Nesosilicate. Staurolite Fe 2 Al 9 O 6 (SiO 4 ) 4 (OH) 2

Transcription

1 Nesosilicate Olivine (Mg,Fe) 2 SiO 4 series: Forsterite Mg 2 SiO 4 - Fayalite Fe 2 SiO 4 NOTE: The use of parenthesis within a chemical formula indicates elements which may be found in variable proportions within a given mineral structure. In olivine, the silicate tetrahedra are connected by either Mg 2+ or Fe 2+ cations. Diagnostic features: hard (6.5-7: harder than knife) but grains are easily detached from the rock; no cleavage but pronounced conchoidal fracture; colour is green but can tend towards yellow or brown. This mineral is found mostly in dark-coloured igneous rocks, like the Hawaiian basalts. Colour: A perfectly pure forsterite would be colourless. The presence of some Fe 2+ in most olivine is responsible for its green colour. Fe-poor varieties can be yellow, and Fe-rich varieties can be brown. The yellow-green, gem-quality variety of olivine is known as PERIDOT. Habit: its stubby prismatic crystals usually look like small grains disseminated through the rock. Look alikes: epidote, diopside. Unlike epidote, green olivine is not found with quartz. It lacks the twinning and prismatic cleavage that are commonly seen in diopside. Garnet group A 3 B 2 Si 3 O 12 (nesosilicate) Diagnostic features: high hardness ( , usually harder than a knife), equant (isometric) habit, vitreous luster, no distinct cleavage. Habit: equant, consisting of a dodecahedron, a trapezohedron or a combination of these (shown below). Faces may be striated. Colour: highly variable with composition. Many garnets have compositions intermediate between the endmembers listed below. (For the test, simply identify any member of this group as "garnet".) Pyrope, Mg 3 Al 2 Si 3 O 12 is usually deep red (typical of igneous rocks formed at great depths, under high pressure). Almandine, Fe 3 Al 2 Si 3 O 12 is red to black. Spessartine, Mn 3 Al 2 Si 3 O 12 is brownish red to pink. Grossular, Ca 3 Mg 2 Si 3 O 12 is colourless to yellow, green or brown (common, often associated with calcite). Andradite, Ca 3 Fe 2 Si 3 O 12 is found in shades of yellow, green or brown to black. Uvarovite Ca 3 Cr 2 Si 3 O 12 has an emerald-green colour. Right: Garnet often occurs in schists, i.e. metamorphic rocks showing a strong foliation caused by the alignment of platy mica crystals or prismatic amphiboles. Topaz Al 2 SiO 4 (F, OH) 2 Nesosilicate Diagnostic features: high hardness (8), perfect basal {001} cleavage, and prismatic crystals with a rhombic cross-section. Habit: commonly euhedral (i.e. well-developed faces), in prismatic crystals terminated by dipyramids, {0kl} and {h0l} prisms and basal pinacoids. The faces of the vertical prism give each crystal its rhombic cross section. These faces are commonly striated vertically. Colour: very variable, from colourless to pink, yellow, green or blue. Rarely dark. Luster: vitreous, especially on a freshly broken cleavage face. Nesosilicate Staurolite Fe 2 Al 9 O 6 (SiO 4 ) 4 (OH) 2 Diagnostic features: characteristic cross-shaped twins and crystal form, high hardness (7-7.5 is harder than a knife) and occurrence in metamorphic schists and gneisses. Habit: the {110} prism (see below) gives it a characteristic rhombic outline. Cruciform twins are common. There is no distinct cleavage. Colour: red brown to brownish black. Luster: vitreous to resinous when fresh, dull to earthy when altered or impure. Often found with garnet and mica in metamorphic rocks. Topaz is a mineral formed by the fluorine-rich vapours given off during the last stages of crystallization of siliceous igneous rocks. It is found in cavities of silica-rich lavas (rhyolites) and granites, and in pegmatites (very coarse-grained igneous rock). Associated with quartz, mica and feldspar and commonly found with tourmaline, apatite and fluorite.

