Gypsum

Formula: CaSO4 · 2H2O

Colour: Colourless to white, often tinged other hues due to impurities; colourless in transmitted light.

Lustre: Vitreous, Sub-Vitreous, Silky, Pearly, Dull

Hardness: 2

Specific Gravity: 2.312 – 2.322

Crystal System: Monoclinic

Member of: Gypsum Supergroup

Name: First known mention is by Theophrastus about 300-325 BCE from the Greek γυψοζ (gypsos) meaning plaster.

Isostructural with: Ardealite, Brushite, Pharmacolite

The most common sulphate mineral. Found as both massive material, including the alabaster variety; and clear crystals, the selenite variety; and, parallel fibrous, the satin spar variety. Typically colourless to white, transparent crystals, thick tabular to lenticular, sometimes prismatic.

Gypsum is a widespread calcium sulfate hydrate that is found in a number of forms, and is of great economic importance. It often occurs in well-developed crystals such as those from the Cave of Swords, Naica, Chihuahua, Mexico where numerous transparent swordlike selenite gypsum crystals reach lengths of several meters.. Selenite, from the Greek word selene, meaning the moon, is the name of transparent crystals of gypsum, but is often incorrectly used for gypsum in general. Single crystals can be blocky with a slanted parallelogram outline, tabular bladed or in a long, thin shape like a ram’s horn. Twinned crystals are common, and frequently form characteristic “swallowtails” or “fishtails”. Gypsum is also found in a parallel, fibrous variety with a silky luster, called satin spar. The massive, fine-grained variety is called alabaster, which has been used for century for statuary and carvings. In ancient times, the term was applied to other minerals of similar appearance; for example, the “alabaster “ containers recovered from Tutankhamun’s tomb, were, in fact, made of calcite (Onyx). Rosette-shaped crystals are called desert roses and are more common and less dense than the barite “sand Crystals” that they resemble. Gypsum occurs in extensive beds formed by the evaporation of ocean brine, along with other minerals similarly formed including anhydrite and halite. It has low solubility and so is the first mineral to separate from evaporating sea water. Once it has been deposited, it is commonly blown into dunes. One of the most spectacular is the white sands dunes of New Mexico that expand over 225 square miles of dazzling white gypsum sand that make up 60 feet high dunes.

Gypsum also occurs as an alteration product of sulfides in ore deposits; as disseminates crystals and rosette-shaped aggregates in sedimentary deposits, including sand and clay; and as deposits around volcanic fumeroles.

Gypsum has been an important economic mineral since the time of the ancient Egyptian civilization. It was mined to the west of Alexandria, near Suez, in Al Fayyum, and near the Red Sea coast.It was used for mortar and plaster in the Giza necropolis, and it was used elsewhere to plaster walls,cover bodies, make staues and masks, as an adhesive, and as a filler in pigments. The Romans discovered that heating gypsum to 600 DF (300DC) made a plaster that sets hard when mixed with water. This plaster was used for building ansd is still widely used today.