Carbonates are good examples of minerals with anisodesmic bonds Ca[CO32-] (example of CO3 radical in carbonate mineral) CO32- (triangular coordination polyhedron) ev = 4/3 which is greater than ½ the valence of the anion

• C has a +4 charge; O has a -2 charge; Ca has a +2 charge
  
  
• in the CO₃ radical, 1⅓ of a negative charge is contributed by each of the 3 oxygens (net -4 charge) to balance the +4 charge of the carbon
• the residual -⅔ charge on each of the 3 oxygens (net charge -2) balances with the +2 charge of the Ca
• the greater charge allocated to the carbon makes the bond of oxygen to carbon stronger than bond of oxygen to Ca²⁺

• Rhombohedral Carbonates
  • Calcite Group:
    • Calcite - CaCO₃
    • Magnesite - MgCO₃
    • Rhodochrosite - MnCO₃
    • Siderite - FeCO₃
    • Smithsonite - ZnCO₃ (not common)
  • Dolomite Group
    • Dolomite - CaMg(CO₃)₂
    • Ankerite - Ca(Mg,Fe)(CO₃)₂
    • Kutnohorite - CaMn(CO₃)₂ (not common)
• Aragonite Group (Orthorhombic Carbonates)
  • Aragonite - CaCO₃
  • Witherite - BaCO₃ (not common)
  • Strontianite - SrCO₃ (not common)
  • Cerrusite - PbCO₃ (not common)
• Hydrated Carbonate Group (Monoclinic Carbonates)
  • Azurite - Cu₃(CO₃)₂(OH)₂
  • Malachite - Cu₂(CO₃)(OH)₂

• Complete solid solution exists between Siderite (Fe²⁺ = 0.92Å) & Magnesite (Mg²⁺ = 0.86Å)
• Complete solid solution exists between Siderite (Fe²⁺ = 0.92Å) & Rhodochrosite (Mn²⁺ = 0.96Å)
• Limited solid solution exists between Magnesite (Mg²⁺ = 0.86Å) & Rhodochrosite (Mn²⁺ = 0.96Å)
  
  
• Solid solution between Calcite (Ca²⁺ = 1.14Å) & other members is very limited because Ca is substantially bigger than the other cations.

• As in the Calcite Group, there is a complete solid solution between Dolomite (CaMg(CO₃)₂) and Ankerite (CaFe(CO₃)₂).

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