Dr Helen Lang Dept of Geology Geography West Virginia University FALL 2014 GEOLOGY 284 MINERALOGY Remember Bowen s Reaction Series hi T olivine Ca plagioclase orthopyroxene temperature clinopyroxene NaCa plagioclase amphibole Hb biotite Na plagioclase cooling alkali feldspar muscovite low T quartz residual phases Mafic Minerals in Igneous Rocks Olivine isolated tetrahedral sil Pyroxenes single chain silicates Enstatite orthopyroxene Augite clinopyroxene Amphiboles double chain silicates Hornblende Micas sheet silicates Biotite Muscovite OK not mafic We ll start with Olivine Olivine occurs in high temperature mafic hi Mg low Si igneous rocks Commonly the first mineral to crystallize from a basaltic magma Olivine also makes up most of the Earth s Mantle metamorphic rock We ll start with Olivine Formula Mg Fe 2SiO4 An Isolated Tetrahedral Silicate complete solid solution between two end members Mg2SiO4 Forsterite Fo and Fe2SiO4 Fayalite Fa Olivine Structure Relatively simple Isolated SiO44tetrahedra Divalent cations Mg2 Fe2 share tetrahedral oxygens balance charge and link tetrahedra Olivine Structure Movie http socrates berkeley edu eps2 wisc geo360 olivine mov Olivine Properties Vitreous luster glassy Olive green color No good cleavages H 6 5 G 3 2 Optical high relief moderately high birefringence Olivine Xenoliths from the Mantle Clear Gem quality Olivine Peridot Olivine in thin section Bowen s Reaction Series hi T olivine Ca plagioclase orthopyroxene temperature clinopyroxene NaCa plagioclase amphibole Hb biotite Na plagioclase cooling alkali feldspar muscovite low T quartz residual phases Pyroxenes Common in mafic and intermediate igneous rocks Commonly crystallize from basaltic magma after olivine Bowen s reaction series Pyroxenes are Single Chain Silicates Pyroxenes General Formula Ca2 Mg2 Fe2 2Si2O6 Two kinds Orthopyroxenes Mg Fe 2Si2O6 have perpendicular axes Enstatite Clinopyroxenes Ca Mg Fe Si2O6 have inclined axes Augite and Diopside Pyroxenes are Single Chain Silicates Bigger cations Ca2 if present in octahedral coordination Smaller cations Mg2 and Fe2 in octahedral coordination Strips or I beams in the pyroxene structure are tightly held together Cleavages at 90o go between I beams where bonds are weaker Movie Pyroxene I beam http socrates berkeley edu eps2 wisc geo360 diopibeam mov Orthopyroxene movie http socrates berkeley edu eps2 wisc geo360 opx mov note no net tilt Diopside movie http socrates berkeley edu eps2 wisc geo360 diopside mov note tilt HRTEM Image of Pyroxene Pyroxene Compositions and the Pyroxene Quadrilateral Diopside CaMgSi2O6 Enstatite Mg2Si2O6 clinopyroxenes Augite Orthopyroxene orthopyroxenes Hedenbergite CaFeSi2O6 Ferrosilite Fe2Si2O6 Pyroxene Properties General Stubby Prisms Vitreous luster translucent Two perfect prismatic cleavages at approximately 90o to eachother Enstatite Mg orthopyroxene Pyroxene Relief Color and Cleavage in PPL Orthopyroxene Properties Enstatite orthopyroxene Color gray bronze brown Colorless to tan in thin section High relief Low birefringence Parallel extinction Orthopyroxene has low birefringence Clinopyroxene has higher second order birefringence photo Smith College John Brady Clinopyroxene Properties Diopside Color light green Colorless in thin section Moderate birefringence inclined extinction Augite Dark green to black Tan light brown or green in thin section Moderate birefringence inclined extinction Augite Augite moderate birefringence twinning and cleavage Bowen s Reaction Series hi T olivine Ca plagioclase orthopyroxene temperature clinopyroxene NaCa plagioclase amphibole Hb biotite Na plagioclase cooling alkali feldspar muscovite low T quartz residual phases Hornblende Hb is the main amphibole in igneous rocks crystallizes after olivine and pyroxenes from mafic magmas and at lower temperature is most common in intermediate composition igneous rocks Amphiboles General Formula Na K 0 1 Ca Na Fe Mg 2 Mg Fe Al 5 Si Al 8O22 OH 2 large medium small tetrahedral cations Simple Amphibole Tremolite Ca2Mg5Si8O22 OH 2 Double Chain Silicates Inosilicates note Si8O22 12 in formula they re hydrous OH bearing Minerals Amphiboles are DoubleChain Silicates Medium cations Ca Small cations Fe Mg Large cations Na K or empty Amphibole Structure View Tremolite Movie http socrates berkeley edu eps2 wisc geo360 tremolite mov Igneous Amphiboles Igneous Amphiboles Big Hornblende Crystals called Phenocrysts Igneous Amphiboles Tightly bonded I beams neg charged chains bonded by strip of many positive cations Cleavages at 60o and 120o go between I beams where bonds are weaker Hornblende Properties Black or dark green Vitreous luster translucent H 5 6 G 3 0 3 5 two perfect prismatic cleavages at about 60o and 120o to eachother sometimes splintery cleavage commonly in prisms with flattened hexagonal cross sections Hornblende s Optical Properties Various shades of brown red brown green blue green or tan in thin section Moderate to strong pleochroism is typical Moderate birefringence commonly obscured by its color Two good cleavages at 60o and 120o to eachother You have to be looking down the prism to see 2 crossing sets of cleavages in many views you will see only one set of cleavages Hornblende shape cleavage birefringence IG7 Crater Lake cumulate Photo Billie Jo Fortney Hornblende color and cleavage Kaersutite Na Ti rich igneous amphibole with strong red brown pleochroism Digression to talk about Asbestos The term asbestiform describes a mineral habit characterized by long thin strong flexible fibers equivalent to hairs or whiskers Uses of asbestos Asbestos Minerals amphiboles sheet silicate Amphiboles are chain silicates and are inherently and predictably elongate or fibrous It s not surprising that most asbestos minerals are amphiboles Crocidolite blue Asbestos Other Fibrous Amphiboles Actinolite Chrysotile white asbestos Is a form of serpentine Mg3Si2O5 OH 4 a sheet silicate Sheet silicates are usually flaky formed of thin flexible sheets or leaflike How can a sheet silicate be fibrous Serpentine is a tetrahedraloctahedral t o Sheet Silicate Mg3Si2O5 OH 4 Si2O5 tetrahedral t sheet Mg surr by O and OH forms octahedral o sheet The trouble with serpentine the o sheet is bigger than the t sheet In normal serpentine Antigorite the sheets are wavy or corrugated In asbestos serpentine Chrysotile the sheets roll to accommodate the misfit HRTEM pictures down fibers Chrysotile white Asbestos Most of the asbestos used worldwide is Chrysotile 95 in