in relief on rotation of the stage. Furthermore, its refractive index direction (low RI direction) when parallel to the polarizer shows a negative relief when compared to the mounting medium of the thin section. Calcite can be distinguished from Aragonite by the lack of rhombohedral cleavage and biaxial nature of Aragonite. Magnesite MgCO3 Magnesite is a common alteration product of Mg-rich minerals on altered igneous and metamorphic rocks. Like calcite, it shows perfect rhombohedral cleavage, but unlike calcite, it does not readily effervesce in dilute HCl. It does, however, effervesce in hot HCl. These properties and its association with Mg-rich minerals and rocks make it distinguishable from Calcite. Siderite FeCO3 Siderite forms complete solid solution series with Magnesite, although the environment in which the two minerals occur usually determines that either Mg-rich Magnesite or Fe-rich Siderite will form, and one rarely sees intermediate end members. In hand specimen, siderite is usually brown colored and effervesces only in hot HCl. In thin section it resembles Calcite, but has a much higher refractive index than Calcite and is commonly pale yellow to yellow

49 brown in color without the analyzer inserted. Rhodochrosite MnCO3 Rhodochrosite is the Mn bearing carbonate, and is thus found only in environments where there is an abundance of Manganese. It is relatively rare and occurs as hydrothermal veins and as an alteration product of Mn rich deposits. In hand specimen it show a distinctive pink color along with the rhombohedral cleavage common to the Calcite group minerals. Hot HCl is required to make the mineral effervesce. The Aragonite Group The Aragonite group of minerals are all orthorhombic, and can thus be distinguished from minerals of the calcite group by their lack of rhombohedral cleavage. Aragonite (CaCO3) is the most common mineral in this group. Aragonite is the higher pressure form of CaCO3 but, nevertheless occurs and forms at surface temperatures and pressures. When found in metamorphic rocks it is a good indicator of the low temperature, high pressure conditions of metamorphism, and is thus commonly found in Blueschist Facies metamorphic rocks along with Glaucophane. Water containing high concentrations of Ca and carbonate can precipitate Aragonite. Warm water favors Aragonite, while cold water favors calcite, thus Aragonite is commonly found as a deposit of hot springs. Aragonite can also form by biological precipitation, and the pearly shells of many organisms are composed of Aragonite. Fine needle-like crystals of Aragonite are produced by carbonate secreting algae. Properties In hand specimen, Aragonite, like calcite effervesces in cold HCl. But, unlike Calcite, Aragonite does not show a rhombohedral cleavage. Instead it has single good {010} cleavage. It is usually transparent to white in color and forms in long bladed crystals. Twinning is common on {110}, and this can produce both cyclical twins, which, when present, make it look pseudohexagonal, and single twins. In thin section Aragonite is distinguished by its high birefringence, showing high order white interference colors, its biaxial character with a 2V of about 18o, and extinction parallel to the {010} cleavage. The Dolomite Group Dolomite - CaMg(CO3)2 and Ankerite - CaFe(CO3)2 form a complete solid solution series, although because Mg-rich environments are much more common than Fe-rich environments, Mg-rich dolomites are much more common than Ankerites. Ankerite is common mineral in Pre-Cambrian iron formations. Dolomite is a common constituent of older limestones, probably the result of secondary replacement of original calcite. It is also found as dolomitic marbles, and in hydrothermal veins..

50 Dolomite is a unique chemical composition, as can be seen in the Magnesite - Calcite phase diagram shown here. Two solvi exist at low temperatures. Thus, any high Mg-calcite dolomite solid solutions that might exist at high temperatures would form nearly pure calcite and pure dolomite at surface temperatures, and similarly, any Magnesite - Dolomite solid solutions that might exist at high temperatures would form nearly pure Magnesite and pure Dolomite at low temperatures. Thus, Magnesite and Dolomite commonly occur together, as do Calcite and Dolomite. PropertiesDolomite, and therefore rocks containing large amounts of dolomite, like dolostones, is easily distinguished by the fact that dolomite only fizzes in cold dilute HCl if broken down to a fine powder. Also, dolostones tend to weather to a brownish color rock, whereas limestones tend to weather to a white or gray colored rock. The brown color of dolostones is due to the fact that Fe occurs in small amounts replacing some of the Mg in dolomite. In thin section it is more difficult to distinguish from calcite, unless it is twined. In order to facilitate its identification in thin section, the sections are often stained with alizarin red S. This turns calcite pink, but leaves the dolomite unstained. If calcite and dolomite are twinned, they are easily distinguishable from one another. Calcite shows twin lamellae that are parallel to the rhombohedral cleavage traces and parallel to the long direction of the cleavage rhombs. Thus, the lamellae bisect the acute angle between the cleavages. Dolomite also has twins parallel to the cleavage faces and parallel to the long direction of the rhombs, but also has twin lamellae that are parallel to the short dimension of the rhomb. Thus, dolomite would also show twin lamellae that would bisect the obtuse angle between the cleavage traces. Accessory Minerals Zircon ZrSiO4 Zircon is a common accessory mineral in nearly all kinds of rocks, particularly the more siliceous igneous rocks, like granites, granodiorites, and syenites. Still, it is not often found in thin section because it is so hard that it gets plucked out during the grinding of the section. Zircon usually contains high amounts of radioactive elements like U and Th. Thus, when it is

51 found as inclusions in minerals like biotite, it produces pleochroic haloes in the biotite as seen in thin section. Because it contains high concentrations of U and Th, it is very useful in obtaining U-Pb and Th-Pb radiometric dates on old rocks. It is very resistant to weathering and may also survives during metamorphism, allowing for dates to be obtained on the original rock prior to metamorphism (often called the protolith). In hand specimen Zircon usually occurs as tiny reddish colored crystals. In thin section, it shows extremely high relief, with = to and = to and is uniaxial positive. Zircon has high birefringence, with interference colors in the higher orders (lots of reds, pinks and light greens). It is commonly colorless to pale brown or pinkish brown in polarized light without the analyzer. Generally it occurs as small crystals with relief higher than almost anything else in the thin section. This latter property should tip you off to its presence. Sphene (Titanite) CaTiSiO4(OH) Sphene is another common accessory mineral in plutonic igneous rocks like granites, granodiorites, and syenites. It is also found as larger crystals in metamorphic gneisses and chlorite bearing schists. In hand specimen as an accessory mineral, it is usually seen as small wedge-shaped crystals with a resinous to adamantine luster and brown to yellow brown color. In thin section, Sphene, has a relief similar to that of zircon, and is usually found in small crystals with an elongated diamond shape. It is generally brownish in color, shows a well developed {110} cleavage, and high order interference colors. Apatite Ca5(PO4)3(OH,F) Apatite is another very common and almost ubiquitous (always present) accessory mineral in igneous rocks and many metamorphic rocks. If the rock contains any phosphorous it is usually found in apatite. Apatite is hexagonal, hence uniaxial with a negative optic sign. Its refractive indices = to and = to are higher than both quartz and nepheline, giving apatite a higher relief than these minerals. Its birefringence, expressed as 1o gray interference colors is similar to that of quartz and nepheline. Quartz, however, is optically positive. Nepheline, while optically negative, shows much lower relief than does apatite. The crystal form of apatite is usually distinctive. If cut parallel to {0001}, it usually has a hexagonal outline. If cut parallel to the C axis, it appears as doubly terminated prisms. COMPILED BY GDC HANDWARA