2 Nesosilicate Andalusite Al 2 SiO 5 This is one of three geologically important aluminosilicate polymorphs. Andalusite, sillimanite and kyanite are identical in composition but the geometry of their internal arrangement is different. Andalusite is an indicator of low-pressure metamorphism. Diagnostic features: hard (7.5, harder than knife), prismatic crystals with nearly square cross-sections. Colour: A perfectly pure andalusite would be colourless but the natural mineral can be anything from reddish brown to olive-green. The variety chiastolite includes carbon-rich material on some faces during its growth, giving rise to a cross-like pattern visible on cut and polished crystals (see below, on the left). Habit: usually forms distinct prismatic crystals, with nearly square cross-sections, terminated by {001}. The prismatic {110} cleavage is good but not perfect (not as obvious as in hornblende, for example). Look-alikes: Orthoclase. Nesosilicate Sillimanite Al 2 SiO 5 This is one of three geologically important aluminosilicate polymorphs. Andalusite, sillimanite and kyanite are identical in composition but the geometry of their internal arrangement is different. Sillimanite is an indicator of a medium- to high-grade of metamorphism. Diagnostic features: slender crystals with one direction of cleavage. Can be difficult to tell apart from tremolite. Colour: A perfectly pure sillimanite would be colourless. The mineral is typically light-coloured, brown, palegreen or white. Weathering tends to turn the surface rusty. Habit: long, slender crystals without clear terminations. Often in parallel groups, frequently fibrous. Hardness: 6-7 (scratches the knife sometimes). Look-alikes: Wollastonite, tremolite, anthophyllite can all crystallize as fibrous masses. Wollastonite and tremolite are frequently associated with calcite. (below: sillimanite-garnet schist) Nesosilicate Kyanite Al 2 SiO 5 This is one of three geologically important aluminosilicate polymorphs. Andalusite, sillimanite and kyanite are identical in composition but the geometry of their internal arrangement is different. Kyanite is generally an indicator of high-pressure metamorphism. Diagnostic features: blue colour, bladed crystals, good cleavage and different hardness in different directions. Habit: long, tabular or bladed crystals with perfect {100} cleavage. Hardness: 5 parallel to the length of the crystals, 7 at right angle to the length. Colour: usually blue, with different shades from rim to center. Luster is vitreous to pearly. Look-alikes: none. + Mixed nesosilicate-sorosilicate Epidote Ca 2 (Al,Fe)Al 2 O(SiO 4 )(Si 2 O 7 )(OH) Diagnostic features: moderately high hardness (6-7: harder than knife); perfect platy cleavage parallel to the b axis; peculiar green colour. Colour: a peculiar yellow-green or pistachio-green that often distinguishes from other minerals that it may resemble. Habit: tabular or prismatic monoclinic crystals are commonly elongated and striated parallel to the b axis. Look-alikes: Olivine, augite. Unlike green olivine, epidote can be found with quartz. The many edges of faces give the epidote crystals the appearance of being striated along their long axis. Epidote is an abundant and widespread metamorphic mineral, occurring in calcium-rich rocks derived from impure limestone or in igneous rocks. Clinozoisite is a pale coloured variety of epidote that has very little Fe; it occurs as a secondary mineral in metamorphosed igneous rocks.

3 Cyclosilicate (ring silicate) Beryl, Be 3 Al 2 Si 6 O 18 Diagnostic features: high hardness (7.5-8, harder than quartz and knife), hexagonal prismatic habit, and a poor platy cleavage. Habit: hexagonal prisms terminated by a pinacoid (less commonly by dipyramidal faces) are common. Striations, when present, are parallel to the long axis of the prism. Colour: usually bluish green or light yellow, but it may be emerald green, pink, white or colourless. Various names are given to gem-quality crystals: AQUAMARINE is the clear blue-green variety, coloured by small amounts of Fe, EMERALD is coloured deep green by chromium or vanadium, MORGANITE is pale pink to deep rose, and GOLDEN BERYL is clear yellow. (You do not need to remember these variety names for the test.) Cyclosilicate (ring silicate) Tourmaline (Na,Ca)(Li, Mg, Al) 3 (Al, Fe, Mn) 6 (BO 3 ) 3 Si 6 O 18 (OH) 4 Diagnostic features: high hardness (7-7.5, can be harder than quartz but always harder than the knife), trigonal prismatic habit with a slightly rounded triangular section, conchoidal fracture. Habit: trigonal prisms, often terminated by trigonal pyramids. The prism is usually vertically striated. Colour: can take a wide range of colours. One of the most common variety is schorl, coloured black by Fe. Pink tourmaline is elbaite, coloured by Li. There are many more. Tourmaline sometimes varies in colour from one end of the prism to another, or from the core to the rim. Crystals that are pink inside with a green outer rim are sometimes called watermelon tourmaline. Look-alikes: beryl, apatite. pyroxenes (single-chain silicates) series: Diopside CaMgSi 2 O 6 - Hedenbergite CaFeSi 2 O 6 Diagnostic features: prismatic habit, octogonal section, moderate hardness (5.5, about same as