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Chapter 25

Chapter 25. Oxides and hydroxides Review of ionic crystals. Introduction. 200 minerals; 10% of all mineral species Oxides, A n (B p )O m : Quartz – 12 vol % of earths crust; discussed with the silicates

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Chapter 25

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  1. Chapter 25 Oxides and hydroxides Review of ionic crystals

  2. Introduction • 200 minerals; 10% of all mineral species • Oxides, An(Bp)Om: • Quartz – 12 vol% of earths crust; discussed with the silicates • Ice – Seasonal phase, major mineral of Earth and Mars polar caps; also large part of other planets • CO2 – present as a mineral on Mars, other planets and meteorites • Fe-oxides only 0.2 vol% of crust - Magnetite, Hematite major iron ores • More than 40 elements found in oxide form • Hydroxides, An(Bp)(OH)m: • gibbsite, goethite, diaspore • 25 elements occur in hydroxide form

  3. Introduction • Oxides and hydroxides occur in two types: • Simple • Single element as cation • Oxides: Periclase (MgO); Corundum (Al2O3) • Hydroxides: Gibbsite (Al(OH)3); Brucite (Mg(OH)2); Manganite (Mn2+Mn4+O2(OH)2) • Complex • Two or more main cations • Oxides: Spinel (MgAl2O4); Perovskite (CaTiO3) • Less important hydroxides: Romanechite (BaMn2+Mn94+O20.3H2O)

  4. Reviewing the ionic crystal structure • Structure names named after first mineral described for that structure i.e.: NaCl crystallizes in the ‘halite structure’ • Ionic bonds rules: • Close packing (or almost closed packing) • Anions forms regular coordination polyhedra • Cations generally smaller than anions • Four simplest ionic structures for A-X compounds: • Halite, Nickeline, Sphalerite, Wurtzite • CsCl structure for large cations • Common ionic structure for AnBmXp: spinel structure; perovskite structure; rutile structure; brucite and gibbsite structure

  5. Simplest ionic structures:A-X compounds • Table 25.3 • Minerals such as: • Corundum (Al2O3) • Hematite (Fe2O3) • Ilmenite (FeTiO3)

  6. Spinel structure • Table 25.4 • Minerals: Magnetite, Spinel, Chromite, etc. • Tetrahedral and octahedral polyhedra forms that are deformed due to non-ideal close packing of oxygen

  7. Perovskite structure • Table 25.5 • Minerals: Perovskite, Loparite, Silicate perovskite • Cubic close packing of oxygen; one oxygen missing in every second layer – filled by Ca2+ - coordinated by 12 oxygens • Can accommodate large cations such as REE in this large cavity • Transforms under high pressure – distorting structure if large cation is smaller than oxygen

  8. Rutile structure • Body centered tetragonal unit cell • Ribbons of edge sharing TiO68-octahedra that link at free corners • Cassiterite (SnO2) • Pyrolusite (MnO2) • Stishovite (SiO2)

  9. Brucite and gibbsite structure • Stacked layers of octahedral sheets • Brucite: all octahedra occupied • Trioctahedral • Gibbsite: one out of three vacant • Dioctahedral

  10. Important oxide minerals • Cuprite Cu2O • Corundum Al2O3 • Hematite Fe23+O3 • Ilmenite FeTiO3 • Periclase MgO • Magnetite Fe2+Fe23+O4 • Chromite FeCr2O4 • Ringwoodite Mg2SiO4 • Rutile TiO2 • Anatase TiO2 • Pyrolusite MnO2 • Uraninite UO2 • Perovskite CaTiO3

  11. Important hydroxide minerals • Brucite Mg(OH)2 • Gibbsite Al(OH)3 • Diaspore AlOOH • Boehmite AlOOH • Manganite MnOOH

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