52 LECTURE NOTES 1ST SEMESTER UNIT 4 Pleochroism With the upper polar removed, many coloured anisotropic minerals display a change in colour - this is pleochroism or diachroism.it Produced because the two rays of light are absorbed differently as they pass through the coloured mineral and therefore the mineral displays different colours. Pleochroism is not related to the interference colours. UNIAXIAL OPTICS Uniaxial minerals have only one optic axis, and belong to the hexagonal and tetragonal systems. Minerals in this group include: nepheline NaAlSiO4 apatite Ca5(PO4)3(F,Cl,OH) calcite CaCO3 dolomite (Ca,Mg)CO3 quartz SiO2 zircon ZrSiO4 tourmaline - borosilicate

53 On rotating the calcite rhomb one dot remained stationary but the other dot rotated with the calcite about the stationary dot. The ray corresponding to the image which moved is called the Extraordinary Ray - epsilon. The ray corresponding to the stationary image, which behaves as though it were in an isotropic mineral is called the Ordinary Ray - omega.

54 The vibration direction of the ordinary ray lies in the {0001} plane of the calcite and is at right angles to the c-axis. The extraordinary ray vibrates perpendicular to the ordinary ray vibration direction in the plane which contains the c-axis of the calcite. If instead of using a calcite rhomb we had used a slab of calcite which had been cut in a random orientation and placed that on the dots, two images would still appear. If the random cuts were such that they were perpendicular to the c-axis, then light travelling through the calcite, along the c-axis would produce only one image andwould not become polarized.

55 The c-axis coincides with the optic axis, which is the direction through the mineral along which light propogates without being split into two rays. For calcite, 1. The index of refraction for the ordinary ray is uniform omega = 1.658, regardless of the direction through the grain that the light follows. 2. The index of refraction for the extraordinary ray, epsilon, is variable ranging from to The index is dependant on the direction that the light travels through the mineral. o If light travels perpendicular to c-axis, epsilon = o If the light travels along the the c-axis, epsilon = o For intermediate directions through the grain epsilon will fall between the two extremes. Calcite is used as an example of the formation of the two rays because of the large difference between the refractive indices (birefringence (delta)). for calcite, delta = For minerals with a lower birefringence, e.g. quartz, delta = 0.009, the two images are still produced but show very little separation. The quartz would have to be 2025X as thick as the calcite to see the same separation of the dots. UNIAXIAL OPTIC SIGN

56 In Calcite omega > epsilon, versus In other minerals, e.g. quartz, omega < epsilon, versus This difference in this refractive index relationship provides the basis for defining the optic sign of uniaxial minerals. omega < epsilon Optically positive uniaxial minerals Optically negative uniaxial minerals omega > epsilon Alternatively, if extrordinary ray is the slow ray, then the mineral is optically positive. if extraordinary ray is the fast ray, then the mineral is optically negative. epsilon refers to the maximum or minimum index of refraction for the extraordinary ray, the value recorded in the mineral descriptions in the text. epsilon' refers to an index of refraction for the extraordinary ray which is between omega and epsilon. For uniaxial minerals any orientation will provide omega, but only one orientation, cut parallel to the c-axis will yield epsilon maximum. This orientation is the one which exhibits the highest interference colour as delta (birefringence), is greatest, and therefore retardation (DELTA) is greatest. (DELTA = d(ns-nf)) LIGHT PATHS THROUGH UNIAXIAL MINERALS nepsilon refers to the maximum or minimum index of refraction for the extraordinary ray,. nepsilon' refers to an index of refraction for the extraordinary ray which is between nomega and nepsilon. For uniaxial minerals any orientation will provide nw, but only one orientation, cut parallel to the c-axis will yield nepsilon maximum. This orientation is the one which exhibits the highest interference colour as delta (birefringence), is greatest, and therefore DELTA (retardation) is greatest (DELTA = d(ns-nf))

57 Hexagonal and tetragonal systems are characterized by a high degree of symmetry about the c-axis. Within the 001 or 0001 plane, at 90 to the c-axis, uniform chemical bonding in all directions is encountered. Light Paths Through a Mineral Light travelling along the c-axis is able to vibrate freely in any direction within the 001 or 0001 plane. No preferred vibration direction allows light to pass through the mineral as if it were isotropic, this orientation has the lowest interference colour - black to dark grey. If the light passes at some angle to the c-axis, it encounters a different electronic configuration and is split into two rays of different velocities. The vibration vector of the ordinary ray is parallel to the 001 or 0001 plane, i.e. perpendicular to the c-axis. The extraordinary ray vibrates across these planes, parallel to the c-axis. The ordinary ray has the same velocity regardless of the path it takes, because it always vibrates in the same electronic environment. The extraordinary ray velocity varies depending on the direction. If the light travels nearly parallel to the c-axis, the extraordinary ray vibrates ~ parallel to 001 or 0001, so that nepsilon'~nomega. If the light travels at right angles to the c-axis, the extraordinary ray vibrates across the 001 or 0001 plane and nepsilon is most different from nomega. For intermediate angles to the c-axis: nomega > nepsilon' and, nepsilon' > nepsilon. Whether the extraordinary ray has a higher or lower RI than the ordiniary ray depends on the chemical bonding and the crystal structure. In the lab you will determine the indices of refraction for a uniaxial mineral using grain mounts and the immersion method.

58 UNIAXIAL INDICATRIX The indicatrix is a geometric figure, constructed so that the indices of refraction are plotted as radii that are parallel to the vibration direction of light. In isotropic minerals the indicatrix was a sphere, because the refractive index was the same in all directions. In uniaxial minerals, because nomega and nepsilon are not equal, the indicatrix is an ellipsoid, the shape of which is dependant on its orientation with respect to the optic axis. In positive uniaxial minerals, the Z indicatrix axis is parallel to the ccrystallographic axis and the indicatrix is a prolate ellipsoid, i.e. it is stretched out along the optic axis.

59 All light travelling along the Z axis (optic axis), has an index of refraction of nomega, whether it vibrates parallel to the X or Y axis, or any direction in the XY plane. The XZ and the YZ planes through the indicatrix are identical ellipses with nomega and nepsilon as their axes, with the radii of the ellipses equal to the magnitude of the RI for the ray. Plotting the indices of light travelling in all directions produces the prolate ellipsoid, whose axis of revolution is the optic axis, for uniaxial positive minerals; nomega < nepsilon. For optically negative minerals the X indicatrix axis corresponds to the optic axis and the indicatrix is an oblate ellipsoid, i.e. flattened along the optic axis, and nomega > nepsilon In each case, for positive and negative minerals the circular section through the indicatrix is perpendicular to the optic axis and has a radius = nomega. The radius of the indicatrix along the optic axis is always nepsilon. Any section through the indicatrix which includes the optic axis is called a principal section, and produces an ellipse with axes nomega and nepsilon. A section through the indicatrix perpendicular to the optic axis produces a circular section with radius nomega.

