Anhydrite, unlike gypsum, is an anhydrous calcium sulfate (CaSO4) - it's gypsum without the water.  Compared with gypsum, anhydrite is harder and less common.  The specimen below is a partial spray of pale bluish, tabular anhydrite crystals. 


Water added to powdered anhydrite will result in the formation of gypsum crystals.  Gypsum that has its water driven off will convert to anhydrite.  Gypsum-anhydrite and anhydrite-gypsum conversions do happen naturally.


This mineral occurs in some evaporite successions with rock salt (composed of halite) and rock gypsum (composed of gypsum).  It is also associated with the cap areas of some salt domes (salt diapirs).  Evaporation of seawater will result in deposition of beds of rock salt and rock gypsum.  With burial & compaction, the gypsum tends to convert to anhydrite.  With subsequent uplift & erosion & near-surface exposure, the anhydrite reconverts to gypsum, which then can be dissolved away by water.  These mineral conversions involve a volume change.  Evaporite beds having undergone such conversions will break up surrounding layers.


Anhydrite (9.8 cm across) from Naica, Chihuahua State, Mexico.



Photo gallery of anhydrite



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