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Anhydrite
Chemical Formula	CaSO4
Species	Sulfates
Crystal System	Orthorhombic
Mohs Scale	3
Specific Gravity	2.97
Color	Colorless to pale blue or Purple if transparent; white, mauve, rose, pale brown or gray from included impurities
Streak	White
Luster	Vitreous, Greasy, Pearly
Refractive Index	n = 1.567 - 1.574 n = 1.574 - 1.579 n = 1.609 - 1.618
Diaphaneity	Transparent, Translucent
Cleavage	PerfectOn perfect; on nearly perfect; on good to imperfect.
Fracture	Irregular/Uneven, Splintery
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
Geological Setting：Sedimentary evaporite deposits, cap rock of salt domes.

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 hardness is 3.5 and the specific gravity 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 vitreous. 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 ~200°C under normal atmospheric conditions. Anhydrite is commonly associated with calcite, halite, and sulfides such as galena, chalcopyrite, molybdenite and pyrite in vein deposits.

Occurrence

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.

From an aqueous solution calcium sulfate is deposited as crystals of gypsum, but when the solution contains an excess of sodium or potassium chloride anhydrite is deposited if temperature is above 40 °C. This is one of the several methods by which the mineral has been prepared artificially, and is identical with its mode of origin in nature, the mineral is common in salt basins.

Tidal flat nodules

Anhydrite occurs in a tidal flat environment in the Persian Gulf sabkhas as massive diagenetic replacement nodules. Cross sections of these nodular masses have a netted appearance and have been referred to as chicken wire anhydrite. Nodular anhydrite occurs as replacement of gypsum in a variety of sedimentary depositional environments.

Salt dome cap rocks

Massive amounts of anhydrite occur when salt domes form a caprock. Anhydrite is 1–3% of the salt in salt domes and is generally left as a cap at the top of the salt when the halite is removed by pore waters. The typical cap rock is a salt, topped by a layer of anhydrite, topped by patches of gypsum, topped by a layer of calcite. Interaction with oil can reduce SO₄ creating calcite, water, and hydrogen sulfide (H₂S).

Naming history

The name anhydrite was given by A. G. Werner in 1804, because of the absence of water of crystallization, as contrasted with the presence of water in gypsum. Some obsolete names for the species are muriacite and karstenite; the former, an earlier name, being given under the impression that the substance was a chloride (muriate). A peculiar variety occurring as contorted concretionary masses is known as tripe-stone, and a scaly granular variety, from Volpino, near Bergamo, in Lombardy, as vulpinite; the latter is cut and polished for ornamental purposes.