Blue halite from the Devonian of Canada. (Left: 2.6 cm across; right: 2.9 cm across)

A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5500 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

The halides are the "salt minerals", and have one or more of the following anions: Cl-, F-, I-, Br-.

Halite is true salt, which consists of sodium chloride (NaCl). This is the same chemical long used as flavoring on food & as a preservative. It has a nonmetallic luster, typically clearish/coloress, and is relatively soft (H = 2.5). Halite forms cubic crystals and has cubic cleavage (= 3 cleavage planes meeting at 90º angles). Halite is most readily identified by its strongly salty taste.

Halite has economic value. In addition to its use in food, salt is traditionally used in large quantities in wintertime to prevent roadways from icing up. Halite is principally mined from ancient rock salt successions. Rock salt is a chemical sedimentary rock composed of halite and formed by evaporation of seawater.

The blue coloration in the halite shown above is the result of radiation from potassium-40 in nearby "potash" salts (= sylvite). Irradiation ultimately results in excess free sodium metal in the halite, turning it blue.

Stratigraphy: Prairie Evaporite Formation, upper Elk Point Group, Middle Devonian

Locality: Potash Saskatchewan-Lanigan Mine (PCS-Lanigan Mine), near the town of Lanigan, south-central Saskatchewan, Canada

Photo gallery of halite: