Important Non-Silicates

Hematite-Corundum

• Ilmenite FeTiO3 is similar

Halite and Galena

Pyrite

Diamond

Diamond

Sphalerite

Chalcopyrite

Halite and Calcite

Calcite

Fluorite

• Uraninite UO2 is isostructural

Goethite

Goethite

Goethite

Lepidocrocite

Lepidocrocite

Bauxite and Limonite

Bauxite Minerals Limonite Minerals

Gibbsite Al(OH)3

Boehmite AlO(OH) Lepidocrocite FeO(OH)

Diaspore AlO(OH) Goethite FeO(OH)

Perovskite

Perovskite in the Mantle

• Perovskite is CaTiO3

• Enstatite (Mg Pyroxene) = MgSiO3

• Can collapse to perovskite structure at high pressure

Rutile

Rutile

Rutile

• Cassiterite SnO2 is isostructural

The Spinels

• AB2O4

• A is divalent, B is trivalent

• Oxygens in cubic close-packed structure

• In a close-packed structure, there is one octahedral site for every oxygen and two tetrahedral

• Ideally B occupies half of octahedral sites and A occupies 1/8 of tetrahedral sites

Normal and Inverse Spinels

• In normal spinels, B (trivalent) occupies octahedral sites and A (divalent) occupies tetrahedral sites

• In inverse spinels, A (divalent) occupies octahedral sites and B (trivalent) occupies tetrahedral sites

• In real spinels there is a lot of crossover

Important Spinels

• Spinel (normal) MgAl2O4

• Hercynite (normal) FeAl2O4

• Magnetite (inverse) Fe++Fe+++2O4

• Chromite (inverse) FeCr2O4

• Ulvospinel (inverse) Fe2TiO4

• Franklinite (inverse) ZnFe+++ 2O4

• Maghemite (inverse) Fe2O3

Spinel

Spinel

Spinel

Spinel

Spinel in the Mantle

• Spinel = AB2O4

• Forsterite (Mg olivine) = Mg2SiO4

• Can collapse to SiMg2O4 at high pressure

• Si in octahedral sites, not tetrahedral!