60 A random section through the indicatrix will produce an ellipse with axes nomega and nepsilon The indicatrix is oriented so that the optic axis is parallel to the c crystallographic axis. Random Section Vibration Directions Random section through the uniaxial indicatrix will give nomega and nepsilon'. Light travelling from the origin of the indicatrix outwards, construct a wave normal to the wave front. A slice through the centre of the indicatrix, perpendicular to the wave normal forms an ellipse with axes of nomega and nepsilon. omega vibrates at 90 to the optic axis = short axis of the ellipse epsilon' vibrates parallel to the optic axis = long axis of the ellipse. The magnitude of the axes = nomega and nepsilon BIREFRINGENCE AND INTERFERENCE COLOURS Birefringence, difference between the index of refraction of the slow and fast rays and the interference colours for uniaxial minerals is dependant on the direction that light travels through the mineral.

61 1. In a sample which has been cut perpendicular to the optic axis, the bottom and top surfaces will be parallel. The angle of incidence for the light entering the crystal = 0 and the wave front are not refracted at the interface and remain parallel to the mineral surface. o A cut through the indicatrix, parallel to the bottom of the mineral, will yield the indices and vibration directions of the light. A slice through the indicatrix is a circular section, with radius nomega. o No preferred vibration direction, so light passes along the optic axis as an ordinary ray and retains whatever vibration direction it had originally. o Between crossed polars the light passing through the mineral is completely absorbed by the upper polar and will remain black on rotation of the stage, The birefringence = 0.

62 2. Cutting the sample such that the optic axis is parallel to the surface of the section the following is observed. o The indicatrix section is a principle section, as it contains the optic axis. The indicatrix forms an ellipse with axes = nomega and nepsilon, with the incident light being split into two rays such that: the ordinary ray vibrates perpendicular to the optic axis, the extraordinary ray vibrates parallel to the optic axis. o The birefringence is at a maximum, and in thin section this grain orientation will display the highest interference colour.

63 3. A mineral cut in a random orientation, with normally incident light; o The ordinary ray produced has an index, nomega and vibrates perpendicular to the optic axis. o The extraordinary ray has an index nepsilon' and vibrates in the plane containing the optic axis. o nepsilon < nomega maximum or minimum, the birefringence is intermediate between the two extremes. EXTINCTION IN UNIAXIAL MINERALS Uniaxial minerals will exhibit all four types of extinction discussed earlier. The type is dependent on: 1. the orientation that the mineral is cut 2. the presence of cleavage(s) in the grain Tetragonal minerals 1. Zircon ZrSi04- poor prismatic

64 2. Rutile Ti02 - good prismatic o are prismatic and either elongate or stubby II to c axis. o display prismatic (parallel to c) o or pinacoidal (perpendicular to c) cleavage. Depending on how the crystal is cut, and how its indicatrix is cut, dictates what will be seen in thin section. Hexagonal Minerals Quartz - SiO2 - no cleavage Apatite - Ca5(PO4)3(F,C1,OH) - rare pinacoidal, prism Calcite - CaC03-1 of two cleavages rhombohedral Nepheline - NaAlSiO4 - no cleavage Hexagonal minerals will exhibit the following forms prisms, pinacoids, pyramids and rhombohedrons which will exhibit prismatic, pinaciodal and rhombohedral cleavages. The birefringence, interference colours and any cleavage displayed by hexagonal minerals is a function of how the grain has been cut. PLEOCHROISM IN UNIAXIAL MINERALS Pleochroism is defined as the change in colour of a mineral, in plane light, on rotating the stage. It occurs when the wavelengths of the ordinary & extraordinary rays are absorbed differently on passing through a mineral, resulting in different wavelengths of light passing the mineral. Coloured minerals, whether uniaxial or biaxial, are generally pleochroic. To describe the pleochroism for uniaxial minerals must specify the colour which corresponds to the ordinary and extraordinary rays. e.g. Tourmaline, Hexagonal mineral o omega = dark green o epsilon = pale green

65 If the colour change is quite distinct the pleochroism is said to be strong. If the colour change is minor = weak pleochroism. For coloured uniaxial minerals, sections cut perpendicular to the c axis will show a single colour, corresponding to ordinary ray. Sections parallel to the c crystallographic axis will exhibit the widest colour variation as both omega and epsilon are present. BIAXIAL MINERALS Include orthorhombic, monoclinic and triclinic systems, all exhibit less symmetry than uniaxial and isotropic minerals. Minerals in these crystal systems exhibit variable crystal structure, resulting in variable chemical bonding. The crystallographic properties of orthorhombic, monoclinic and triclinic minerals are specified by means of the unit cell measured along the three crystallographic axes. It is also necessary to specify 3 different indices of refraction for biaxial minerals: alpha, beta, gamma are used in text. where alpha < beta < gamma A variety of other conventions have been used or suggested, make sure that you are aware of the convention used in the text you are using, if it is not Nesse. The maximum birefringence of a biaxial mineral is defined by (gamma - alpha) Clarification 1) It takes 3 indices of refraction to describe optical properties of biaxial minerals, however, light that enters biaxial minerals is broken into two rays - FAST and SLOW. 2) Ordinary - extraordinary terminology is not used. Both

66 rays behave as the extraordinary ray did in uniaxial minerals. The rays are both extraordinary and are referred to as SLOW RAY and FAST RAY. o o slow = gamma', between beta and gamma (higher RI) gamma > gamma' > beta fast = alpha', between alpha and beta (lower RI) alpha < alpha' < beta BIAXIAL INDICATRIX The biaxial indicatrix is similar to the uniaxial indicatrix, except now there are three principal indices of refraction instead of two. The biaxial indicatrix is constructed by plotting the principal indices along 3 mutually perpendicular axes. nalpha plotted along X nbeta plotted along Y ngamma plotted along Z again, nalpha < nbeta < ngamma So that the length of X<Y<Z.

67 Indicatrix is a triaxial ellipsoid elongated along the Z axis, and flattened along the X axis. Indicatrix has 3 principal sections, all ellipses: X - Y axes = nalpha & nbeta X - Z axes = nalpha & ngamma Y - Z axes = nbeta & ngamma Random sections through the indicatrix also form ellipses. The uniaxial indicatrix exhibited a single circular section, a biaxial indicatrix exhibits two circular sections with radius = nbeta; the circular sections intersect along the Y indicatrix axis, which also has a radius of nbeta. Look at the X - Z plane in the above image. The axes of the ellipse are = nalpha & ngamma. The radii vary from nalpha through nbeta to ngamma. Remember that nalpha < nbeta < ngamma, so a radii = nbeta must be present on the X - Z plane. The length of indicatrix along the Y axis is also nbeta, so the Y axis and radii nbeta in X - Z plane defines a circular section, with radius nbeta.

68 In the biaxial indicatrix the directions perpendicular to the circular sections define the OPTIC AXES of the biaxial mineral. Optic axes lie within the X - Z plane, and this plane is the OPTIC PLANE. The acute angle between the optic axes is the optic or 2V angle. The indicatrix axis, either X or Z, which bisects the 2V angle is the ACUTE BISECTRIX or Bxa. The indicatrix axis, either X or Z, which bisects the obtuse angle between the optic axes is the OBTUSE BISECTRIX or Bxo. The Y axis is perpendicular to the optic plane and forms the OPTIC NORMAL. OPTIC SIGN

69 For biaxial minerals optic sign is dependant on whether the X or Z indicatrix axis is the acute bisectrix. if Bxa is X, mineral is -ve if Bxa is Z, mineral is +ve

70 In the special case where 2V = 90, mineral is optically neutral. Another convention used is to identify the angle between the optic axes bisected by the X axis as the 2VX angle; and the Z axis as 2VZ angle. These two angles can vary from 0 to 180, such that the following relationship holds: 2VX + 2VZ = 180 Using this convention the optic sign is determined by the following: if 2VZ < 90, the mineral is +ve. if 2VZ > 90, the mineral is -ve. Light travelling through biaxial minerals is split into two rays FAST and SLOW rays which vibrate at 90 to each other.

