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Carbonates and other 'ates'

These minerals all contain oxyanions:

  • (CO3)2- carbonate
  • (NO3)2- nitrate
  • (BO3(OH)4)2- etc. borates
  • (SO4)2- sulphate
  • (AsO4)3- arsenate
  • (PO4)3- phosphate
  • (VO4)3- vanadate
  • (WO4)2- tungstate
  • (MO4)2- molybdate
  • (CrO4)2- chromate
  • (UO2)2+ uranyl

Of these, the carbonates, sulphates and phosphates are the most important. The metal oxygen bond in the oxyanion is very covalent. The bond between cations and the oxyanions, however, are ionic.

Oxidized environments in the Earths Crust...

  1. Precipitation from seawater:
    • Calcite, CaCO3
    • Aragonite, CaCO3
    • Dolomite, CaMg(CO3)2
    • Celestite, SrSO4
  2. Evaporites:
    • Gypsum, CaSO4.2H2O
    • Anhydrite, CaSO4
    • Epsomite, MgSO4.7H2O
    • Soda-Niter, NaNO3
    • Borates
  3. Hydrothermal deposition at high temperature
    • Scheelite, CaWO4
    • Wolframite, (Fe,Mn)WO4
  4. Oxidized zone of sulfide ore deposits
    • Barite, BaSO4
    • Jarosite, KFe3(SO4)2(OH)6
    • Siderite, FeCO3
    • Azurite, Cu3(CO3)2(OH)2
    • Malachite, Cu2(CO3)(OH)2
    • Wulfenite, PbMoO4
    • Vivianite Fe3(PO4)2.8H2O
    • All arsenates
    • Anglesite PbSO4

  5. Pegmatites
    • Monazite, (Ce,La, Y, Th) PO4
    • Xenotime, YPO4
    • Apatite, Ca5(PO4)3(F,Cl,OH)
  6. Sandstone-type Uranium deposits (oxidation of UO2)
    • Carnotite, K2(UO2)2(VO4)2.3H2O

The Carbonates

The Calcite Structure

Structure of calcite: note the CaO6 polyhedra and the planer CO3 groups.

The system CaCO3-FeCO3-MgCO3 shows a strong parallel with the pyroxene and amphibole quadrilaterals.

The aragonite structure

Calcite undergoes a polymorphic phase transition to aragonite (orthorhombic): In the aragonite structure, the coordination number of Ca has increased to 9.

Structure of aragonite: note the CaO10 polyhedra and the planer CO3 groups.

Sulphate Minerals: Gypsum and Anhydrite

The gypsum structure contains H2O molecules which hold the CaSO4 sheets together.

Structure of gypsum: note the CaO7 polyhedra (blue) the tetrahedral SO4 groups (yellow) and the water molecules.

At elevated temperature, gypsum dehydrates to anhydrite:

Structure of anhydrite: note the CaO7 polyhedra (blue) the tetrahedral SO4 groups (yellow).

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