[Chemical composition] The composition does not change much, and sometimes a small amount of Sr and Ba replaces Ca.
[Crystal structure] orthorhombic system; 
a 0 =0.699 nm, b 0 =0.700 nm, c 0 =0.624 nm; Z=4.
[Form] Crystals are rare. Usually, it is elongated along the a-axis or the c-axis, and is sometimes in the form of a column. According to (011) contact twin or polycrystalline twin. The aggregate is fibrous, dense granular or massive (Figure H-20).
Figure H-20 anhydrite crystal
[Physical properties] white, often slightly light blue, light gray or light red, dyed red, color or gray by iron oxide or clay ; streaks white or light grayish white; crystal colorless and transparent; glass luster, solution The surface is pearly. Cleavage {010} is complete, {100}, {001} is medium ({100} is better than {001}). Hardness 3 to 3.5. The relative density is 2.8 to 3.0.
[Cause] anhydrite and the occurrence of major chemical deposition product is formed in a large number of salt lakes, often coexist with gypsum. Anhydrite is unstable under surface conditions and converted to gypsum.
Occasionally appeared in the hydrothermal veins and volcanic lava holes. In some sulphide deposits, there may be a small amount of oxidized zone (such as Northwest China).
[Characteristics] Identification and aromatic mineral barite its relatively low density, the cleavage directions (perpendicular to each other cleavage Group 3) and the optical constants of the phase difference; difference between the particulate calcium carbonate is magnesium HCl dropwise non-foaming; and The difference between gypsum is that the hardness is large and the nails are not immovable.
[Main use] It is roughly the same as plaster.
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