2.7 Nesosilicates - Orthosilicates Isolated tetrahedron...

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Unformatted text preview: Orthosilicates Isolated tetrahedron Common examples Olivine, garnet, and zircon Al2SiO5 polymorphs, staurolite, topaz, titanite Oxygen coordinate with other anions Olivine Group Structure Octahedron occupied with Fe2+, Ca2+, Mg2+ Two distinct sites: M1 and M2 M1 are stretched out along c axis edge sharing M2 less distorted Distortion decreases symmetry orthorhombic Crystals can be elongate along c axis Fe and Mg octahedral sites Fig. 161 Composition Complete solid solution between forsterite (Mg) and fayalite (Fe) Fe and Mg contents cause variations in physical properties Can be used to identify composition Zoning can be common Mn end members as well rare Ca can be around 50% of cations 2Vx Index of Refraction Specific gravity Birefringence d spacing (130) Fig. 16.2 Structure and composition M1 = distorted, so smaller than M2 M2 = regular octahedron Controls distribution of cations M2 only site for Ca, 1.12 M1 and M2 both Fe = 0.78 and Mg = 0.72 Distorted, small site, Ca will not fit If sufficient Ca present when olivine forms, all M2 sites filled with Ca. Ca = 50 mole % Fe + Mg = 50 mole % Ca Mg Fe Olivine solid solution at high T (Plagioclase 1553 to 1118 C) Fig. 163 Occurrence Common mafic and ultramafic mineral Mantle Metamorphosed carbonate rocks Factoids Olivine unstable at high pressure of mantle Convert to more dense phase (~6% more dense) Cubic closest packing or oxygen Very similar to spinel structure (metal oxide) Change in structure causes: Seismic velocity increases at 400 km (greater density) Deep earthquakes in subduction zones, >70 km depth too hot for earthquakes Garnet Group Structure Single Si tetrahedron bonded by octahedron and distorted 8fold sites Octahedron: Ysites = Al, Fe3+, (Cr) 8 fold: Xsites = Mg, Fe2+, Mn, Ca Composition General formula = X3Y2(SiO4)3 Typically Al fixed in Y site, elements vary in Xsite Ca fixed in X site, elements vary in y site X distorted cube, 8 fold, Larger cations Fe2+, Mg2+, Ca2+ Y octahedral, smaller cations Al3+, Fe3+, (Cr) Fig. 165 Two major groups Pyralspite: Al in Y structural site Ugrandite: Ca2+ in X structural site Xsite = Pyrope (Mg), Almandine (Fe), Spessertine (Mn) Grossular (Al), Andradite (Fe), Uvarovite (Cr, rare) Hydrogrossular missing Si, charge balanced from bonding H+ to each oxygen Ca3Al2(SiO4)3x(OH)4x There is limited solid solution between end members "Pyralspite" Garnets Al in Y site Almandine Fe Pyrope Mg "Pyralspite" (Mg,Fe,Mn) Al Spessartine Mn Grossular CaAl Andradite CaFe Pyralspite Variations in n, d spacing, specific gravity for pyrope, spessartine, and almandine Index of refraction grossular and andradite Occurrence Wide variety of occurrences best indicator of type of garnet Pyrope (Mg, Al) ultramafic igneous rocks Almandine (Fe, Al) mica schists and gneiss Spessartine (Mn, Al) felsic igneous rocks like pegmatites, granite, and rhyolite Grossular (Ca, Al) and Andradite (Ca, Fe) metamorphosed carbonate rocks Factoids Characteristic mineral of metamorphic rocks May be chemically zoned depending on growth characteristics Determine timetemperaturepressure history of growth Zircon Zr in 8fold coordination with silica tetrahedron Widely distributed in igneous, sedimentary and metamorphic rocks Very resistant to weathering (sedimentary) Importance: Source of ore for Zr, used in reactors, refractory bricks etc. Contains U, but little "common" Pb Extremely valuable mineral for dating rocks Used to find "oldest" rocks, e.g. 3.9 by Aluminum Silicates Common in pelitic metamorphic rocks Formula: Al2SiO5 or AlAlOSiO4 The second formula shows two distinct Al sites Also that they are orthosilicates Structure reconstructive polymorphs Edge sharing chains of aluminum octahedra Linked by Si and Al polyhedrons Al coordination varies between polymorphs Octahedral chains 5 fold 4 fold 6 fold Fig. 169 Andalusite Sillimanite "extra" Al in 5fold coordination Open structure with low specific gravity VI AlVAlOIVSiO4 "extra" Al in tetrahedral coordination VI AlIVAlOIVSiO4 "extra" Al in octahedral coordination VI AlVIAlOIVSiO4 Most dense of all polymorphs Kyanite Al in 6 fold coordination High P = high density Al in 4 fold coordination High T = low density Al in 5 fold coordination Intermediate density Fig. 1610 Metamorphic Grades Metamorphic Facies Fig. 119, 1110 Occurrence Common in pelitic metamorphic rocks Extensive use for determining PT conditions Boundaries of phase diagrams not clear: Less common in igneous rocks Compositional problems, e.g. Fe or Mn in structures Unclear if primary or secondary minerals Other common orthosilicates Titanite (sphene) CaTiOSiO4 Topaz Al2(F,OH)2SiO4 ...
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This note was uploaded on 07/06/2011 for the course GLY 5245 taught by Professor Staff during the Spring '11 term at University of Florida.

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