Anhydrite

n. (Mineralogy) an anhydrous calcium sulfate

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 (barite) 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 colour 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. Anhydrite is commonly associated with calcite, halite, and sulfides such as galena, chalcopyrite, molybdenite, and pyrite in vein deposits.
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adj. anhydrous, lacking water

n. anhydrite, anhydride, substance formed by dehydration (Chemistry)

(n.)
A mineral of a white or a slightly bluish color, usually massive. It is anhydrous sulphate of lime, and differs from gypsum in not containing water (whence the name).
  

General Information:
 Chemical Formula:
CaSO4
 Composition:
(Molecular Weight = 136.14 gm)
Calcium 29.44 % Ca
Sulfur 23.55 % S 
Oxygen 47.01 % O 
 Empirical Formula:
Ca(SO4)
 Environment:
Sedimentary beds, gangue in ore veins, and in traprock zeolite occurrences.
 Locality:
World wide occurrences where marine evaporite beds occur. Often interbedded with halite.
 Name Origin:
From the Greek anhydros, meaning "waterless" (In contrast to Gypsum, which contains water).
Physical Properties:
 Cleavage:
[010] Perfect, [100] Perfect, [001] Good
 Color:
colorless, white, bluish white, violet white, or dark gray.
 Density:
2.96 - 2.98, Average = 2.97
 Diaphaniety:
Transparent to Subtransparent to translucent
 Habits:
Massive - Granular - Common texture observed in granite and other igneous rock., Massive - Fibrous - Distinctly fibrous fine-grained forms., Plumose - "Mica-like" minerals forming agg regates of plume-like forms.
 Hardness:
3.5 - Copper Penny
 Luminescence:
None.
 Luster:
Vitreous - Pearly
 Streak:
white
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