anhydrite

{{Infobox mineral | name = Anhydrite | category = Sulfate mineral | boxwidth = | boxbgcolor =#b7dfda | image = Anhydrite_HMNH1.jpg | imagesize = 260px | alt = | caption = Anhydrite, from Chihuahua, Mexico | formula = CaSO4 |IMAsymbol=Anh | strunz = 7.AD.30 | dana = 28.3.2.1 | system = Orthorhombic | class = Dipyramidal (mmm) H–M symbol: ( ) | symmetry = Amma | unit cell = a = 6.245(1) Å, b = 6.995(2) Åc = 6.993(2) Å; Z = 4 | color = Colorless to pale blue or violet if transparent; white, mauve, rose, pale brown or gray from included impurities | 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 = 3.5 | luster = Pearly on {010} vitreous to greasy on {001} vitreous on {100} | refractive = nα = 1.567–1.574 nβ = 1.574–1.579 nγ = 1.609–1.618 | opticalprop = Biaxial (+) | 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 = 56–84° | streak = White | gravity = 2.97 | melt = | fusibility = 2 | diagnostic = | solubility = | diaphaneity = Transparent to translucent | other = Some specimens fluoresce; many more fluoresce after heating | references = }} Anhydrite, or anhydrous calcium sulfate, is a mineral with the chemical formula 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 under normal atmospheric conditions. Anhydrite is commonly associated with calcite, halite, and sulfides such as galena, chalcopyrite, molybdenite, and pyrite in vein deposits.

==Occurrence== left|thumb|Crystal structure of anhydrite 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.