knife) and colour (white to green). Cleavage is imperfect, at angles of 87 and 93. A well-developed parting parallel to the basal pinacoid c is sometimes visible. Habit: prismatic crystals are common. The cross section seen perpendicular to the prism is usually eight-sided, because the faces m, a and b in the prism zone make angles of nearly 45 with each other. Colour: white to green, varying with the amount of Fe present. Cleavage: prismatic, parallel to the length of the prisms, at angles of 87 and 93. Look-alikes: altered or smaller crystals can be mistaken for olivine (lacks good cleavage) and/or augite (usually darker). pyroxenes (single-chain silicates) Augite: (Ca,Na)(Fe,Mg,Al)(Al,Si) 2 O 6 Diagnostic features: stubby prismatic habit with an octogonal cross-section, moderate hardness (5.5, about same as a steel blade) and dark colour (deep green to black). Cleavage is imperfect, at angles of 87 and 93. Habit: stubby prismatic crystals very similar to those of diopside. The cross section seen perpendicular to the prism is often eight-sided, because the faces m, a and b in the prism zone make angles of nearly 45 with each other. Colour: deep green to black. Its luster is rarely as vitreous as that of hornblende. Cleavage: prismatic, parallel to the length of the prisms, at angles of 87 and 93. Streak: colourless (unless the mineral contains inclusions of oxide minerals).

4 Pyroxenes (single-chain silicates) Spodumene: LiAlSi 2 O 6 Diagnostic features: flattened prismatic habit, vertical striations and light colour. The crystal surface alters easily, giving it a dull, woody aspect which is very different from the vitreous luster of a fresh parting surface. Be careful: it is easy to mistake one cleavage direction and the parting with the angle typical of amphiboles. Habit: prismatic crystals, often flattened on {100} and usually deeply striated vertically. Parting (a smooth surface along which the crystal has broken) is sometimes well developed on {100}, the largest parallelohedron on the drawing below. Hardness: (harder than knife, but possible softer on weathered surfaces). Colour: typically light coloured but in various shades of gray, yellow, green or pink. Among the gem varieties, kunzite is lilac and hiddenite is emerald-green. NOTE: its unusual composition, i.e. the presence of lithium, is due to the composition of watery fluids concentrated in the last stages of crystallization of a magma. Spodumene generally crystallizes from this magmatic waters with other Li-bearing minerals like rubellite (a variety of pink tourmaline) and lepidolite. Amphiboles (double-chain silicates) Anthophyllite (Mg, Fe) 7 Si 8 O 22 (OH) 2 Diagnostic features: "clove-brown" colour (i.e. beige brown like clove, the spice) and often softer than its "official" hardness (5.5-6: barely scratched by a knife) because it alters so easily to talc,. Prismatic to fibrous habit (fairly similar to those of actinolite and tremolite). The perfect prismatic cleavage of amphibole (two planes at angles of 124 and 56 ) gives a splintery look to broken surfaces. Habit: prismatic to fibrous. The crystals are generally too small to recognize the cleavage angle or rhombic cross section. Colour: beige to brown, less commonly shades of green. Luster should be vitreous but is often pearly or silky because the mineral is partly altered to talc. As in all amphiboles, the prismatic cleavage is parallel to the direction of elongation of the crystal. amphiboles (double-chain silicates) amphiboles (double-chain silicates) Tremolite, Ca 2 Mg 5 Si 8 O 22 (OH) 2 - Actinolite, Ca 2 Fe 5 Si 8 O 22 (OH) 2 Diagnostic features: moderate hardness (5-6: barely scratched by a knife), perfect prismatic cleavage (with angles of 124 and 56 ) gives a splintery look to broken surfaces. Found in prismatic crystals, sometimes bladed. Colour: varies with iron content. Pure tremolite is white but even a small amount of iron gives it a greenish tinge. The green colour of pure actinolite is so dark that the crystals looks black. Luster, generally vitreous. Silky in fibrous varieties. Habit: Slender bladed prisms or fibrous radiating bunches. Cleavage: Obvious parallel to their lengths; the two sets intersect at nearly 120º on basal sections Hardness: should be moderate (5-6). May be difficult to test because the crystals tend to break or detach rather than powder. Both minerals alter easily to talc which is much softer. Look-alikes (Actinolite): Hornblende. Look-alikes (Tremolite): Anthophyllite, sillimanite and wollastonite. Hornblende, NaCa 2 (Mg, Fe, Al) 5 (Si, Al) 8 O 22 (OH) 2 Diagnostic features: moderate hardness (5-6: barely scratched by a knife), perfect prismatic cleavage (two planes at angles of 124 and 56 ) which gives a splintery look to its broken surfaces. Found as prismatic crystals, dark green to black. Habit: prismatic crystals with a six-sided, nearly rhombic cross section. Colour: dark green to black. Vitreous luster. Streak: white (colourless, unless the mineral contains small inclusions of opaque oxide minerals). Look-alikes: augite. Hornblende is the most common of the amphiboles and occurs in a variety of igneous rocks. The prismatic cleavage is parallel to the direction of elongation of the crystal.