71 The vibration directions of the FAST and SLOW rays are defined, or determined, by the axes of the ellipse or section through the indicatrix, which is oriented at 90 to the wave normal. The Refractive Index corresponding to the FAST ray will be between nalpha and nbeta, and is referred to as nalpha'. The Refractive Index corresponding to the SLOW ray will be between nbeta and & ngamma, and is referred to as ngamma'. With this convention the following relationship will be true for all biaxial minerals: 1. X - will always correspond to the fast ray and will have the lowest RI. o RI = nalpha, always fast 2. Y - will be either the fast or the slow ray depending on which other indicatrix axis it is withand its refractive index will be between the lowest and highest RI for the mineral. o RI = nbeta, either fast or slow 3. Z - will always correspond to the slow ray and will have the highest RI. o RI = ngamma, always slow. COMPILED BY GDC HANDWARA

72 LECTURE NOTES 1ST SEMESTER UNIT 4 REFLECTION AND REFRACTION At the interface between the two materials, e.g. air and water, light may be reflected at the interface or refracted (bent) into the new medium. For Reflection the angle of incidence = angle of reflection.

73 For Refraction the light is bent when passing from one material to another, at an angle other than perpendicular. A measure of how effective a material is in bending light is called the Index of Refraction (n), where: Index of Refraction in Vacuum = 1 and for all other materials n > 1.0. Most minerals have n values in the range 1.4 to 2.0. A high Refractive Index indicates a low velocity for light travelling through that particular medium. Snell's Law

74 Snell's law can be used to calculate how much the light will bend on travelling into the new medium. If the interface between the two materials represents the boundary between air (n ~ 1) and water (n = 1.33) and if angle of incidence = 45, using Snell's Law the angle of refraction = 32. The equation holds whether light travels from air to water, or water to air. In general, the light is refracted towards the normal to the boundary on entering the material with a higher refractive index and is refracted away from the normal on entering the material with lower refractive index. In labs, you will be examining refraction and actually determine the refractive index of various materials. ISOTROPIC INDICATRIX To examine how light travels through a mineral, either isotropic or anisotropic, an indicatrix is used. INDICATRIX - a 3 dimensional geometric figure on which the index of refraction for the mineral and the vibration direction for light travelling through the mineral are related. Isotropic Indicatrix Indicatrix is constructed such that the indices of refraction are plotted on lines from the origin that are parallel to the vibration directions. It is possible to determine the index of a refraction for a light wave of random orientation travelling in any direction through the indicatrix. 1. a wave normal, is constructed through the centre of the indicatrix 2. a slice through the indicatrix perpendicular to the wave normal is taken. 3. the wave normal for isotropic minerals is parallel to the direction of propagation of light ray.

75 4. index of refraction of this light ray is the radius of this slice that is parallel to the vibration direction of the light. For isotropic minerals the indicatrix is not needed to tell that the index of refraction is the same in all directions. Anisotropic minerals differ from isotropic minerals because: 1. the velocity of light varies depending on direction through the mineral; 2. they show double refraction. When light enters an anisotropic mineral it is split into two rays of different velocity which vibrate at right angles to each other. In anisotropic minerals there are one or two directions, through the mineral, along which light behaves as though the mineral were isotropic. This direction or these directions are referred to as the optic axis. Hexagonal and tetragonal minerals have one optic axis and are optically UNIAXIAL. Orthorhombic, monoclinic and triclinic minerals have two optic axes and are optically BIAXIAL. Calcite Rhomb Displaying Double Refraction Light travelling through the calcite rhomb is split into two rays which vibrate at right angles to each other. The two rays and the corresponding images produced by the two rays are apparent in the above image. The two rays are: 1. Ordinary Ray, labelled omega w, nw = Extraordinary Ray, labelled epsilon e, ne = Vibration Directions of the Two Rays The vibration directions for the ordinary and extraordinary rays, the two rays which exit the calcite rhomb, can be determined using a piece of polarized film. The polarized film has a single vibration direction and as such only allows light, which has the same vibration direction as the filter, to pass through the filter to be detected by your eye. 1. Preferred Vibration Direction NS With the polaroid filter in this orientation only one row of dots is visible within the area of the calcite rhomb covered by the filter. This row of dots corresponds to the light ray which has a vibration direction parallel to the filter's preferred or permitted vibration direction and as such it passes through the filter. The other light ray

76 represented by the other row of dots, clearly visible on the left, in the calcite rhomb is completely absorbed by the filter. 2. Preferred Vibration Direction EW With the polaroid filter in this orientation again only one row of dots is visible, within the area of the calcite coverd by the filter. This is the other row of dots thatn that observed in the previous image. The light corresponding to this row has a vibration direction parallel to the filter's preferred vibration direction. It is possible to measure the index of refraction for the two rays using the immersion oils, and one index will be higher than the other. 1. The ray with the lower index is called the fast ray o recall that n = Vvac/Vmedium If nfast Ray = 1.486, then VFast Ray = 2.02X1010 m/sec 2. The ray with the higher index is the slow ray 10 o If nslow Ray = 1.658, then VSlow Ray = 1.8 1x10 m/sec Remember the difference between: vibration direction - side to side oscillation of the electric vector of the plane light and propagation direction - the direction light is travelling. Electromagnetic theory can be used to explain why light velocity varies with the direction it travels through an anisotropic mineral. 1. Strength of chemical bonds and atom density are different in different directions for anisotropic minerals. 2. A light ray will "see" a different electronic arrangement depending on the direction it takes through the mineral. 3. The electron clouds around each atom vibrate with different resonant frequencies in different directions. Velocity of light travelling though an anisotropic mineral is dependant on the interaction between the vibration direction of the electric vector of the light and the resonant frequency of the electron clouds. Resulting in the variation in velocity with direction. Can also use electromagnetic theory to explain why light entering an anisotropic mineral is split into two rays (fast and slow rays) which vibrate at right angles to each other. PACKING

77 As was discussed in the previous section we can use the electromagnetic theory for light to explain how a light ray is split into two rays (FAST and SLOW) which vibrate at right angles to each other. The above image shows a hypothetical anisotropic mineral in which the atoms of the mineral are: 1. closely packed along the X axis 2. moderately packed along Y axis 3. widely packed along Z axis The strength of the electric field produced by the electrons around each atom must therefore be a maximum, intermediate and minimum value along X, Y and Z axes respectively, as shown in the following image.

