The soft mineral in your walls also grows crystals 11 metres long
Gypsum is one of the softest common minerals and ends up in drywall, chalk and plaster. In a Mexican mine cave kept at 58 degrees Celsius, though, it grew crystals up to 11 metres long, the biggest weighing 55 tonnes. In the 1820s, American farmers even smuggled it from Nova Scotia.
Chemically, gypsum is calcium sulfate with two molecules of water bound in, and it rates only 2 on the Mohs hardness scale. Its structure explains a lot: layers of calcium and sulfate ions are held together by sheets of water through weak hydrogen bonds, so the crystal splits cleanly along those sheets. Heat drives the water out, first producing the hemihydrate known as plaster and then anhydrite. Add water to the burned powder and within a few dozen minutes it turns back into gypsum and hardens, which is why it is so useful for casts. Plaster of Paris takes its name from the quarries of Montmartre.
It comes in many guises. Clear, cleavable crystals are called selenite, after the Greek word for the Moon, though they contain no selenium. Silky fibres are satin spar; fine white stone is alabaster, carved by cultures from ancient Egypt to medieval Nottingham and at times preferred even to marble. In deserts it forms flower-like desert roses packed with sand. Unusually for a salt, it becomes less soluble as water warms. Because rain slowly dissolves it, gypsum sand is rare, but New Mexico's White Sands holds about 710 square kilometres of it, protected as a national monument in 1933. Gypsum dunes have also been spotted near the north pole of Mars and confirmed by the Opportunity rover.
Most deposits are evaporites left by drying lakes and seas, some from as far back as the Archaean eon. At Naica in Chihuahua, a stable, mineral-rich hot cave let crystals grow for around half a million years. Coal power stations that scrub sulfur from their exhaust also produce large amounts of synthetic gypsum, pure enough for wallboard and cement.
Its uses stretch back millennia. On Bronze Age Crete, builders swapped gypsum in for scarce timber. In the mid-18th century the German clergyman Johann Friderich Mayer promoted it as fertiliser, since it supplies calcium and sulfur without changing soil acidity, and demand in the United States grew so fierce that smuggling led to the so-called Plaster War of 1820. Today gypsum board lines walls and ceilings, adding fire resistance and insulation.
Source: Gypsum
