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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

Chapter 25

Oxides and hydroxides

Review of ionic crystals

introduction
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
introduction1
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)
reviewing the ionic crystal structure
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
simplest ionic structures a x compounds
Simplest ionic structures:A-X compounds
  • Table 25.3
  • Minerals such as:
    • Corundum (Al2O3)
    • Hematite (Fe2O3)
    • Ilmenite (FeTiO3)
spinel structure
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
perovskite structure
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
rutile structure
Rutile structure
  • Body centered tetragonal unit cell
  • Ribbons of edge sharing TiO68-octahedra that link at free corners
  • Cassiterite (SnO2)
  • Pyrolusite (MnO2)
  • Stishovite (SiO2)
brucite and gibbsite structure
Brucite and gibbsite structure
  • Stacked layers of octahedral sheets
  • Brucite: all octahedra occupied
    • Trioctahedral
  • Gibbsite: one out of three vacant
    • Dioctahedral
important oxide minerals
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
important hydroxide minerals
Important hydroxide minerals
  • Brucite Mg(OH)2
  • Gibbsite Al(OH)3
  • Diaspore AlOOH
  • Boehmite AlOOH
  • Manganite MnOOH
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