78 With a random wavefront the strength of the electric field, generated by the mineral, must have a minimum in one direction and a maximum at right angles to that. Result is that the electronic field strengths within the plane of the wavefront define an ellipse whose axes are; 1. at 90 to each other, 2. represent maximum and minimum field strengths, and 3. correspond to the vibration directions of the two resulting rays. The two rays encounter different electric configurations therefore their velocities and indices of refraction must be different. There will always be one or two planes through any anisotropic material which show uniform electron configurations, resulting in the electric field strengths plotting as a circle rather than an ellipse. Lines at right angles to this plane or planes are the optic axis (axes) representing the direction through the mineral along which light propagates without being split, i.e., the anisotropic mineral behaves as if it were an isotropic mineral.

79 INTERFERENCE PHENOMENA the colours for an anisotropic mineral observed in thin section, between crossed polars are called interference colours and are produced as a consequence of splitting the light into two rays on passing through the mineral. RETARDATION Monochromatic ray, of plane polarized light, upon entering an anisotropic mineral is split into two rays, the FAST and SLOW rays, which vibrate at right angles to each other. Development of Retardation Due to differences in velocity the slow ray lags behind the fast ray, and the distance represented by this lagging after both rays have exited the crystal is the retardation - D The magnitude of the retardation is dependant on the thickness (d) of the mineral and the differences in the velocity of the slow (Vs) and fast (Vf) rays. The time it takes the slow ray to pass through the mineral is given by: during this same interval of time the fast ray has already passed through the mineral and has travelled an additional distance = retardation.

80 substituting 1 in 2, yields rearranging The relationship (ns - nf) is called birefringence, given Greek symbol lower case d (delta), represents the difference in the indices of refraction for the slow and fast rays. In anisotropic minerals one path, along the optic axis, exhibits zero birefringence, others show maximum birefringence, but most show an intermediate value. The maximum birefringence is characteristic for each mineral. Birefringence may also vary depending on the wavelength of the incident light. INTERFERENCE AT THE UPPER POLAR Now look at the interference of the fast and slow rays after they have exited the anisotropic mineral.fast ray is ahead of the slow ray by some amount = D.Interference phenomena are produced when the two rays are resolved into the vibration direction of the upper polar. Interference at the Upper Polar - Case 1

81 1. Light passing through lower polar, plane polarized, encounters sample and is split into fast and slow rays. 2. If the retardation of the slow ray = 1 whole wavelength, the two waves are IN PHASE. 3. When the light reaches the upper polar, a component of each ray is resolved into the vibration direction of the upper polar. 4. Because the two rays are in phase, and at right angles to each other, the resolved components are in opposite directions and destructively interfere and cancel each other. 5. Result is no light passes the upper polar and the grain appears black. Interference at the Upper Polar - Case 2 1. If retardation of the slow ray behind the fast ray = ½ a wavelength, the two rays are OUT OF PHASE, and can be resolved into the vibration direction of the upper polar. 2. Both components are in the same direction, so the light constructively interferes and passes the upper polar.

82 MONOCHROMATIC LIGHT If our sample is wedged shaped, as shown above, instead of flat, the thickness of the sample and the corresponding retardation will vary along the length of the wedge. Examination of the wedge under crossed polars, gives an image as shown below, and reveals:

83 1. dark areas where retardation is a whole number of wavelengths. 2. light areas where the two rays are out of phase, 3. brightest illumination where the retardation of the two rays is such that they are exactly ½, 1½, 2½ wavelengths and are out of phase. The percentage of light transmitted through the upper polarizer is a function of the wavelength of the incident light and retardation. If a mineral is placed at 45 to the vibration directions of the polarizers the mineral yields its brightest illumination and percent transmission (T). POLYCHROMATIC LIGHT Polychromatic or White Light consists of light of a variety of wavelengths, with the corresponding retardation the same for all wavelengths. Due to different wavelengths, some reach the upper polar in phase and are cancelled, others are out of phase and are transmitted through the upper polar. The combination of wavelengths which pass the upper polar produces the interference colours, which are dependant on the retardation between the fast and slow rays. Examining the quartz wedge between crossed polars in polychromatic light produces a range of colours. This colour chart is referred to as the Michel Levy Chart and may be found as Plate I in Nesse. At the thin edge of the wedge the thickness and retardation are ~ 0, all of the wavelengths of light are cancelled at the upper polarizer resulting in a black colour. With increasing thickness, corresponding to increasing retardation, the interference colour changes from black to grey to white to yellow to red and then a repeating sequence of

84 colours from blue to green to yellow to red. The colours get paler, more washed out with each repetition. In the above image, the repeating sequence of colours changes from red to blue at retardations of 550, 1100, and 1650 nm. These boundaries separate the colour sequence into first, second and third order colours. Above fourth order, retardation > 2200 nm, the colours are washed out and become creamy white. The interference colour produced is dependant on the wavelengths of light which pass the upper polar and the wavelengths which are cancelled. The birefringence for a mineral in a thin section can also be determined using the equation for retardation, which relates thickness and birefringence. Retardation can be determined by examining the interference colour for the mineral and recording the wavelength of the retardation corresponding to that colour by reading it directly off the bottom of Plate I. The thickness of the thin section is ~ 30 µm. With this the birefringence for the mineral can be determined, using the equation: See the example below.

85 This same technique can be used by the thin section technician when she makes a thin section. By looking at the interference colour she can judge the thickness of the thin section. The recognition of the order of the interference colour displayed by a mineral comes with practice and familiarity with various minerals. In the labs you should become familar with recognizing interference colours. EXTINCTION Now we want to examine other properties of minerals which are useful in the identification of unknown minerals. Anisotropic minerals go extinct between crossed polars every 90 of rotation. Extinction occurs when one vibration direction of a mineral is parallel with the lower polarizer. As a result no component of the incident light can be resolved into the vibration direction of the upper polarizer, so all the light which passes through the mineral is absorbed at the upper polarizer, and the mineral is black.

86 Upon rotating the stage to the 45 position, a maximum component of both the slow and fast ray is available to be resolved into the vibration direction of the upper polarizer. Allowing a maximum amount of light to pass and the mineral appears brightest. The only change in the interference colours is that they get brighter or dimmer with rotation, the actual colours do not change. Many minerals generally form elongate grains and have an easily recognizable cleavage direction, e.g. biotite, hornblende, plagioclase. The extinction angle is the angle between the length or cleavage of a mineral and the minerals vibration directions. The extinction angles when measured on several grains of the same mineral, in the same thin section, will be variable. The angle varies because of the orientation of the grains. The maximum extinction angle recorded is diagnostic for the mineral. Types of Extinction 1. Parallel Extinction The mineral grain is extinct when the cleavage or length is aligned with one of the crosshairs. The extinction angle (EA) = 0 e.g. o orthopyroxene

87 o 2. biotite Inclined Extinction The mineral is extinct when the cleavage is at an angle to the crosshairs. EA > 0 e.g. o o 3. clinopyroxene hornblende Symmetrical Extinction The mineral grain displays two cleavages or two distinct crystal faces. It is possible to measure two extinction angles between each cleavage or face and the vibration directions. If the two angles are equal then Symmetrical extinction exists. EA1 = EA2 e.g. o o 4. amphibole calcite No Cleavage Minerals which are not elongated or do not exhibit a prominent cleavage will still go

88 extinct every 90 of rotation, but there is no cleavage or elongation direction from which to measure the extinction angle. e.g. o o quartz olivine Exceptions to Normal Extinction Patterns Different portions of the same grain may go extinct at different times, i.e. they have different extinction angles. This may be caused by chemical zonation or strain. Chemical zonation The optical properties of a mineral vary with the chemical composition resulting in varying extinction directions for a mineral. Such minerals are said to be zoned. e.g. plagioclase, olivine

89 LECTURE NOTES 1ST SEMESTER UNIT 4 RELIEF Refractometry involves the determination of the refractive index of minerals, using the immersion method. This method relys on having immersion oils of known refractive index and comparing the unknown mineral to the oil. If the indices of refraction on the oil and mineral are the same light passes through the oil-mineral boundary un-refracted and the mineral grains do not appear to stand out. If noil <> nmineral then the light travelling though the oil-mineral boundary is refracted and the mineral grain appears to stand out.

