Gypsum
A soft sulfate mineral used in plaster, fertilizer, and sculpture.
Gypsum is a soft mineral made of calcium sulfate with two water molecules attached, written as CaSO4·2H2O. It is mined in large amounts and used as a fertilizer and as the key ingredient in plaster and drywall. It can also form clear, see-through crystals called selenite. Gypsum usually forms when water evaporates from salty water or when the mineral anhydrite takes on water. On the Mohs hardness scale, gypsum ranks at 2, meaning it is soft enough to be scratched with a fingernail.
The word "gypsum" comes from the Greek word *gypsos*, meaning "plaster." Because the quarries in Paris's Montmartre district supplied burnt gypsum for many uses, this dehydrated form became known as plaster of Paris. When water is added, plaster of Paris turns back into regular gypsum within a few dozen minutes, hardening into a solid useful for casting and construction. In Old English, gypsum was called *spærstān*, or "spear stone," because of its pointed crystal shapes. This is why the term "spar" in mineralogy refers to any non-ore mineral that forms spear-like projections. In the mid-1700s, German clergyman and farmer Johann Friderich Mayer studied and promoted gypsum as a fertilizer. It provides sulfur for plant growth, and by the early 1800s it was seen as nearly miraculous. American farmers wanted it so badly that a lively smuggling trade with Nova Scotia sparked the "Plaster War" of 1820.
Gypsum dissolves moderately in water—about 2.0 to 2.5 grams per liter at 25°C—and, unlike most salts, it becomes less soluble as the water gets warmer. When heated in air, gypsum loses water and first becomes calcium sulfate hemihydrate (bassanite, often called plaster), and with more heat turns into anhydrous calcium sulfate (anhydrite). Its solubility in salty water or brines depends heavily on the amount of sodium chloride (table salt) present. The structure of gypsum has layers of calcium and sulfate ions tightly bound together, separated by sheets of water molecules held by weaker hydrogen bonds. This gives the crystal perfect cleavage along those sheets.
In nature, gypsum appears as flat, often twinned crystals and as transparent, cleavable masses called selenite. Selenite can form some of the largest crystals known, up to 12 meters (39 feet) long. Despite the name, selenite contains no selenium; both words come from the ancient Greek word for the Moon. Selenite can also be silky and fibrous, known as satin spar. It may be granular or compact, and in hand-sized samples it ranges from transparent to opaque. A very fine-grained white or lightly tinted variety called alabaster is prized for ornamental work. In dry areas, gypsum can form flower-like shapes, often opaque and embedded with sand grains, called desert roses.
Gypsum is a common mineral, found in thick, widespread evaporite beds within sedimentary rocks. Deposits exist in strata as old as the Archaean eon. It forms from lake and sea water, hot springs, volcanic vapors, and sulfate solutions in veins. Hydrothermal anhydrite in veins often turns into gypsum when groundwater hydrates it near the surface. Gypsum is often found with halite and sulfur, and it is the most common sulfate mineral. Pure gypsum is white, but impurities can give it many colors. Because gypsum dissolves in water over time, it rarely forms sand. However, White Sands National Park in New Mexico has a 710-square-kilometer (270-square-mile) area of white gypsum sand—enough to supply the U.S. construction industry with drywall for 1,000 years. Commercial mining there was permanently blocked in 1933 when President Herbert Hoover made the dunes a protected national monument. Gypsum's solubility can also create karst landscapes, like the UNESCO World Heritage Site Evaporitic Karst and Caves of Northern Apennines in Italy, an unusually well-preserved gypsum karst terrain. Gypsum also forms as a byproduct when sulfide minerals like pyrite oxidize, producing sulfuric acid that reacts with calcium carbonate. Its presence indicates oxidizing conditions. Under reducing conditions, sulfate-reducing bacteria can turn the sulfates back into sulfides, sometimes leading to elemental sulfur in oil-bearing formations like salt domes, where it can be mined using the Frasch process. Coal-fired power plants with flue gas desulfurization produce large amounts of gypsum as a byproduct. Orbital images from the Mars Reconnaissance Orbiter have shown gypsum dunes near Mars's north pole, later confirmed on the ground by the Mars Exploration Rover Opportunity.
