Features of the Mineral Gypsum

Appearance

• Gypsum forms in clear, tabular crystals that grow up to 3.3 feet in length. Often they form in swallowtail or spearhead twins. Transparent crystals and cleavage fragments of selenite, another form of gypsum, are common. It may also produce fibrous aggregates of elongated, satiny crystals called satin spars. As well, it can be found in granular and compact, waxy-appearing masses and may be banded alabaster, yet another form of gypsum. Besides clear, gypsum comes in white, gray, yellow or brown. It can also occur in rosette aggregates, incorporating sand. This formation is known as a desert rose.

Physical Properties

• Gypsum is a soft and lightweight mineral. On the Mohs Hardness scale, in which a diamond is 10, gypsum is a 2. It possesses perfect cleavage, breaking into slightly flexible but inelastic plates and very fine flakes. Gypsum is typically transparent with a vitreous or silky luster. It is often pearly on cleavage faces. And it can be fluorescent in ultraviolet light and is soluble in hydrochloric acid and hot water.

Environment

• As a typical sedimentary evaporite mineral, gypsum forms through direct precipitation either from saline-based waters or by alteration of anhydrite. Anhydrite is formed through a process in which gypsum is heated and moisture is removed. Gypsum can also form by direct sublimation from sources like fumaroles (holes near volcanoes from which vapor rises) or as precipitate from hot volcanic springs. It is also found in blocks in clay. Giant crystals found in clay come from sites near Bologna and Pavia in Italy and from sulfur mines in places such as Sicily and Chihuahua, Mexico. Gypsum is also found in Chile and in Utah, among other places.

Uses

• Gypsum is used in making Plaster of Paris for construction. It is also used as a retarder in Portland cement. As well, it serves as a flux for pottery and as a fertilizer. Certain varieties of alabaster are commonly used in interior decorating as well as for sculpture. Alabaster is also popular for polished slabs. Gypsum deposits find many uses in industry, including use in certain types of stucco and as filler materials for paper and rubber.