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Oxides

Lecture 18 (11/29/2006) Systematic Description of Minerals Part 2: Oxides, Hydroxides Halides, Carbonates, Sulfates, and Phosphates. Oxides. Three main groups based on metal cation/oxygen ratios Hematite Group (X 2 O 3 ) Corundum X=Al +3 most phases hexagonal Hematite X=Fe +3

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Oxides

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  1. Lecture 18 (11/29/2006)Systematic Description of MineralsPart 2:Oxides, Hydroxides Halides, Carbonates, Sulfates, and Phosphates

  2. Oxides Three main groups based on metal cation/oxygen ratios Hematite Group (X2O3) Corundum X=Al+3 most phases hexagonal Hematite X=Fe+3 Ilmenite X=1Fe+2+1Ti+4 Rutile Group (XO2) Rutile X=Ti x- +4 cation Pyrolusite X=Mn most phases tetragonal Cassiterite X=Sn Uraninite X=U Spinel Group (XY2O4) Spinel X=Mg, Y=Al X- +2 ; Y- +3 cation Magnetite X=Fe+2, Y=Fe+3 most phases isometricChromite X=Fe+2, Y=Cr or orthorhombic Chrysoberyl X=Be, Y=Al Ulvospinel X=Ti+4, Y=Fe+2

  3. Fe-Ti oxides Fe+2 Fe+3

  4. Spinel Group (XY2O4) Gem-quality Spinels

  5. Other Common Oxides Corundum (Al2O3) Rutile (TiO2) Hardness=9 Rutile needles in Quartz Pyrolusite (MnO2) Blue = Sapphire Red = Ruby Chrysoberyl (BeAl2O4) Pyrolusite dendrites on fracture faces Mass of botryoidal Pyrolusite Cyclic twinning in Chrysoberyl

  6. Hydroxides (OH)- main anionic group forming octahedrally coordinated sheets with weak bonds between Two structural types: Brucite-type – trioctahedral sheets (all octahedral cation sites are filled) Gibbsite-type – dioctahedral sheets (only two of three octahedral sites are filled)

  7. Common Types of Hydroxides Brucite Mg(OH)2 Gibbsite Al(OH)3 Manganite MnO(OH) Diaspore AlO(OH) Goethite FeO(OH) Bauxite Al-hydroxide* *hybrid mix of diaspore, gibbsite, and boehmite (AlO(OH))

  8. Halides • Simple compounds composed of large halogen anions (Cl, Br, F, I) • Typically isometric • Dominantly ionic bonding • Properties – low hardness, high melting points, poor conductors (except at high temperatures)

  9. Common Halides Halite (NaCl) Sylvite (KCl) Flourite (CaF2)

  10. Mineral Groups formed with Anionic Complexes Carbonates Sulfates Phosphates Silicates Amount of residual charge indicates relative strength bonds with cations, which reflected in the hardness of the mineral

  11. Carbonates Aragonite (High-P) Orthorhombic Calcite (Low-P - Hexagonal) Most are Hexagonal

  12. Other Carbonates Azurite - Cu3(CO3)2(OH)2 (Blue) Malachite – Cu2CO3(OH)2 (Green) Rhodochrosite – MnCO3

  13. Borates Kernite – Na2B4O6(OH)2·3H2O Borax - Na2B4O5(OH)4·8H2O Ulexite – NaCaB5O6(OH)6·5H2O H – 3; SG – 1.95 H – 2-2.5; SG – 1.7 H – 1-2.5; SG – 1.96 “Television Rock”

  14. Sulfates HYDROUS ANHYDROUS H: 2 SG: 2.32 H: 3-3.5 SG: 2.9 Gypsum – CaSO4·2H2O Anhydrite – CaSO4 H: 4 SG: ~2.7 H: 3-3.5 SG: 4.5 Alunite – KAl3(SO4)2(OH)6 Barite – BaSO4

  15. Tungstates & Molybdates Woframite – (Fe,Mn)WO4 SG: 7-7.5 Scheelite – CaWO4 SG: ~6 Wulfenite – PbMoO4 SG: 6.8

  16. Phosphates • Apatite – Ca5(PO4)3(F,Cl,OH) • prismatic hexagonal crystals • common in igneous rocks and hydrothermal deposits - variable colors

  17. Other Common Phosphates Monazite – (Ce,La,Y,Th)PO4 Ore mineral for Rare Earth Elements Useful mineral in U-Pb and Th age dating Wavellite – Al3(PO4)2(OH)3·5H2O Radiating globular aggregates Turquoise – CuAl6(PO4)4(OH)8·5H2O

  18. Next Lecture 11/29/06 Systematic Description of Minerals Part 3: Silicates Read: Klein Chap. 11

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