ROCK GYPSUM
Rock gypsum (a.k.a. gyprock) is a chemical sedimentary rock. It is an example of an evaporite - it forms
by the evaporation of water (usually seawater) and the precipitation of
dissolved minerals. Rock salt & rock
gypsum often occur together in evaporitic successions. Rock gypsum is
composed of the mineral gypsum (CaSO4·2H2O -
hydrous calcium sulfate). Heating of gypsum or rock gypsum drives off the
water, leaving only calcium sulfate behind (the mineral anhydrite).
Adding water to anhydrite results in the formation of gypsum again.
Rock gypsum, unlike rock salt, does not have a salty
taste, and is softer (H = 2) - it can be scratched with a fingernail.
Rock gypsum’s color is often a mottled whitish-light grayish-light brownish.
It is usually microcrystalline and powdery looking (it's much finer-grained
than typical rock salt deposits). Rock gypsum superficially resembles
chalk. Chalk is calcitic, and so will
bubble in acid - rock gypsum does not bubble in acid.
Rock gypsum (Gyprock)