5 amphiboles (double-chain silicates) Glaucophane NaCa 2 (Mg, Fe, Al) 5 (Si, Al) 8 O 22 (OH) 2 Diagnostic features: blue colour and fibrous habit. Habit: slender acicular crystals (with typical amphibole cleavage) or an asbestiform (i.e. flexible fibers) habit. Colour: blue colour. Silky luster when asbestiform, vitreous when acicular. Hardness: 6 (same or slightly harder than a knife) but cannot be tested accurately on asbestiform samples! Occurrence: found mostly in high-pressure metamorphic rocks (once buried deep in subduction zones), but also seen in some igneous rocks. Phyllosilicate Talc Mg 3 Si 4 O 10 (OH) 2 Diagnostic features: low hardness (1: softest on Moh s scale, scratched by fingernail), soapy feel and pearly luster. Habit: crystals are rare. Found most commonly in foliated or compact masses. When talc separates in plates, the plates are not elastic, i.e. once it has been bent, the plate does not snap back. Colour: white, gray or greenish. Its luster is non-metallic and often described as pearly or silky. Look-alikes: Serpentine and fine grained muscovite (sericite). Soapstone, used for carving, is a rock largely made of compact, fine-grained talc. chlorite group Chlorite (Mg, Fe) 3 (Si, Al) 4 O 10 (OH) 2 (Mg, Fe) 3(OH) 6 Diagnostic features: pale green colour appears when scratched, micaceous habit and cleavage, folia are not elastic. Habit: Large crystals are rare. Found most commonly as small scales dispersed in metamorphic rocks, giving them a greenish colour. The perfect {001} cleavage often makes it easy to detach small platelets by scraping the rock with the knife. Chlorite is often found as pseudomorphs of other ferromagnesian silicate minerals, and the crystals may be submicroscopic. Scratching the specimen will leave a pale green groove. Colour: various shades of green (pale to nearly black), depending on the relative amounts of Mg and Fe. Other properties: low hardness (2-2.5: close to fingernail). May be hard to test if small chloride crystals are dispersed among harder minerals in a rock, or if a thin layer of chlorite covers a much harder mineral. This mineral is a common product of the alteration of other ferromagnesian minerals (e.g., olivine, augite, hornblende, actinolite and some garnets among those seen last week). The green colour of many igneous rocks is due to the alteration to chlorite of the original amphiboles (e.g. hornblende) and/or pyroxenes (e.g. augite) in these rocks. The green colour of many schists and slates (metamorphic rocks) is due to disseminated chlorite. serpentine group Antigorite/lizardite Mg 3 Si 2 O 5 (OH) 4 Diagnostic features: variegated green colour (i.e. not uniform) and greasy to waxlike luster. Habit: usually massive and fine-grained (the individual crystals cannot be distinguished). Colour: often variegated, with mottling in lighter and darker shades of green. Hardness: more variable than most minerals, 3-5 (usually 4) but softer than olivine (6.5-7). The text calls these minerals polymorphs of chrysotile (the fibrous mineral of this group), but crystallographers would disagree. Recent techniques have shown that antigorite and lizardite differ slightly in composition and structure from chrysotile. However the differences are too subtle to be recognized in hand specimen. This is why the more general term serpentine is used for hand specimens that contain either antigorite or lizardite. Minerals of the serpentine group are a common alteration mineral of olivine and magnesian pyroxenes or amphiboles.

