Gypsum (mineral)
Gypsum is a widely occurring hydrated calcium sulfate mineral, recognized as the most common sulfate mineral found in sedimentary rocks, often in association with other minerals like rock salt and dolomite. It forms white or colorless prismatic crystals and is notable for its soft texture, with a hardness rating of 1.5 to 2 on the Mohs scale. Gypsum is primarily utilized in the construction industry for making plasters, wallboard, and tiles, and it also plays a significant role in the production of cements, as well as in agriculture as a soil conditioner and fertilizer.
When heated, gypsum transforms into plaster of Paris, which is known for its quick-setting properties, making it ideal for various construction applications. The mineral is mined extensively in numerous countries, with significant deposits located in the United States, Canada, Iran, and Australia. Gypsum's diverse forms include alabaster, satin spar, and selenite, each with unique properties and uses, ranging from ornamental crafts to medical applications. Historically, gypsum has been used since ancient times by various cultures for decorative purposes and construction techniques, underscoring its long-standing significance in human creativity and industry.
Gypsum (mineral)
Where Found
Gypsum is the most common sulfate mineral. It is widely distributed in sedimentary rocks, frequently occurring with limestones and shales. It is commonly associated with minerals such as rock salt, anhydrite, dolomite, calcite, sulfur, pyrite, galena, and quartz. Gypsum is mined extensively in many parts of the world.
Primary Uses
Gypsum is used in the construction industry, especially for the manufacture of plasters, wallboard, and tiles. It is also used in cements, as a filler in paper and paints, and as a fertilizer and soil conditioner.
Technical Definition
Gypsum is a hydrated calcium sulfate (CaSO4·2H2O). Its average molecular weight is 172.18, and its specific gravity is 2.32. This mineral forms white or colorless prismatic crystals; impurities may add a grayish, reddish, yellowish, bluish, or brownish tint. Its hardness on the Mohs scale is 1.5 to 2. Gypsum has a characteristic three-way cleavage; that is, it breaks along three different crystallographic planes. It is insoluble in water and soluble in acids. When heated to between 190° and 200° Celsius, gypsum loses three-quarters of its water of crystallization to become calcium sulfate hemihydrate (2CaSO4·H2O), also known as plaster of paris. Heating to more than 600° Celsius drives off all water to produce anhydrous or dead-burned gypsum.
Description, Distribution, and Forms
Gypsum, a widely distributed sedimentary deposit, is a soft, colorless, or light-colored mineral that can be scratched with the fingernail. Its crystals often form arrowhead-shaped or swallowtail-shaped twins (two individual crystals joined along a plane). When heated to drive off much of its water of crystallization, gypsum is transformed into plaster of paris (so named because of the famous gypsum deposits of the Montmartre district of Paris, France). When reduced to a powder and mixed with water, plaster of paris forms a slurry that sets quickly and gradually re-forms again as tiny interlocking crystals of gypsum. Its properties as a natural plaster make gypsum an important resource for construction and other industries. In 2008, the United States produced about 12.7 million metric tons of gypsum, and total world production was about 151 million metric tons.
Gypsum, the most common sulfate mineral, is widely distributed in sedimentary rocks. It forms thick, extensive evaporite beds, especially in rocks of Permian and Triassic age. In the United States, gypsum is present in rocks of every geologic era except the Cambrian. Because gypsum is normally deposited before anhydrite and salt during the evaporation of seawater, it often underlies beds of these minerals. Other minerals with which gypsum is frequently associated include dolomite, calcite, sulfur, pyrite, galena, quartz, and petroleum source rocks. Massive layers of gypsum frequently occur interbedded with limestones and shales, and lens-shaped bodies or scattered crystals are found in clays and shales. Gypsum is common in volcanic regions, particularly where limestones have been acted upon by sulfur vapors. It is also found in association with sulfide ore bodies. Extensive gypsum deposits are found in many localities throughout the world, including the United States, Great Britain, Thailand, Iran, Canada, China, France, and Australia. In Arizona and New Mexico there are large deposits in the form of wind-blown sand.
Gypsum occurs in nature in five varieties: gypsum rock, a bedded aggregate consisting mostly of the mineral gypsum; gypsite, an impure, earthy variety that is found in association with gypsum-bearing strata in arid regions;alabaster, a massive, fine-grained form, white or delicately shaded and often translucent; satin spar, a white, translucent mineral with a fibrous structure and a silky luster; and selenite, a transparent, colorless, crystalline variety.
Gypsum is rarely found in its pure form. Deposits may contain quartz, sulfide minerals, carbonates, and clayey and bituminous materials. Gypsum dehydrates readily in nature to form anhydrite (CaSO4), a mineral with which it is often associated; bassanite (2CaSO4·H2O) forms much more rarely. High-temperature and low-humidity environments favor the formation of anhydrite. Anhydrite can also hydrate to form gypsum. Gypsum deposits formed by the alteration of anhydrite may show folding due to the increased volume of the mineral in its hydrated state.
History
The Chinese, Assyrians, and Greeks made decorative carvings from gypsum. The Greek philosopher Theophrastus (371-287 b.c.e.) wrote of burning gypsum to create plaster. Gypsum’s properties as a plaster were also known to the early Egyptians, who used a crude gypsum plaster in such building projects as the pyramids. Gypsum gained widespread use as a soil conditioner in eighteenth century Europe. The development of a commercial method for retarding the setting of gypsum plaster in 1885 made it possible to use gypsum for more construction applications.
Obtaining Gypsum
Gypsum is generally obtained through open-pit mining, although some underground mining is performed where the material is of a high quality or is close to the consuming market. Gypsum may be crushed and ground for use in dihydrate form, heated to produce plaster of paris, or completely dehydrated to form anhydrous gypsum.
Uses of Gypsum
Unaltered gypsum is commonly used to slow the rate of setting in portland cement. Other major uses include the manufacture of wallboard, gypsum lath, and artificial marble products. Its sulfate contents make it useful for agriculture, where it serves as a soil conditioner and fertilizer. Gypsum is used as a white pigment, filler, or glaze in paints, enamels, pharmaceuticals, and paper. It is also used in making crayons, chalk, and insulating coverings for pipes and boilers. Other uses are as a filtration agent and a nutrient in yeast growing.
Plaster of paris is used for builder’s plaster and the manufacture of plaster building materials such as moldings and panels. In medicine, plaster of paris is used for surgical casts, bandages, and supports and for taking dental and other impressions. The anhydrous form of gypsum is used in cement formulations; in metallurgy; in the manufacture of tiles, plate glass, pottery, and paints; and as a paper filler. Because of its water-absorbing nature, it is also used as a drying agent.
Alabaster, a form of gypsum that can be carved and polished with ease because of its softness, is fashioned into ornamental vessels, figures, and statuary. Satin spar is used in jewelry and other ornaments.
Bibliography
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