90 RELIEF - the degree to which a mineral grain or grains appear to stand out from the mounting material, whether it is an immersion oil, Canada balsam or another mineral. When examining minerals you can have: 1. Strong relief o mineral stands out strongly from the mounting medium, o whether the medium is oil, in grain mounts, or other minerals in thin section, o for strong relief the indices of the mineral and surrounding medium differ by greater than 0.12 RI units. 2. Moderate relief o mineral does not strongly stand out, but is still visible, o indices differ by 0.04 to 0.12 RI units. 3. Low relief o mineral does not stand out from the mounting medium, o indices differ by or are within 0.04 RI units of each other. A mineral may exhibit positive or negative relief: +ve relief - index of refraction for the material is greater than the index of the oil. - e.g. garnet ve relief nmin < noil - e.g. fluorite It is useful to know whether the index of the mineral is higher or lower that the oil. This will be covered in the second lab section - Becke Line and Refractive Index Determination. BECKE LINE In order to determine whether the idex of refraction of a mineral is greater than or less than the mounting material the Becke Line Method is used. BECKE LINE - a band or rim of light visible along the grain

91 boundary in plane light when the grain mount is slightly out of focus. Becke line may lie inside or outside the mineral grain depending on how the microscope is focused. To observe the Becke line: 1. use medium or high power, 2. close aperture diagram, 3. for high power flip auxiliary condenser into place. Increasing the focus by lowering the stage, i.e. increase the distance between the sample and the objective, the Becke line appears to move into the material with the higher index of refraction. The Becke lines observed are interpreted to be produced as a result of the lens effect and/or internal reflection effect. LENS EFFECT Most mineral grains are thinner at their edges than in the middle, i.e. they have a lens shape and as such they act as a lens. If nmin > noil the grain acts as a converging lens, concentrating light at the centre of the grain. If nmin < noil, grain is a diverging lens, light concentrated in oil.

92 INTERNAL REFLECTION This hypothesis to explain why Becke Lines form requires that grain edges be vertical, which in a normal thin section most grain edges are believed to be more or less vertical. With the converging light hitting the vertical grain boundary, the light is either refracted or internally reflected, depending on angles of incidence and indices of refraction. Result of refraction and internal reflection concentrates light into a thin band in the material of higher refractive index. If nmin > noil the band of light is concentrated within the grain. If nmin < noil the band of light is concentrated within the oil.

93 BECKE LINE MOVEMENT The direction of movement of the Becke Line is determined by lowering the stage with the Becke Line always moving into the material with the higher refractive index. The Becke Line can be considered to form from a cone of light that extends upwards from the edge of the mineral grain. Becke line can be considered to represent a cone of light propagating up from the edges of the mineral. If nmin < noil, the cone converges above the mineral. If nmin > noil, the cone diverges above the mineral.

94 By changing focus the movement of the Becke line can be observed. If focus is sharp, such that the grain boundaries are clear the Becke line will coincide with the grain boundary. Increasing the distance between the sample and objective, i.e. lower stage, light at the top of the sample is in focus, the Becke line appears: in the mineral if nmin >noil or in the oil if nmin << noil Becke line will always move towards the material of higher RI upon lowering the stage. A series of three photographs showing a grain of orthoclase: 1. Photo 1 The grain is in focus, with the Becke line lying at the grain boundary. 2. Photo 2 The stage is raised up, such that the grain boundary is out of focus, but the Becke line is visible inside the grain. 3. Photo 3 The stage is lowered, the grain boundary is out of focus, and the Becke line is visible outside the grain. When the RI of the mineral and the RI of the mounting material are equal, the Becke line splits into two lines, a blue line and an orange line. In order to see the Becke line the microscope is slightly out of focus, the grain appears fuzzy, and the two Becke lines are visible. The blue line lies outside the grain and the orange line lies inside the grain. As the stage is raised or lowered the two lines will shift through the grain boundary to lie inside and outside the grain, respectively.

95 Index of Refraction in Thin Section It is not possible to get an accurate determination of the refractive index of a mineral in thin section, but the RI can be bracket the index for an unknown mineral by comparison or the unknown mineral with a mineral whoseri is known. Comparisons can be made with: 1. epoxy or balsam, material (glue) which holds the sample to the slide n = Quartz o nw = o ne = Becke lines form at mineral-epoxy, mineral-mineral boundaries and are interpreted just as with grain mounts, they always move into higher RI material when the stage is lowered. OPTICS In Isotropic Materials - the velocity of light is the same in all directions. The chemical bonds holding the material together are the same in all directions, so that light passing through the material sees the same electronic environment in all directions regardless of the direction the light takes through the material. Isotropic materials of interest include the following isometric minerals: 1. Halite - NaCl 2. Fluorite - Ca F2 3. Garnet X3Y2(SiO4)3, where: 2+ o X = Mg, Mn, Fe, Ca o Y = Al, Fe3+, Cr 4. Periclase - MgO If an isometric mineral is deformed or strained then the chemical bonds holding the mineral together will be effected, some will be stretched, others will be compressed. The result is that the mineral may appear to be anisotropic.

96 COMPILED BY GDC HANDWARA

97 LECTURE NOTES 1ST SEMESTER UNIT 4 PHYSICAL PROPERTIES OF MINERALS 1. Introduction The physical characteristics of minerals include traits which are used to identify and describe mineral species. These traits include color, streak, luster, density, hardness, cleavage, fracture, tenacity, and crystal habit. Certain wavelengths of light are reflected by the atoms of a mineral's crystal lattice while others are absorbed. Those wavelengths of light which are reflected are perceived by the viewer to possess the property of color. Some minerals derive their color from the presence of a particular element within the crystal lattice. The presence of such an element can determine which wavelengths of light are reflected and which are absorbed. This type of coloration in minerals is termed idiochromatism; different samples of an idiochromatic mineral species will all display the same color. Other minerals are colored by the presence of certain elements in mixture. Different samples of such a species may exhibit a range of similar colors. Still other mineral species may usually be colorless, but may display several different and startling colors when trace amounts of impurities, or elements which are not an integral part of the crystalline lattice, are present. Coloration which is caused by the presence of an element foreign to the crystal lattice, whether in mixture or in trace amounts, is termed allochromatism. Certain elements are strong pigmenting agents and may lend vivid colors to specimens when they are present, whether as a part of the crystal lattice, in mixture, or as an impurity. These elements are termed the chromophores. Streak is the color which a mineral displays when it has been ground to a fine powder. Trace amounts of impurities do not tend to affect the streak of a mineral, so this characteristic is usually more predictable than color. Two different specimens of the same species may be expected to possess the same streak, whereas they may display different colors. Minerals are either opaque or transparent. A thin section of an opaque mineral such as a metal will not transmit light, whereas a thin section of a transparent mineral will. Typically those minerals which possess metallic bonding are opaque whereas those where ionic bonding is prevalent are transparent. Relative differences in opacity and transparency are described as luster. The characteristic of luster provides a qualitative measure of the amount and quality of

