Here is your essay on Carbonate minerals

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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:

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(i) Calcite. CaC03. (ii) Dolomite. CaMg(C03)2.

(iii) Magnesite. MgC03. (iv) Siderite. FeC03.

(v) Rhodochrosite. MnC03. (vi) Smithsonite. ZnCOs.

(vii) Ankerite. (Ca, Mg, Fe)C03.

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Physical properties:

(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.

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(iv)Cleavage. Perfect, rhombohedral (3 sets).

(v) Hardness. Calcite-3, in others it is 3.5 to 5.

(vi)Sp. gravity. Low.

Optical properties:

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(i)Non-pleochroic colourless. (ii) Ref. index. Low.

(iii) Bi-refringence. High, in calcite it is 0.18.

(iv) Twinning. Polysynthetic.

(v) Twinkling is usually observed.

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(vi) Extinction. Depends on the orientation of the section.

Varieties:

(i) Iceland spar. Transparent ealcite.

(ii) Satin spar. Fibrous calcite.

2. Orthorhombic carbonates:

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.

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