Geology 284 Mineralogy Fall 2007 Dr Helen Lang West Virginia University Mafic Minerals in Igneous Rocks Mafic Minerals in Igneous Rocks We ll follow Bowen s Reaction Series Olivine isolated tetrahedral silicate Pyroxenes single chain silicates o Enstatite orthopyroxene o Augite clinopyroxene Amphiboles double chain silicates o Hornblende Micas sheet silicates o Biotite o Muscovite OK it s not mafic but is much like biotite 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 much of the Earth s Mantle metamorphic rock Formula Mg Fe 2SiO4 An Isolated Tetrahedral Silicate complete solid solution with two end members Mg2SiO4 Forsterite Fo and Fe2SiO4 Fayalite Fa Olivine Structure see Fig 5 25 Olivine Structure Movie Olivine Properties Vitreous luster Olive green color No good cleavages H 6 5 G 3 2 Optical high relief high birefringence Olivine Xenoliths from the Mantle Clear Gem quality Olivine Peridot Olivine in thin section Pyroxenes Common in mafic and intermediate igneous rocks Commonly crystallize from basaltic magma after olivine Bowen s reaction series Pyroxenes are Single Chain Silicates General Formula Ca2 Mg2 Fe2 2Si2O6 Two kinds Orthopyroxenes Mg Fe 2Si2O6 have perpendicular axes o Enstatite Clinopyroxenes Ca Mg Fe Si2O6 have inclined axes o Augite and Diopside Pyroxenes are Single Chain Silicates See Handout Movie Pyroxene I beam Orthopyroxene movie Clinopyroxene movie HRTEM Image of Pyroxene Pyroxene Compositions and the Pyroxene Quadrilateral Diopside CaMgSi2O6 Hedenbergite CaFeSi2O6 clinopyroxenes Enstatite Mg2Si2O6 Ferrosilite Fe2Si2O6 orthopyroxenes Pyroxene Properties General Stubby Prisms Vitreous luster translucent Two perfect prismatic cleavages at approximately 90o to eachother Pyroxene Cleavage Relief and Color Orthopyroxene Properties Enstatite orthopyroxene Color gray bronze brown Colorless in thin section High relief Low birefringence Parallel extinction Enstatite 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 Hornblende Hb is the main amphibole in igneous rocks crystallizes after olivine and pyroxenes from mafic magmas or at lower temperature is most common in intermediate composition igneous rocks Amphiboles General Formula o Na K 0 1 Ca Na Fe Mg 2 Mg Fe Al 5 Si Al 8O22 OH 2 o large small tetrahedral cations Simple Amphibole Tremolite o medium Ca2Mg5Si8O22 OH 2 Double Chain Silicates Inosilicates Hydrous OH bearing Minerals Amphiboles are Double Chain Silicates see handout Amphibole Structure see handout View Tremolite Movie Igneous Amphiboles Digression to talk about Asbestos The term asbestiform describes a mineral habit characterized by long thin strong flexible fibers equivalent to hairs or whiskers Asbestos Minerals see handout Amphiboles are chain silicates and are inherently and predictably elongate or fibrous Acicular to Fibrous Amphiboles 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 leaf like How can a sheet silicate be fibrous Serpentine is a tetrahedral octahedral t o Sheet Silicate In normal serpentine Antigorite the sheets are wavy or corrugated see handout In asbestos serpentine Chrysotile the sheets roll to accommodate the misfit see handout Serpentine Asbestos Chrysotile Most of the asbestos used world wide is Chrysotile 95 in U S Chrysotile is much less dangerous than amphibole asbestos in fact there is little evidence that chrysotile poses any health hazard to those who are exposed casually e g in buildings Back to Igneous Amphiboles Hornblende Properties Black or dark green Vitreous luster translucent H 5 6 two perfect prismatic cleavages at about 60o and 120o to eachother sometimes G 3 0 3 5 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 typical Moderate birefringence commonly obscured by its color Two good cleavages at 60o and 120o to eachother You have to be looking down the prisms to see 2 crossing sets of cleavages in many sections you will see only one set of parallel cleavages Hornblende cleavage color pleochroism Kaersutite Na Ti rich igneous amphibole has very strong red brown pleochroism