Anhydrite
| Anhydrite | |
|---|---|
Anhydrite, Chihuahua, Mexico
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| General | |
| Category | Sulfate mineral |
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Formula (repeating unit) |
Anhydrous calcium sulfate:CaSO4 |
| Strunz classification | 7.AD.30 |
| Dana classification | 28.3.2.1 |
| Crystal system | Orthorhombic |
| Crystal class | Dipyramidal (mmm) H–M symbol: (2/m 2/m 2/m) |
| Space group | Amma |
| Unit cell |
a = 6.245(1) Å, b = 6.995(2) Å c = 6.993(2) Å; Z = 4 |
| Identification | |
| Color | Colorless to pale blue or violet if transparent; white, mauve, rose, pale brown or gray from included impurities |
| Crystal habit | Rare tabular and prismatic crystals. Usually occurs as fibrous, parallel veins that break off into cleavage fragments. Also occurs as grainy, massive, or nodular masses |
| Twinning | Simple or repeatedly on {011} common; contact twins rare on {120} |
| Cleavage | [010] perfect [100] perfect [001] good, resulting in pseudocubic fragments |
| Fracture | Conchoidal |
| Tenacity | Brittle |
| Mohs scale hardness | 3.5 |
| Luster | Pearly on {010} vitreous to greasy on {001} vitreous on {100} |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 2.97 |
| Optical properties | Biaxial (+) |
| Refractive index |
nα = 1.567–1.574 nβ = 1.574–1.579 nγ = 1.609–1.618 |
| Birefringence | δ = 0.042–0.044 |
| Pleochroism | For violet varieties X = colorless to pale yellow or rose Y = pale violet or rose Z = violet. |
| 2V angle | 56–84° |
| Fusibility | 2 |
| Other characteristics | Some specimens fluoresce; many more fluoresce after heating |
| References | |
Anhydrite is a mineral—anhydrous calcium sulfate, CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with the orthorhombic barium (baryte) and strontium (celestine) sulfates, as might be expected from the chemical formulas. Distinctly developed crystals are somewhat rare, the mineral usually presenting the form of cleavage masses. The Mohs hardness is 3.5 and the specific gravity is 2.9. The color is white, sometimes greyish, bluish, or purple. On the best developed of the three cleavages, the lustre is pearly; on other surfaces it is glassy. When exposed to water, anhydrite readily transforms to the more commonly occurring gypsum, (CaSO4·2H2O) by the absorption of water. This transformation is reversible, with gypsum or calcium sulfate hemihydrate forming anhydrite by heating to around 200 °C (400 °F) under normal atmospheric conditions. Anhydrite is commonly associated with calcite, halite, and sulfides such as galena, chalcopyrite, molybdenite, and pyrite in vein deposits.
Anhydrite is most frequently found in evaporite deposits with gypsum; it was, for instance, first discovered, in 1794, in a salt mine near Hall in Tirol. In this occurrence, depth is critical since nearer the surface anhydrite has been altered to gypsum by absorption of circulating ground water.
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