98 light which is reflected from a mineral's exterior surfaces. Luster thus describes how much the mineral surface 'sparkles'. The property of density is defined as mass per unit volume. Certain trends exist with respect to density which may sometimes aid in mineral identification. Native elements are relatively dense. Minerals whose chemical composition contains heavy metals, or atoms possessing an atomic number greater than iron (Fe, atomic number 26), are relatively dense. Species which form at high pressures deep within the earth's crust are in general more dense than minerals which form at lower pressures and shallower depths. Dark-colored minerals are typically fairly dense whereas light-colored ones tend to be less dense. Hardness is defined as the level of difficulty with which a smooth surface of a mineral specimen may be scratched. Hardness has historically been measured according to the Mohs scale. Mohs' method relies upon a scratch test to relate the hardness of a mineral specimen to the hardness of one of a set of reference minerals. Hardness may also be measured according to the more quantitative but less accessible diamond indentation method. Cleavage refers to the splitting of a crystal along a smooth plane. A cleavage plane is a plane of structural weakness along which a mineral is likely to split. The quality of a mineral's cleavage refers both to the ease with which the mineral cleaves and to the character of the exposed surface. Not every mineral exhibits cleavage. Fracture takes place when a mineral sample is split in a direction which does not serve as a plane of perfect or distinct cleavage. A mineral fractures when it is broken or crushed. Fracture does not result in the emergence of clearly demarcated planar surfaces; minerals may fracture in any possible direction. The characteristic of tenacity describes the physical behavior of a mineral under stress or deformation. Most minerals are brittle; metals, in contrast, are malleable, ductile, and sectile. The term crystal habit describes the favored growth pattern of the crystals of a mineral species. The crystals of particular mineral species sometimes form very distinctive, characteristic shapes. Crystal habit is also greatly determined by the environmental conditions under which a crystal develops. 2. Color: When different wavelengths of visible light are incident upon the eye they are perceived as being of different colors. Three different varieties of color receptors in the eye correspond to light possessing wavelengths of approximately 660 nm (red), 500 nm (green), and 420 nm (blueviolet). The eye then interprets the color of incident light according to which color receptors have been stimulated. For example, if monochromatic light which stimulated the red and green color receptors equally and did not affect the blue-violet receptors was detected, then the eye would

99 interpret this light as possessing a wavelength halfway between those of red and green light. The eye would therefore register an incident light wave with a wavelength of approximately 580 nm and the viewer would percieve the incoming light as yellow. Incident polychromatic light which stimulated the red and green color receptors equally and did not affect the blue-violet ones would also be interpreted as yellow light, regardless whether or not the incoming light actually contained a component with a wavelength close to 580 nm. The incident polychromatic light might possess only a red and a green component of equal intensity; it would nevertheless be interpreted by the eye as yellow light. The phenomenon called color is thus a description of the differentiation by the eye between various wavelengths and combinations of wavelengths of visible light. When light is incident upon a mineral specimen, some wavelengths are absorbed by the atoms of the crystal lattice while others are reflected. Those wavelengths which were not absorbed are reflected off of the mineral's surfaces and enter the eye of the viewer. The color which is perceived by the viewer depends on the wavelengths of light which are reflected rather than absorbed by the mineral. The property of color in minerals is thus due to the absorption of particular wavelengths of light and the reflection of others by the atoms of the crystal lattice. The color exhibited by certain mineral species may depend upon which crystallographic axis is transmitting the light. Such species may demonstrate several different colors as light is transmitted along various different axes. This phenomena of directionally selective absorption is termed pleochroism. Idiochromatism and the Chromophores The color of many mineral species is derived directly from the presence of one or more of the elements which constitute the crystal lattice. The color of such minerals is a fundamental property directly related to the chemical composition of the species. Minerals which exhibit this type of coloration are called idiochromatic minerals. Idiochromatic coloration is a property possessed by a mineral species as a whole. In such species color can successfully be utilized as a means of identification. Ions of certain elements are highly absorptive of selected wavelengths of light. Such elements are called chromophores; they possess strong pigmenting capabilities. The elements vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), and copper (Cu) are chromophores. A mineral whose chemical formula stipulates the presence of one or more of these elements may possess a vivid and distinctive color. Examples of idiochromatic minerals abound. For instance, the copper carbonate malachite is consistently green; the copper carbonate azurite and the copper silicate chrysocolla are each a distinctive and predictable blue. Rhodochrosite is always red or pink; samples of sulphur are a bright, recognizable yellow. Each of these distinctive colors is due to the fact that the chemical composition which defines the mineral species specifies inclusion of one of the chromophores

100 within the lattice structure. Allochromatism Most minerals which are composed entirely of elements other than the chromophores are nearly colorless. However, certain specimens are sometimes observed to possess vivid coloration. Color in such instances is due to the presence of an impurity. If one of the chromophores is present within a mineral whose chemical formula does not include it, then the foreign element constitutes an impurity or a defect in the lattice structure. Coloration in minerals which is due to the presence of a foreign element is termed allochromatism. In such cases the color of the mineral may differ radically from the nearly colorless shade expected of the species. Some minerals demonstrate a range of colors due to the presence in mixture of one of the chromophores. For example, the substitution of a quantity of iron for zinc atoms within the crystal lattice of sphalerite (ZnS) implements a change from white to yellow in the color of the mineral. Proportionally larger inclusions of iron will progressively result in a brown and eventually a black mineral specimen. In such cases the color of the sample is directly proportional to the amount of the pigmenting element which is present in the crystal lattice. Not all allochromatism in minerals is due to presence of substantial amounts of a chromophore in mixture, however. The property of color may sometimes be highly dependent on the inclusion of trace amounts of impurities. The presence of even a minute quantity of a chromophore within the crystal lattice can cause a mineral specimen to exhibit vivid color. For example, trace inclusions of chromium (Cr) in beryl are responsible for the deep green of emerald, while the purple of amethyst is due to trace amounts of iron (Fe) in quartz and the pink of rose quartz is due to trace inclusions of titanium (Ti). Samples of the mineral corundum which include tiny amounts of chromium are deep red, and the gem is then called a ruby, while samples containing iron or titanium impurities produce blue gems termed sapphire. Trace amounts of an impurity do not affect the basic chemical composition or the chemical formula of a mineral, and thus do not affect its classification as a species. Trace amounts of the various chromophores, however, can cause several samples of a single species to differ radically in color. (Beryl, corundum, and quartz provide examples of this possibility.) Because it varies so widely, color is a property which is sometimes of little use in identification. However, the idiochromatic minerals are consistently of distinctive color. The green of malachite, the blue of azurite, the pink of rhodocrosite, and the yellow of sulphur are easily recognized and are therefore quite useful in the identification of these species. 3. Streak Streak is the color of a mineral substance when it has been ground to a fine powder. Typically an edge of the sample will be rubbed across a porcelain plate, leaving behind a 'streak' of finely ground material. The material in a streak sample thus consists of a powder composed of

