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Comprehensive Overview of Oxides and Hydroxides in Ionic Crystals

This chapter delves into the significance of oxides and hydroxides, which comprise roughly 10% of all mineral species, highlighting their role in the Earth's crust and beyond. Key minerals discussed include quartz, ice, and various metal oxides like magnetite and hematite. The structure and types of ionic crystals are examined, with detailed insights on simple and complex oxides and hydroxides. The text also covers important minerals like corundum, gibbsite, and perovskite, emphasizing their crystal structures and classification, as well as their occurrences in planetary geology.

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Comprehensive Overview of Oxides and Hydroxides in Ionic Crystals

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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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