The carbonate minerals do not form a homogeneous group while some are hexagonal in crystallisation, others are orthorhombic crystals and still some others are massive in nature.
1. Hexagonal carbonates:
The more important members are:
(i) Calcite. CaC03. (ii) Dolomite. CaMg(C03)2.
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(iii) Magnesite. MgC03. (iv) Siderite. FeC03.
(v) Rhodochrosite. MnC03. (vi) Smithsonite. ZnCOs.
(vii) Ankerite. (Ca, Mg, Fe)C03.
Physical properties:
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(i) All are rhombohedral crystals.
(ii) Colour. Calcite is colourless, dolomite is often tinged with yellow, Magnesite is white or colourless, Siderite is yellowish brown, Rhodochrosite is pink, white-green colour is found in Smithsonite and in case of Ankerite.
(iii) Lusture. Vitreous to silky in fibrous varieties.
(iv)Cleavage. Perfect, rhombohedral (3 sets).
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(v) Hardness. Calcite-3, in others it is 3.5 to 5.
(vi)Sp. gravity. Low.
Optical properties:
(i)Non-pleochroic colourless. (ii) Ref. index. Low.
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(iii) Bi-refringence. High, in calcite it is 0.18.
(iv) Twinning. Polysynthetic.
(v) Twinkling is usually observed.
(vi) Extinction. Depends on the orientation of the section.
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Varieties:
(i) Iceland spar. Transparent ealcite.
(ii) Satin spar. Fibrous calcite.
2. Orthorhombic carbonates:
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The common members are
(i) Aragonite. CaCO3 (iii) Cerrusite. PbCO3.
(ii) Witherite. BaCO2. (iv) Strontianite. SrCO3.
Physical properties:
(i) Colour:
White, yellowish and grey.
(ii) Lusture:
Vitreous but cerrusite shows admantine lusture.
(iii) Hardness:
5 in case of aragonite, 3 to 4 in case of cerrusite,witherite, and strontianite.
(iv) Sp. Gravity:
High in cerrusite (6), in others 3 to 4.
(v) Cleavage:
Distinct in some cases and poor in others.
Optical properties:
Nothing important.
Occurrence:
Carbonates are usually secondary minerals, commonly occur as gangue minerals in ores of hydrothermal origin. But in sedimentary rocks they are quite important.
Important characteristics:
All the carbonates react actively with acids.