Commercial gypsum is mined in Araripina and Grajaú in Brazil; in Pakistan, Jamaica, Iran (the world's second-largest producer), and Thailand.
- chemical_formula
- CaSO4·2H2O
- mohs_hardness
- 2
- solubility
- ~2.0–2.5 g/L at 25°C, retrograde solubility
- common_varieties
- selenite, alabaster, satin spar, desert rose
- primary_uses
- fertilizer, plaster, drywall, sculpture
- notable_deposits
- White Sands National Park (USA), Naica Mine (Mexico), Montmartre (Paris)
Lore & Background
The word gypsum is derived from the Greek word gypsos, meaning 'plaster.' Because the quarries of the Montmartre district of Paris long furnished burnt gypsum (calcined gypsum) used for various purposes, this dehydrated gypsum became known as plaster of Paris. Upon adding water, after a few dozen minutes, plaster of Paris becomes regular gypsum (dihydrate) again, causing the material to harden or 'set' in ways useful for casting and construction. Gypsum was known in Old English as spærstān, 'spear stone,' referring to its crystalline projections; the word spar in mineralogy, by comparison to gypsum, refers to any crystalline, non-metallic mineral that is easily cleavable or lustrous. In the mid-18th century, the German clergyman and agriculturalist Johann Friderich Mayer investigated and publicized gypsum's use as a fertilizer. Gypsum may act as a source of sulfur for plant growth, and in the early 19th century, it was regarded as an almost miraculous fertilizer.
Reader's Guide
Gypsum is significant as one of the most common sulfate minerals, with thick and extensive evaporite beds in association with sedimentary rocks. Its moderate water solubility and retrograde solubility make it distinct among salts, and it can form karst landscapes, such as the UNESCO World Heritage Site Evaporitic Karst and Caves of Northern Apennines in Italy. Gypsum is also formed as a by-product of sulfide oxidation, and its presence indicates oxidizing conditions. Commercially, gypsum is mined in many countries, including Brazil, Pakistan, Jamaica, Iran, Thailand, Spain, Germany, Italy, England, Ireland, Canada, and the United States. Large open pit quarries exist in Fort Dodge, Iowa, and Plaster City, California. Crystals of gypsum up to 11 m long have been found in the Naica Mine of Chihuahua, Mexico. Synthetic gypsum is produced as a waste product or by-product in industrial processes, such as flue gas desulfurization at coal-fired power plants, desalination, and phosphate fertilizer production (phosphogypsum). Orbital pictures from the Mars Reconnaissance Orbiter have indicated gypsum dunes in the northern polar region of Mars, later confirmed at ground level by the Mars Exploration Rover Opportunity.
Did You Know?
- Gypsum exhibits retrograde solubility, becoming less soluble at higher temperatures.
- The largest gypsum crystals found in nature are up to 12 m (39 ft) long, in the form of selenite.
- Gypsum was known in Old English as spærstān, meaning 'spear stone,' due to its crystalline projections.
Naming, Mythology & Chemical Identity
The name selenite traces a long linguistic path: from Greek selēnítēs líthos, literally "moon stone," through Latin selenites and Middle English selenite, arriving at the modern term. The lunar association arose from a historical belief that the mineral's appearance waxed and waned in sympathy with the Moon's phases. Despite the suggestive spelling, selenite contains no selenium whatsoever; both names independently derive from the Greek selḗnē, meaning Moon. From the 15th century onward, the word specifically designated the transparent crystalline form of gypsum, although in contemporary usage it has broadened to cover satin spar, desert roses, and gypsum flowers as well. What unites all these varieties is a single chemical identity: calcium sulfate dihydrate, CaSO4·2H2O, in which every formula unit incorporates two water molecules. This shared composition underlies the family's remarkably diverse appearances.
