Dr Helen Lang Dept of Geology Geography West Virginia University FALL 2013 GEOLOGY 284 MINERALOGY Metamorphic Rocks and Minerals Metamorphism means Change Rocks Change as Conditions Change When changes in conditions are not too dramatic we call changes in sedimentary rocks diagenesis up to about 200oC When new minerals which are never observed to form at Earth s surface form we call the changes metamorphism Metamorphism is Changes in a rock s mineralogy texture or composition without melting in the solid state Any kind of rock can become a Metamorphic Rock Parent rock of a metamorphic rock is called its protolith Meta igneous rocks Originally igneous have been changed Meta sedimentary rocks Originally sedimentary have been changed Poly metamorphic rocks meta metamorphic rocks Already metamorphic changed again What causes Metamorphism Increase in temperature T T increases with depth in the Earth Increase in pressure P P increases with depth in the Earth from the load of overlying rock Deformation changes rock texture Movement of fluids like water vapor Metamorphic Minerals The two most important factors controlling mineralogy of metamorphic rocks are 1 2 the composition of the rock the available ingredients the grade intensity of metamorphism We account for rock composition by grouping rocks into general compositional classes Pelitic rocks Psammites Mafic rocks Carbonates Ultramafic Granitic clay rich sediments shales sandstones basalts and gabbros limestones mantle rocks granites We ll consider important minerals in each of these rock types separately Psammites meta sandstones start with Quartz and Feldspar metamorphic rock has Quartz and Feldspar coarser grain size rock may be foliated layering caused by deformation called Quartzite or Quartzofeldspathic Gneiss Not much change with metamorphism Metamorphosed Granites Start with Quartz Feldspar and Micas Metamorphic rock has Quartz Feldspar and Micas Rock may be foliated gneissic layering Metamorphic rocks called granitic gneiss Again not much change with metamorphism Metamorphosed Limestone If you start with just Calcite and or Dolomite not much change Grain size increases Limestone Marble If protolith limestone has quartz silt or clay minerals many more minerals are possible Dirty limestones display interesting changes with metamorphic grade Metamorphosed Shales Pelitic Rocks Sedimentary protolith has Quartz silt and Clay minerals Clays contain K Fe Mg Ca and especially Al in addition to SiO2 and OH from water These elements can make many new and interesting minerals Their mineralogy also changes dramatically with metamorphic grade or intensity We ll spend some time on these Some Minerals we already talked about occur in Pelitic Metamorphic Rocks What silicate subgroup Quartz Plagioclase Muscovite Biotite What are the new and interesting minerals Garnet Ca Mg Fe Mn 3Al2Si3O12 Staurolite Fe2Al9Si4O23 OH Andalusite Al2SiO5 Al2OSiO4 Sillimanite Al2SiO5 Al2OSiO4 Kyanite Al2SiO5 Al2OSiO4 Notice the essential Al which comes from clay minerals like kaolinite Al4 Si4O10 OH8 All of these minerals are isolated tetrahedral silicates All are porphyroblastic big visible crystals except sillimanite Garnet Structure Garnet Ca Mg Fe Mn 3Al2Si3O12 isolated tetrahedral silicate SiO4 4 tetrahedra AlO6 9 octahedra Ca2 Mg2 Fe2 or Mn2 Garnet Properties Commonly dodecahedral almost always equant H 7 G 3 54 Resinous luster Color varies with composition Fe Mg garnets pink to red Ca rich garnets are colorless orange yellow brown or green dodecahedron Optical hi relief isotropic More Garnet Examples Garnet trapezohedra another equant many sided form Gore Mountain Garnet Mine NY garnet mined for sandpaper Garnet Porphyroblasts in thin section Euhedral garnet porphyroblasts in PPL Staurolite Fe2Al9Si4O23 OH Isolated tetrahedral silicate Stubby Prisms commonly euhedral Simple penetration crosstwins are common Resinous luster Brown color H 7 7 5 G 3 75 Twinned Prismatic Staurolite Optical Properties of Staurolite High relief Low birefringence maximum 1st order yellow Clear yellow or light brown color Pleochroic Commonly exhibits a sieve like texture due to abundant quartz inclusions like Swiss cheese Staurolite in a Thin Section The Aluminosilicate Polymorphs Al2SiO5 Andalusite Sillimanite Kyanite All are isolated tetrahedral silicates Al2OSiO4 These minerals are very important for telling pressure during metamorphism View Andalusite Movie http socrates berkeley edu eps2 wisc geo360 andalusite mov Pressure The Al2SiO5 Phase Diagram The triple point 500oC 4 kilobars 4000 atm 12 km depth Kyanite SG 3 60 Sillimanite SG 3 23 Andalusite SG 3 18 Temperature Kyanite Properties Blue bladed crystals can be clear or white H 5 along blades H 7 across blades it s anisotropic SG 3 60 One perfect cleavage Luster vitreous pearly Kyanite Examples Kyanite Optical Properties Typically colorless in thin section may be blue and pleochroic High relief low birefringence but higher birefringence than andalusite Monoclinic inclined extinction Bladed shape with one perfect and one good cleavage Kyanite in PPL and XPL Virtual Geology Project Glazner Ratajeski UNC Pressure The Al2SiO5 Phase Diagram The triple point 500oC 4 kilobars 4000 atm 12 km depth Kyanite SG 3 60 Sillimanite SG 3 23 Andalusite SG 3 18 Temperature Andalusite Low pressure form of Al2SiO5 Luster vitreous Color pink or white H 7 5 G 3 18 Stubby square prisms May show a crossed pattern of graphite inclusions called Chiastolite Optically lower relief and lower birefringence than kyanite Andalusite Square prisms in hand specimen Andalusite Square prisms in hand specimen Chiastolite cross graphite inclusions get concentrated along diagonals Sillimanite Properties high T polymorph Acicular to thin prismatic Clear colorless and vitreous May form mats of fine fibers called fibrolite Optically sillimanite has high relief moderate birefringence and parallel extinction Clear needles thin prisms or fibers Sillimanite in Outcrop Rarely visible on outcrop scale Sillimanite in thin section PPL XPL Pressure The Al2SiO5 Phase Diagram The triple point 500oC 4 kilobars 4000 atm 12 km depth Kyanite SG 3 60 Sillimanite SG 3 23 Andalusite SG 3 18 Temperature