ANHYDRITE
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.