Chapter 25
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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

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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

  • ChromiteFeCr2O4

  • Ringwoodite Mg2SiO4

  • RutileTiO2

  • AnataseTiO2

  • PyrolusiteMnO2

  • UraniniteUO2

  • PerovskiteCaTiO3


Important hydroxide minerals

Important hydroxide minerals

  • BruciteMg(OH)2

  • GibbsiteAl(OH)3

  • DiasporeAlOOH

  • BoehmiteAlOOH

  • ManganiteMnOOH


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