6 serpentine group Chrysotile: Mg 3 Si 2 O 5 (OH) 4 BE CAREFUL when handling CHRYSOTILE or ANY FIBROUS MINERAL. Any fibrous mineral will release microscopic fibers if it is scratched or shaken. Breathing mineral fibers or dust irritates the lung tissues. Diagnostic features: fibrous to asbestiform habit, greenish colour. Habit: asbestiform (long, flexible fibers), usually interbedded with other massive minerals of the serpentine group. (The fibers actually consists of layer-like crystals rolled up like miniature carpets.) Colour: shades of green, pearly luster. Muscovite, KAl 2 (AlSi 3 O 10 )(OH) 2 Muscovite is also called white mica and potash mica. Diagnostic features: micaceous habit; perfect platy cleavage; coloured in thick blocks but colourless and transparent in thin sheets. Low hardness (2-2.5). Habit: monoclinic, but euhedral crystals are rare and their cross-section is nearly rhombic or pseudo-hexagonal. Most commonly occurs in scales or sheets without any regular form, simply filling space between surrounding minerals. When it occurs as compact masses of minute scales, as an alteration product of potassic feldspar, it is known as sericite. Colour: transparent and colourless in thin sheets. Thicker plates are commonly smoky brown but they may also be pink, yellow or green. You should detach plates from any dark mica to make sure that it is not this Fe-poor mica. Hardness: 3-4, but hard to test on asbestiform specimens because the fibers separate so easily. This is a common alteration mineral of olivine and magnesian pyroxene or amphibole. It is one of the minerals called "asbestos" (a general term for fibrous silicate minerals used in industry for fire- and earthquake-proofing materials), but probably the least harmful of them all because the fibers can dissolve in the lung fluids. Lepidolite, K(Li, Al) 2-3 (AlSi 3 O 10 )(O, OH, F) 2 Diagnostic features: micaceous habit and cleavage; pink to lilac colour in thick books but colourless and transparent in thin sheets. Low hardness (2-2.5). Habit: monoclinic, but euhedral crystals are rare and their cross-section is nearly rhombic or pseudo-hexagonal. Found most commonly in coarse to fine-grained scaly aggregates. Occurrence: lepidolite is a relatively rare mineral, found in pegmatites, and usually associated with other lithium-bearing minerals such as pink and green tourmaline and spodumene. Biotite, KMg 3 (AlSi 3 O 10 )(OH) 2 Diagnostic features: micaceous habit and cleavage; dark green or brown to black, even in thin sheets. Low hardness (2-2.5). Habit: monoclinic, the cross-section of euhedral crystals is nearly rhombic or pseudo-hexagonal. Also commonly found in coarse to fine-grained scaly aggregates (in metamorphic schists and gneisses). Colour: varies with Fe content, from light yellow (rare) to deep green, brown or black. Thin sheets usually have a smoky colour (differing from the almost colourless muscovite and lepidolite). Luster is splendent (i.e. even brighter than vitreous) on fresh surfaces. Occurrence: biotite is a common mineral, found in metamorphic rocks (schists and gneisses) and a wide range of igneous rocks. It is not found immediately next to muscovite or lepidolite (these other micas crystallize from rocks or magmas than are iron-poor and more aluminous).

