Gypsum

• 1. Introduction

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  Gypsum is so soft you can scratch it with a fingernail. On the Mohs scale of mineral hardness, it sits at a value of 2, defined there by simple scratch comparison. Yet this fragile sulfate mineral has shaped walls, fertilized wheat fields, coagulated tofu, and grown into some of the largest crystals nature has ever produced. Its chemistry is plain calcium sulfate dihydrate, a salt that quietly carries water inside its own structure. Drive that water out with heat and it becomes plaster. Add water back and it sets hard again, ready for casting and construction. How does one humble mineral end up in drywall, in dentistry, in surgical splints, and in soil reclamation projects? Why did American farmers smuggle it across a border? And how did crystals of it reach lengths of twelve metres in a cave in Mexico? The story runs from Ancient Egypt to the northern polar region of Mars.

• 2. Plaster Of Paris

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  The word gypsum comes from the Greek gypsos, meaning plaster, and the Montmartre district of Paris gave the material its most famous name. Quarries there long supplied burnt, calcined gypsum, so the dehydrated mineral became known as plaster of Paris. The transformation is reversible. Add water to plaster of Paris and, after a few dozen minutes, it returns to regular gypsum, the dihydrate, hardening or setting as it goes. That setting behaviour explains why gypsum sits at the centre of so many forms of plaster, drywall, and sidewalk chalk. Old English speakers had their own name for it: sparstan, or spear stone, a nod to its crystalline projections. From that root, the word spar entered mineralogy, describing any non-ore mineral or crystal that grows in spearlike forms by comparison to gypsum itself.

• 3. A Miraculous Fertilizer

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  In the mid-18th century, the German clergyman and agriculturalist Johann Friderich Mayer investigated and publicized gypsum's use as a fertilizer. Gypsum supplies two secondary plant macronutrients, calcium and sulfur, and unlike limestone it generally does not affect soil pH. By the early 19th century it was regarded as an almost miraculous fertilizer for wheat. American farmers wanted Nova Scotia gypsum so badly that a lively smuggling trade grew up across the border. That illicit demand produced the so-called Plaster War of 1820. Gypsum's agricultural reach extends past feeding crops. Added to sodic and acidic soils, it converts the highly soluble sodium metaborate form of boron into less soluble calcium metaborate and reduces the exchangeable sodium percentage. The Zuiderzee Works used gypsum on its recovered land. A gypsum block buried in soil can even report moisture, since measuring its electrical resistance lets growers derive the soil's water potential.

• 4. Crystals And Caves

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  In the form of selenite, gypsum forms some of the largest crystals found in nature, up to twelve metres long. Selenite contains no significant selenium. Both names instead trace to the ancient Greek word for the Moon. This transparent, cleavable mineral can also appear in a silky fibrous form called satin spar, or in a flower-like desert rose with sand grains embedded inside, formed in arid country. The most spectacular examples grew underground in the Naica Mine of Chihuahua, Mexico, where crystals up to eleven metres long were found. They thrived in a rare and stable environment, with temperatures holding at 58 degrees Celsius and the cave filled with mineral-rich water that drove their growth. The largest weighs 55 tonnes and is around 500,000 years old. The same gypsum that builds such giants also dissolves in water over time, which is why it can carve karst landscapes such as the UNESCO World Heritage Site Evaporitic Karst and Caves of Northern Apennines in Italy.

• 5. Beds Of Stone

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  Thick and extensive evaporite beds of gypsum lie in association with sedimentary rocks, with deposits known from strata as far back as the Archaean eon. The mineral precipitates from lake and sea water, from hot springs, from volcanic vapors, and from sulfate solutions in veins. Pure gypsum is white, but impurities can tint local deposits across a wide range of colors. Because gypsum dissolves over time in water, it is rarely found as sand. The exception is striking. White Sands National Park in New Mexico holds a 710 square kilometre expanse of white gypsum sand, enough to supply the US construction industry with drywall for 1,000 years. Commercial exploitation, strongly opposed by area residents, was permanently prevented in 1933 when President Herbert Hoover declared the dunes a protected national monument. The mineral has even been found off-world: orbital images from the Mars Reconnaissance Orbiter revealed gypsum dunes in the northern polar region of Mars, later confirmed at ground level by the Mars Exploration Rover Opportunity.

• 6. Mining The World

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  China dominates gypsum production, with an estimated 132,000 thousand metric tons in 2015, far ahead of Iran, the world's second largest producer. The United States reported reserves of 700,000 thousand metric tons, while the world total of production reached 258,000 thousand metric tons. Large open pit quarries operate in many places. Fort Dodge, Iowa sits on one of the largest gypsum deposits in the world, alongside Plaster City, California, and East Kutai in Kalimantan, Indonesia. Smaller mines serve niche needs. At Kalannie in Western Australia, gypsum is sold to private buyers for additions of calcium and sulfur, and for reducing aluminium toxicities in soil for agriculture. Commercial quantities also appear in the Brazilian cities of Araripina and Grajau, and across Pakistan, Jamaica, Thailand, Spain, Germany, Italy, England, Ireland, and Canada.

• 7. Made By Industry

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  Synthetic gypsum arrives as waste or by-product from a range of industrial processes. Coal-fired power stations with flue gas desulfurization scrubbers produce large quantities, recovered as flue gas desulfurization gypsum. Its main contaminants, including magnesium, potassium, chlorine, fluorine, boron, aluminium, iron, silicon, and selenium, come from the limestone used and the coal burned. Cleaner enough to replace natural gypsum, it serves drywall, water treatment, and cement set retarding. Other industrial streams leave dirtier residues. Phosphate fertilizer production breaks down phosphate rock with acid, yielding phosphogypsum contaminated by fluoride, silica, radioactive radium, and heavy metals such as cadmium. Titanium dioxide manufacture produces titanium gypsum from neutralizing excess acid with lime. Such impurities often bar these wastes from ordinary use, so they sit in stacks indefinitely, risking leaching into water and soil while researchers hunt for new applications. Gypsum scaling also plagues desalination, precipitating onto brackish water membranes and cutting their life and productivity in reverse osmosis and nanofiltration.

• 8. From Walls To Tofu

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  Gypsum board, known as plasterboard, sheetrock, or drywall, finishes walls and ceilings while lending fire resistance, an effect sharpened by added glass fibers. Its negligible heat conductivity gives gypsum plaster insulative qualities, and it serves as a component of Portland cement that prevents the flash setting of concrete. When deforestation made wood scarce on Bronze Age Crete, builders turned to gypsum where wood had been used before. The mineral's softness made it a sculptor's favourite long ago. As alabaster, fine-grained and lightly tinted, it was carved by Ancient Egypt, Mesopotamia, Ancient Rome, the Byzantine Empire, and the Nottingham alabasters of Medieval England, and during the Middle Ages and Renaissance it was preferred even to marble. Medieval scribes mixed it into gesso for illuminated letters gilded with gold. In the kitchen it acts as a tofu coagulant and a source of dietary calcium, hardens brewing water, conditions dough, and keeps grains from clumping in mushroom cultivation. Medicine uses it for surgical splints and dental impression plasters, and tests show it can pull pollutants such as lead or arsenic from contaminated waters.