Crystal Habits & the Four Varieties
Gypsum crystallizes in four distinct habit types, each with its own visual signature. Selenite tends toward tabular, prismatic, or acicular forms, often appearing flawless and sometimes as thin mica-like sheets. Twinned crystals are common, producing shapes described as "swallow tail" or "arrow/spear-head." Satin spar is almost exclusively prismatic and fibrous, growing in parallel bundles that can form long seams attached to host rock. Desert roses present as bladed rosettes, typically bearing a sandy exterior druse, and form in wet sand rather than attached to a matrix. Gypsum flowers are acicular, scaly, and stellate, frequently displaying severe curves and bends that create "ram's horns," "fishtail," or "swallowtail" twin patterns; they are usually found attached to a matrix or base rock within solution caves. All four varieties can be brittle and fragile, and they may form densely in acicular mats or nets. Selenite crystals can be found both attached to a matrix and as entire free-floating crystals, often in clay beds.
Physical Properties & Identification
Gypsum's softness is its most immediately recognizable trait: at hardness 2 on the Mohs scale, it can be scratched by a fingernail. It also exhibits three unequal cleavages, a pearly lustre, and easy fusibility with loss of water. Solubility in hot dilute hydrochloric acid provides another quick identification test. Selenite typically shows vitreous luster, while satin spar displays a characteristic silky sheen. Fibrous satin spar exhibits chatoyancy—the cat's eye effect—and when cut across its fibers and polished on the ends, it creates an optical illusion where printed images beneath appear to float on the stone's surface, earning it the nickname "television stone." Some selenite and satin spar specimens also display fluorescence or phosphorescence. All four varieties are slightly flexible and sectile, meaning they can be cut or peeled, though they are not elastic and will break if bent too far. Desert roses, due to their sandy exterior druse, are marginally harder to scratch but still yield to a fingernail.
Notable Specimens & Historical Uses
Among the most extraordinary mineral specimens ever discovered is a selenite crystal from the Naica Mine's Cave of the Crystals, measuring 12 meters in length and weighing 12 tons. On a smaller but equally remarkable scale, selenite's thin tabular, mica-like sheets were historically employed as window panes, as seen in the church of Santa Sabina in Rome. Satin spar, prized for its chatoyance, is sometimes cut into cabochons to best showcase its silky optical play. The mineral's color palette ranges from colorless and white to gray, with tints of brown, yellow, red, or blue introduced by impurities such as iron oxides or clay minerals. Transparency varies across the varieties: selenite is most often fully transparent, satin spar tends toward translucency, while rosettes and flowers are frequently opaque due to impurities, inclusions, or their sandy druse. These diverse appearances, all arising from the same calcium sulfate dihydrate formula, make the gypsum family one of the most visually varied groups in the mineral collector's world.
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Frequently Asked Questions
Who is Gypsum?
Gypsum is a soft sulfate mineral whose formula, CaSO4·2H2O, tells you it is calcium sulfate locked together with two water molecules. It typically forms when seawater or lake water evaporates, or when anhydrite picks up water from its surroundings, making it one of the most widely mined minerals on Earth.
What are Gypsum's powers/role?
In everyday life Gypsum is the backbone of drywall, plaster, and a cheap agricultural fertilizer, while its translucent crystal form (selenite) is prized for carving and sculpture. On the Mohs scale it sits at a mere 2, so it scratches easily, and it dissolves in water at roughly 2–2.5 g/L at room temperature.
How does Gypsum's story end?
Heated or compressed, Gypsum sheds its two water molecules and reverts to anhydrite, effectively losing the 'dihydrate' identity that defines it. In water it simply dissolves, and its retrograde solubility—becoming less soluble as temperature climbs—makes its behavior a well-known exception among common salts.
Why is Gypsum important?
Without it, the construction industry would lose its primary wall-building material and agriculture would lose an inexpensive soil conditioner, so its economic footprint is enormous. Spectacular deposits such as White Sands National Park in New Mexico and the giant crystal caves of the Naica Mine in Mexico also make it a landmark in mineralogy and tourism.
What varieties does Gypsum have?
The same CaSO4·2H2O formula shows up as glassy selenite, fine-grained alabaster, silky satin spar, and the rosette-shaped desert rose. These look and feel completely different from one another, yet they are all the same mineral expressed in different crystal habits.
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