7 Tectosilicate The potassic feldspars: Orthoclase/microcline KAlSi 3 O 8 Diagnostic features: moderately high hardness (6: barely scratched or harder than knife), two directions of cleavage that are mutually perpendicular (parallel to faces b and c), vitreous luster. Habit: euhedral crystals have a tabular to prismatic habit. Colour: very variable. Often pinkish or light orange but may also be white, pale yellow, reddish, greenish, or gray. The variety AMAZONITE has a blue-green colour related to the presence of small amounts of Pb. Most feldspars contain sodium as well as potassium and are part of the "alkali feldspar" series orthoclase KAlSi 3 O 8 -albite NaAlSi 3 O 8. Once they crystallize from magma, feldspars of intermediate composition undergo a chemical unmixing if they are allowed to cool slowly (in an intrusive rock, for example). The mineral separates into lighter and darker veins, giving rise to a characteristic texture called PERTHITE. Tectosilicate Albite NaAlSi 3 O 8 sodic feldspar/plagioclase: Diagnostic features: moderately high hardness (6: barely harder than knife), two directions of cleavage mutually perpendicular (parallel to faces b and c), vitreous luster, striations due to polysynthetic twinning. Habit: euhedral crystals have a tabular to prismatic habit. Colour: variable. Albite makes up the lighter-coloured veins of Na-felspar (albite, NaAlSi 3 O 8 ) that often separate from the darker-coloured K-feldspar (orthoclase or microcline) during its cooling following igneous crystallization. We already saw that this texture formed by two intergrown feldspar is called PERTHITE. Microcline is the low-temperature version of orthoclase. The bonding angles within their structures are slightly different. Orthoclase and microcline cannot be told apart in hand specimen. Both names (and their synonym, "K-feldspar") will be accepted interchangeably on the test. Tectosilicate Labradorite (Na ~.5,Ca ~.5 )(Al ~1.5,Si ~2.5 )O 8 A member of the plagioclase series (sodic-calcic feldspar): Diagnostic features: moderately high hardness (6: barely harder than knife), two directions of cleavage mutually perpendicular (parallel to faces b and c), vitreous luster, striations due to polysynthetic twinning, dark colour and iridescence visible on {010} (i.e. the faces b). Habit: euhedral crystals have a tabular to prismatic habit similar to that of albite. Colour: dark, due to minuscule inclusions of the mineral magnetite. The iridescence is due to the scattering of light by minuscule lamellae that form during cooling of this feldspar after crystallization. The cause is an unmixing process (exsolution) of the same type as the one forming perthite in microcline. Here, the unmixing gives rise to lamellae of Ca-rich feldspar (anorthite) in a Na-rich feldspar (albite), or vice-versa. Tectosilicate Quartz, SiO 2 There are several polymorphs of quartz. The most common one is low-quartz or alpha-quartz which crystallizes in class 32. A higher-temperature form, "beta-quartz", crystallizes in class 622 but it inverts to alpha-quartz during cooling. Both will be referred to as "quartz" during the laboratory exercises and mineral identification tests. Diagnostic features: hardness (7: not scratched by knife), prismatic habit, conchoidal fracture, vitreous luster. Quartz always leaves a white streak, regardless of its colour (unless it contains inclusions of other minerals). Habit: in euhedral (i.e. well-formed) crystals, the dominant form is generally a hexagonal prism with faces showing striations perpendicular to the c axis. The "pyramidal" ends are actually two rhombohedra (labelled r and z on these drawings). In anhedral (i.e. without faces) specimens, the lack of cleavage and hardness are characteristic. Colour: highly variable, from transparent and colourless to black, with nearly every colour in between! Quartz is always very pure but minute amounts of certain impurities can give it vivid colours. Some colours can be modified by irradiation or heating because they are due to the presence of defects in the structure. The names given to varieties of quartz having specific colours include: amethyst (purple), citrine (yellow), smoky quartz (dark brown), milky quartz (white). Chalcedony is a fibrous variety, often with a botryoidal habit.

8 Sulfide Pyrite FeS 2 Diagnostic features: pale brass yellow colour, relatively high hardness, streak is greenish to brownish black. Habit: crystals are often cubic, and usually striated (one direction on each face). Pyrite can also adopt other regular forms like octahedra and dodecahedra, and some crystals combine two or three forms. Poor cleavage. Colour: pale brass yellow (deeper if tarnished) and metallic luster. Hardness: 6 (unusually high for a sulfide mineral). Look-alikes: massive chalcopyrite and other yellow sulfides are easily confused with massive pyrite until your eye becomes attuned to their distinctly different colours and hardness. This is the most common sulfide, found in sedimentary, metamorphic and igneous rocks as well as hydrothermal deposits. carbonate (rhombohedral) Calcite CaCO 3 Diagnostic features: noticeable reaction (effervescence) with HCl at room temperature, rhombohedral cleavage, moderate hardness (H = 3-3.5). Habit: highly variable but a three-fold or six-fold symmetry is often clearly visible. Crystals can be nearly acicular or flattened plates. Most are combinations of rhombohedra and prism(s) or scalenohedra.. Colour: colourless when pure, but highly variable because of the presence of fluid inclusions, organic matter, inclusions of other minerals or substitution of Ca 2+ ions by various impurities. Cleavage fragments of limpid, colourless calcite crystals displays pronounced double refraction. There is no double refraction if one looks through the crystal down its c axis. Calcite is the most common of two geologically important polymorphs of CaCO 3.