101 randomly oriented microscopic crystals rather than a lattice structure containing the uniformly oriented unit cells which compose a macroscopic crystal. Although color is a property which may vary widely between two different specimens of the same mineral, streak generally varies little from sample to sample. The presence of trace amounts of an impurity may radically affect the property of color in a macroscopic crystal because each unit cell is aligned within the crystal structure, thereby forming a diffraction grating. Minute amounts of a strongly absorptive impurity within the structure may highly affect which wavelengths of light are reflected from this diffraction grating. This change may greatly modify the absorption of certain wavelengths of incoming light, altering the percieved color of the specimen. In a streak sample, however, each of the microscopic crystal grains of the sample is randomly oriented and the presence of an impurity does not greatly affect the absorption of incoming light. Because it is not typically affected by the presence of an impurity, streak is a more reliable identification property than is color. 4. Luster Minerals may be categorized according to whether they are opaque or transparent. A thin section of an opaque mineral such as a metal will not transmit light, whereas a thin section of a transparent mineral will. The absorption index of an opaque mineral is high. Light which is incident upon an opaque mineral such as a metal is unable to propagate through the mineral due to this high rate of absorption, and will thus be reflected. Opaque minerals typically reflect between 20% to 50% or more of the light incident upon them. In contrast, most of the light which is incident upon a transparent mineral passes into and through the mineral; transparent minerals may reflect as little as 5% of the incident light and as much as 20%. Typically those minerals which possess metallic bonding are opaque whereas those where ionic bonding is prevalent are transparent. Relative differences in opacity and transparency are described as luster. The term luster refers to the quantity and quality of the light which is reflected from a mineral's exterior surfaces. Luster provides an assessment of how much the mineral surface 'sparkles'. This quality is determined by the type of atomic bonds present within the substance. It is related to the indices of absorption and refraction of the material and the amount of dispersion from the crystal lattice, as well as the texture of the exposed mineral surface. Minerals are primarily divided into the two categories of metallic and nonmetallic luster. Minerals possessing metallic luster are opaque and very reflective, possessing a high absorptive index. This type of luster indicates the presence of metallic bonding within the crystal lattice of the material. Examples of minerals which exhibit metallic luster are native copper, gold, and silver, galena, pyrite, and chalcopyrite. The luster of a mineral which does not quite possess a metallic luster is termed submetallic; hematite provides an example of submetallic luster.

102 The property of streak can aid in distinguishing whether a specimen has a metallic or a nonmetallic luster. Metals tend to be soft, implying that more powdered material may be obtained from the streak sample of a metal than a nonmetal. Metals are also opaque, transmitting no light. Minerals which possess a metallic luster therefore tend to exhibit a thick, dense, dark streak whereas those which possess a nonmetallic luster tend to produce a thinner, less dense streak which is also lighter in color. Adjectives such as "vitreous', 'dull', 'pearly', 'greasy', 'silky' or 'adamantine' are frequently used to describe various types of nonmetallic luster. Dull or Earthy : Minerals of dull or earthy luster reflect light very poorly and do not shine. This type of luster is often seen in minerals which are composed of an aggregate of tiny grains. Resinous A surface of resinous luster possesses a sheen resembling that of resin. Such materials have a refractive index greater than 2.0. Sphalerite (ZnS) demonstrates a resinous luster. Pearly Pearly luster appears iridescent, opalescent, or pearly. This is typically exhibited by mineral surfaces which are parallel to planes of perfect cleavage. Layer silicates such as talc often demonstrate a pearly luster on cleavage surfaces.

103 Greasy A surface which possesses greasy luster appears to be covered with a thin layer of oil. A lightscattering surface which is slightly rough, such as that of nepheline, may exhibit greasy luster. Silky Silky luster occurs when light is reflected off of an aggregate of fine parallel fibers; malachite and serpentine may both exhibit silky luster. Vitreous Vitreous luster occurs in minerals with predominant ionic bonding and resembles the reflective quality of broken glass. The refractive index of such minerals is 1.5 to 2.0. Many silicates possess this type of luster; quartz and tourmeline both demonstrate vitreous luster.

104 Adamantine or brilliant A brilliant luster such as the sparkling reflection of diamond is known as adamantine. Minerals of adamantine luster have high refractive indices ( ) and are highly dispersive and translucent. Covalent bonding or the presence of heavy metal atoms or transition elements may result in adamantine luster. Metallic lustre Metallic (or splendant) minerals have the lustre of polished metal, and with ideal surfaces will work as a reflective surface. Examples include galena,6[6] pyrite[7] and magnetite

105 Submetallic lustre Submetallic minerals have similar lustre to metal, but are duller and less reflective. A submetallic lustre often occurs in near-opaque minerals with very high refractive indices,2such as sphalerite, cinnabar and cuprite. Waxy lustre Jade Waxy minerals have a lustre resembling wax. Examples include jade[11] and chalcedony.[12] Optical phenomena Asterism Sapphire cabochon Asterism is the display of a star-shaped luminous area. It is seen in some sapphires and rubies, where it is caused by impurities of rutile.[12][13] It can also occur in garnet, diopside and spinel.

106 Aventurescence Aventurine Aventurescence (or aventurization) is a reflectance effect like that of glitter. It arises from minute, preferentially oriented mineral platelets within the material. These platelets are so numerous that they also influence the material's body colour. In aventurine quartz, chromebearing fuchsite makes for a green stone and various iron oxides make for a red stone. Chatoyancy Tiger's eye Chatoyant minerals display luminous bands, which appear to move as the specimen is rotated. Such minerals are composed of parallel fibers (or contain fibrous voids or inclusions), which reflect light into a direction perpendicular to their orientation, thus forming narrow bands of light. The most famous examples are tiger's eye and cymophane, but the effect may also occur in other minerals such as aquamarine, moonstone and tourmaline.

107 Color change..alexandrite Color change is most commonly found in Alexandrite, a variety of chrysoberyl gemstones. Other gems also occur in color-change varieties, including (but not limited to) sapphire, garnet, spinel. Alexandrite displays a color change dependent upon light, along with strong pleochroism. The gem results from small scale replacement of aluminium by chromium oxide, which is responsible for alexandrite's characteristic green to red color change. Alexandrite from the Ural Mountains in Russia is green by daylight and red by incandescent light. Other varieties of alexandrite may be yellowish or pink in daylight and a columbine or raspberry red by incandescent light. The optimum or "ideal" color change would be fine emerald green to fine purplish red, but this is exceedingly rare. Schiller Labradorite Schiller, from German for "twinkle", is the metallic iridescence originating from below the surface of a stone, that occurs when light is reflected between layers of minerals. It is seen in moonstone and labradorite and is very similar to adularescence and aventurescence.[14] 5. Density The property of density is defined as mass per unit volume: µ = m/v The geometric structure of the unit cell of a mineral determines the volume which it occupies. The masses of the atoms which compose the unit cell decree the mass of each cell. The identity of the atoms which compose the unit cell is specified by the chemical formula of the mineral. Density is therefore directly related to both the physical structure of the unit cell and the chemical composition of each species of mineral.