Geology 284 Mineralogy Fall 2010 Dr Helen Lang West Virginia University Micas and Minerals of Pegmatites Biotite and Muscovite Micas one kind of sheet silicate Low on Bowen s Reaction Series Common only in lower temperature felsic igneous rocks like granite Also common in metamorphic rocks Mica Formulas Biotite K Mg2 Fe2 3 AlSi3O10 OH 2 complete solid solution Muscovite KAl3 2 AlSi3O10 OH 2 Mica Structures Simple Mica Structure from Text see handout Show Muscovite Movie thanks to Dr Jillian Banfield U C Berkeley Muscovite Movie http socrates berkeley edu eps2 wisc geo360 muscovite mov Biotite is a Trioctahedral Mica 3 3 of the octahedral spaces are filled with divalent Fe2 or Mg2 cations Biotite Properties Light tan Mg rich to black Fe rich color Hardness 2 5 3 Platy habit Perfect basal cleavage Flexible cleavage sheets May occur in pseudo hexagonal books Biotite Biotite note hexagonal books Biotite Optical properties Brown red brown tan or green in thin section Strongly pleochroic Perfect basal cleavage Parallel or near parallel extinction Bird s eye extinction photo coming Pleochroic haloes around zircon see photo Biotite Pleochroism Bird s eye Extinction in Biotite typical of all micas Muscovite is a dioctahedral Mica 2 3 of octahedral spaces are filled with trivalent Al3 cations Muscovite Properties White to gray or silvery Hardness 2 to 2 5 Platy habit Perfect basal cleavage Flexible cleavage sheets Occurs as books massive aggregates or scattered in a quartz feldspar matrix Muscovite Muscovite with Quartz Large cleavage plates of Muscovite are used for Insulation in electrical apparatus Isinglass windows in furnace and stove doors Muscovite Optical Properties Clear and colorless in thin section PPL Perfect basal cleavage Moderate to high birefringence Bird s eye extinction like all micas Muscovite in thin section Minerals of Pegmatites Large crystals of Quartz Alkali Feldspar and Muscovite which are low on Bowen s Reaction Series Some Pegmatites contain Graphic Granite Some Pegmatites contain rare and interesting minerals like Beryl and Tourmaline 6 membered ring silicates with Be B Li Topaz a fluorine bearing mineral Al2SiO4 F OH 2 Lepidolite purple Li mica Spodumene Li pyroxenoid Tourmaline Na Ca Li Fe Mg Al 3Al6 BO3 3Si6O18 OH 4 color varies based on variation in chemical composition rounded triangular or hexagonal cross section striated sides Beryl Be3Al2Si6O18 View Beryl Structure Movie http socrates berkeley edu eps2 wisc geo360 beryl mov Beryl Be3Al2Si6O18 Topaz Al2SiO4 F OH 2 an isolated tetrahedral silicate Tourmaline Topaz Quartz Albite from Pegmatite