Rock salt (sedimentary halitite) from the Permian of New Mexico, USA. (8.3 cm across)

Sedimentary rocks form by the solidification of loose sediments. Loose sediments become hard rocks by the processes of deposition, burial, compaction, dewatering, and cementation.

There are three categories of sedimentary rocks: 1) Siliciclastic sedimentary rocks form by the solidification of sediments produced by weathering & erosion of any previously existing rocks. 2) Biogenic sedimentary rocks form by the solidification of sediments that were once-living organisms (plants, animals, micro-organisms). 3) Chemical sedimentary rocks form by the solidification of sediments formed by inorganic chemical reactions. Most sedimentary rocks have a clastic texture, but some are crystalline.

Rock salt is a chemical sedimentary rock that forms by the evaporation of water (usually seawater) and the precipitation of dissolved minerals. Chemical sedimentary rocks that form by the evaporation of water are called evaporites. Rock salt & rock gypsum are the two most common evaporites.

Rock salt is composed of the mineral halite (NaCl - sodium chloride). It ranges in color from clearish to grayish to orangish-brown, but sometimes has other colors, such as blue. It has a strongly salty taste, is often coarsely-crystalline, and is relatively soft (H = 2.5).

Rock salt is also known as halitite, which refers to sedimentary evaporite deposits composed of halite (NaCl). Halitite has also been used in the geologic literature to refer specifically to rock salt that's been contact metamorphosed by igneous intrusions.

The attractive rock salt shown above has remarkable bluish-colored halite crystals. The bluish and orangish-brown coloration is caused by radiation from potassium-40 in nearby potash salts (principally sylvite, KCl), resulting in excess free sodium metal in the halite.

Stratigraphy: McNutt Member, middle Salado Formation, Ochoan Stage, upper Upper Permian