Earth Materials II Review Optical Mineralogy and Igneous Minerals

Earth Materials II Review Optical Mineralogy and Igneous Minerals

Refractive Index and Angle of Refraction Refractive Index(R. I. ) = velocity of light in a vacuum velocity of light in a medium The refractive index varies with the wavelength of light. Becke Lines Super Important Refraction Snell s Law sin i = R.I.2 sin r R.I.1

Birefringence difference between maximum and minimum refractive indexes. Retardation amount by which the fast and slow light waves are out of phase. Function of birefringence and thickness of the mineral.

Extinction occurs when one of the vibration directions in the crystal parallels the E-W polarizer. In this case the polarized light is not split into two rays vibrating at right angles to each other. When the E-W vibrating ray encounter the upper polarizer which only permits rays vibrating in the N-S direction to pass, the crystal goes to extinction (becomes dark). The relationship between this angle and crystallographic directions can be an important piece of diagnostic information. Extinction can be parallel, inclined, or symmetrical.

Sign of Elongation

Pleochroism is the change in color that occurs when a mineral is rotated under plane-polarized light. This is due to the selective adsorption of certain wavelengths of light which causes the transmitted light to appear colored. The pleochroic colours are at their maximum when light is polarized parallel with a crystallographic axis. The axes are designated X, Y and Z. An absorption formula records the amount of absorption parallel to each axis in the form of X < Y < Z with the left most having the least absorption and the rightmost the most. Manganese epidote piemontite Biotite absorption is greater when cleavage parallels polarizer. Tourmaline absorbs more light when the long axis of the crystal is perpendicular to the polarizer.

Uniaxial Indicatrix

Uniaxial interference figures

Biaxial Indicatrix and relationship to crystallography

Biaxial indicatrix positive vs negative

Determining optic sign and 2V

Determining optic sign

Igneous Minerals Quartz Orthoclase Plagioclase Mica Amphibole Pyroxene Olivine

T able 7-4. Properties of the silicate crystal classes Class Tetrahedral arrangement # shared corners Che mical unit Si:O Example Nesosilicate Independent tetrahedra 0 SiO 4 4 1:4 Olivine Sorosilicate Two tetrahedra sharing a corner 1 Si 2 O 6 7 1:3.5 Melilite Cyclosilicate Inosilicate Three or more tetrahedra sharing two corners, forming a ring Single chain of tetrahedra sharing two co rners 2 SiO 3 3 1:3 Beryl 2 SiO 3 3 1:3 Augite Double chain of tetrahedra alternately sharing two or three corners 2.5 Si 4 O 6 11 1:2.75 Hornblende Phyllosilicate Sheet of tetrahedra sharing three corners 3 Si 2 O 2 5 1:2.5 Kaolinite Tektosilicate Framework of tetrahedra sharing all four co rners 4 SiO 2 1:2 K-feldspar

Quartz SiO 2 P-T conditions and polymorphs Feldspars K-feldspar (KAlSi 3 O 8 ) Plagioclase (NaAlSi 3 O 8 CaAl 2 Si 2 O 8 ) Coupled substitution: CaAl NaSi

KAlSi 3 O 8 polymorphs: The distinction between the polymorphs is based on the ordering of Al in the tetrahedral sites. Microcline low T polymorph one in every four tetrahedral sites is filled with an Al. Total order Orthoclase moderate T polymorph Al is distributed over two equivalent tetrahedral sites. Partially ordered Sanidine high T polymorph equal probability of finding Al in any of the four tetrahedral sites. Completely disordered. Perthite albite exsolved from microcline or orthoclase. Antiperthite orthoclase exsolved from albite.

Feldspathoids Leucite KAlSi 2 O 6 Nepheline (Na,K)AlSiO 4 Sodalite Na 4 Al 3 Si 3 O 12 Cl Feldspathoids are found in silica undersaturated igneous rocks. Silica deficient relative to the feldspars. For example: KAlSi 2 O 6 + SiO 2 = KAlSi 3 O 8

Single chain silicates - Pyroxenes Enstatite: MgSiO 3 (Mg,Fe)SiO 3 Pigeonite: Ca 0.25 (Mg,Fe) 1.75 SiO 6 Augite: (Ca,Na)(Mg,Fe,Al)(Si,Al)O 2 O 6 Aegirine: NaFe 3+ Si 2 O 6

Alkali olivine basalts one Ca-pyroxene. Tholeiites two pyroxenes, one Ca-rich and the other Ca-poor. Composition of co-existing pyroxenes. Exsolution of augite lamallae in enstatite host. Enstatite is orthorhombic but orientation of lamallae indicate that the original grain was pigeonite (monoclinic).

Double chain silicates - Amphiboles Hornblende: (Na,K) 0-1 Ca 2 (Mg,Fe,Al) 5 (Si,Al) 8 O 22 (OH) 2

Sheet Silicates - micas Biotite: K(Mg, Fe) 3 AlSi 3 O 10 (OH) 2 Dioctahedral gibbsite Al(OH) 3 Trioctahedral brucite Mg(OH) 2

Cyclosilicate Sorosilicate Gehlenite [Ca 2 Al(Al, Si) 2 Si 7 ] Akermanite [Ca 2 MgSi 2 O 7 ]

Nesosilicates

Apatite Chlorapatite Cl Fluorapatite F Hydroxyapatite - OH

Other Minerals Mineral Formula Oxides Chromite FeCr 2 O 4 Hematite Fe 2 O 3 Magnetite Fe 3 O 4 Ilmenite FeTiO 3 Rutile TiO 2 Sulfides Pyrite FeS 2 Pyrrhotite Fe 1-x S Chalcopyrite CuFeS 2