Calcium sulfate

Ca 1
S 1
O 4

Formula unit Lattice

Choose Lattice to see four unit cells. Then drag to rotate; pinch or Ctrl + scroll to zoom. Arrow keys rotate, + − zoom, R resets.

6 atoms in one formula unit of an ionic lattice.

The drawing shows one formula unit of an ionic lattice, not a molecule; calcium sulfate has no molecular shape.

IUPAC name: calcium sulfate
Molar mass: 136.134(28) g/mol
Atoms: 6
SMILES: [O-]S(=O)(=O)[O-].[Ca+2]
InChIKey: OSGAYBCDTDRGGQ-UHFFFAOYSA-L
PubChem CID: 24497

Made of

Ca S O

Calcium sulfate holds water in its crystals in fixed proportions, and each proportion makes a different material. Gypsum, the mineral of wallboard, is a hydrate with two water molecules for every calcium ion and sulfate ion. Its formula is CaSO₄·2H₂O, which the Hazardous Substances Data Bank (HSDB) puts at 27 per cent water, and it rates 2 on the Mohs hardness scale. Plaster of Paris, the hemihydrate CaSO₄·0.5H₂O, keeps one water molecule for every two such pairs.

The mineral anhydrite holds no water, and its composition is the one that the formula CaSO₄, the molar mass of 136.13 g/mol and the formula-unit drawing describe. Calcined, or heated to drive water out, gypsum becomes plaster of Paris; above 600 °C it ends as anhydrous “dead-burned” gypsum. Soluble anhydrite, a third product, pulls in water so strongly that it serves as a desiccant, turning into plaster of Paris as it does.

Structure and bonding

The drawing holds one calcium ion, charge +2, and one sulfate ion, charge −2, as two pieces with no bond between them. The pair is one formula unit of an ionic lattice, the ratio in which the ions occur, not a particle with a life of its own. Inside the sulfate ion the bonds are real. A sulfur atom holds four oxygen atoms, drawn with two double bonds and two single ones, and the drawing marks each singly bonded oxygen with a negative charge.

In the Lattice view the model shows four outlined unit cells of anhydrous calcium sulfate, each the crystal’s smallest repeating block. Each cell counts 4 calcium ions and 4 sulfate ions, the ions on its faces shared with neighbouring cells. Under Coordination, one calcium ion stands out among its 8 nearest oxygen atoms, which belong to 6 different sulfate ions and lie 2.33 to 2.55 Å away.

That measured structure also tests the drawing of the sulfate ion. In it, all four sulfur–oxygen distances come out between 1.48 and 1.49 Å, nearly equal, whichever bonds the drawing makes double. Like the drawing, the crystal holds no water: it is not gypsum.

Where it turns up

White Sands National Park holds the world’s largest gypsum dunefield (National Park Service). Its dunes cover 275 square miles and hold over 4.5 billion tons of sand, about 98 per cent of it pure gypsum. Gypsum sand is rare because gypsum dissolves in water. Rain and snowmelt carry it down from the surrounding mountains. As the water evaporates it crystallises again as selenite, some crystals as large as a bicycle tyre.

Mammoth Cave grows gypsum too. In its drier passages the mineral forms “snowballs” and “flowers”; a flower grows as the gypsum pushes out of the rock and some of its crystals outgrow others. Prehistoric American Indians collected gypsum there 2,000 to 3,000 years ago, along with epsomite and mirabilite. Soot from torches of that period still darkens some of the gypsum.

On Mars, NASA/JPL reports, the Curiosity rover found calcium sulfate, gypsum included, in Mount Sharp, among salty minerals that point to water in the planet’s past.

Why it matters

Wallboard is one of the main US uses of gypsum, and a typical new American home holds over 7 metric tons of it. One grade, Type X, is named for its extra fire retardation. Gypsum also slows the setting of portland cement, where the US Geological Survey (USGS) finds it has no practical substitute.

The country produced an estimated 22 million metric tons of crude gypsum in 2024. On USGS estimates no other country produced more, of 160 million tons worldwide. Synthetic gypsum met about 34 per cent of domestic supply that year. Its main source is flue-gas desulfurisation, the removal of sulfur from coal-fired power plants’ smokestack emissions. Its use has levelled off, largely as power generation moves from coal to natural gas.

On old stone buildings, sulfuric acid reacts with the stone’s calcium carbonate. The gypsum that forms gathers iron and other stone components into a black crust, part of which blisters off and bares fresh stone. Westminster Abbey, the Colosseum, Notre Dame and the Taj Mahal all bear such damage, on Chem1’s account.

Properties

Properties of calcium sulfate Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 24497.
Property	Value	Unit	Source
Formula (Hill order)	CaO₄S
Molar mass	136.134(28) g/mol	g/mol	IUPAC/CIAAW 2024
Calcium atoms	1
Sulfur atoms	1
Oxygen atoms	4
PubChem molecular weight	136.14 g/mol	g/mol	PubChem

Safety

Calcium sulfate is essentially harmless in its everyday forms, and Haz-Map describes it as relatively non-toxic; it does not burn. The caution is its dust. NIOSH, the US National Institute for Occupational Safety and Health, lists it as irritating eyes, skin and upper airways, with inflamed eyes and nose and nosebleeds among its effects. The NIOSH Pocket Guide card for calcium sulfate carries the workplace exposure limits, and the PubChem Laboratory Chemical Safety Summary collects its hazard classifications.

Questions

Why does plaster of Paris harden when water is added?

Plaster of Paris is gypsum with most of its water driven off by heat. Stirred into water, the powder takes that water back, rehydrates and becomes gypsum again, a hard solid, and a whole range of plaster products is formed this way. HSDB records that neither anhydrous calcium sulfate nor gypsum itself sets with water. Dentists use plaster for impressions, casts and dies, and medicine uses it for immobilising casts.

Why is calcium sulfate added to food?

Calcium sulfate is affirmed as generally recognised as safe (GRAS) for adding directly to human food in the United States. Its federal listing, 21 CFR 184.1230, names 14 jobs it may do there, among them firming agent, dough strengthener, leavening agent and nutrient supplement. Haz-Map counts gypsum among calcium supplements in food, and HSDB records it as a dietary calcium source for poultry.

How does gypsum help farm soil?

Quarried gypsum has been spread on fields to loosen the soil, to correct soils rich in sodium or toxic aluminium, and to supply calcium or sulfur where a soil lacks them. In the 1990s, after the Clean Air Act of 1990, scrubbers added to power-plant smokestacks produced growing amounts of gypsum too. The USDA Agricultural Research Service reports that this gypsum stops soil crusting, so more rain soaks in rather than running off.

What is phosphogypsum?

Phosphogypsum is the gypsum-like solid generated in making phosphoric acid: five tons of it for every ton of acid, according to the US Geological Survey. It is nearly identical to natural gypsum but may carry small amounts of sand, phosphate, fluorine and radium. Because of that radium content, federal regulations in the United States restrict both the use of phosphogypsum and research involving it.