The carbonate mineral group form in the environment where water and carbon-di-oxide are present. The (CO3)– complex are basic building units of all carbonate minerals and are responsible for the properties to group. An acid test is widely used for identification of carbonates.
- Carbonates can be divided into two groups, Anhydrous and Hydrous carbonates.
- Anydrous carbonates have three groups, Calcite (Hexagonal), Aragonite (orthorhombic) and Dolomite (Hexagonal)
- Hydrous carbonates include minerals like, Malachite and Azurite (carbonate ore of Cu)
| Calcite Group (Hexagonal) | Dolomite Group (Hexagonal) | Aragonite Group (Orthorhombic) |
| Calcite [CaCO3] Magnesite [MgCO3] Siderite [FeCO3] Rhodochrosite [MnCO3] Smithsonite [ZnCO3] | Dolomite [CaMg(CO3)2] Ankerite [Ca(Mg, Fe)(CO3)2] Kuthnohorite [CaMn(CO3)2] | Aragonite [CaCO3] Witherite [BaCO3] Strontianite [SrCO3] |
Calcite Group
The large clear crystals of calcite are known as Island Spar which comes from island. Calcite structure can be a derivative of NaCl structure. Triangular CO3 groups replace the spherical Cl and Ca is in place of Na. The triangular (CO3) groups causes the resulting structure to be rhombohedral instead of isometric as in NaCl.
Dolomite Group
The composition of dolomite is intermediate between CaCO3 and MgCO3 with Ca : mg= 1 : 1, where Ca and Mg are alternating along C-axis.
- It is a member of trigonal system.
- It forms rhombohedra with markedly curved faces. There is a perfect rhombohedral cleavage and harness is 3.5 to 4.
- Dolomite can be produced directly or through diagenitic replacement of preexisting Carbonate deposits. A processes is known as dolomitization.
- Ca + Mg + 2CO3 = CaMg(CO3)2
- 2 CaCO3 + Mg = CaMg(CO3)2 + Ca
Calcite – Dolomite as Thermometer
At elevated temperatures, calcite coexisting with dolomite becomes more magnesian. At temperatures above about 1000 to 1100 °C a complete solid solution exists between calcite and dolomite only.
The compositions of coexisting dolomite and calcite have been used for an estimation of temperatures of crystallization of rocks containing both carbonates.
Aragonite Group
When the CO3 group is combined with large divalent cations with ionic radii greater than 1.0 Å. The radius ratios generally do not permit stable 6-coordination and Orthorhombic structure result.
- Carbonates with larger cations such as BaCO3 , SrCO3 and PbCO3 have aragonite structure.
- Each Ca is surrounded by 9 closed O2.
- It has a denser structure than calcite.
- It is less stable than calcite under atmospheric condition and much less common.
Hydrous Carbonates
There are two important hydrous carbonate minerals, Malachite Cu2CO3 (OH)2 & Azurite Cu3(CO3)2(OH)2. They are the ore of Cu and generally occurs as secondary minerals in the upper oxidized zone of copper deposits.
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