Gypsum
Gypsum is CaSO4·2H2O, hardness 1.5 to 2, density 2.317, flexible cleavage flakes. Selenite, satin spar, alabaster and desert rose are forms of it, not species.
Gypsum is hydrated calcium sulphate, CaSO4·2H2O: monoclinic, hardness 1.5 to 2, density 2.317, with one perfect cleavage giving flexible, inelastic flakes. Selenite, satin spar, alabaster and desert rose are names for forms of gypsum, not separate species; desert rose can also be baryte. The Handbook records crystals to 17 m.
In short
- One species, four names. The International Gem Society calls colourless, transparent gypsum crystals selenite, the massive fibrous variety satin spar and the massive granular variety alabaster. Its desert rose is rose-shaped tabular crystals of baryte or gypsum with trapped sand. None of the four names is a row on the September 2026 IMA-CNMNC Master List.
- The 12-gram line. Of the seven white-to-colourless minerals in the table, read from their Handbook of Mineralogy sheets, gypsum is the only one under 12 g in water for a 20.00 g piece: 11.37 g. The next lightest is calcite, at 12.62 g. Gypsum is also the only one of the seven at hardness 2 or below, and the only one whose tenacity is flexible.
- Worked composition: on standard atomic weights CaSO4·2H2O is 32.57% CaO, 46.50% SO3 and 20.93% water, exactly the sheet’s ideal column. Calcium is 23.28% of the mass.
- Anhydrite to gypsum, worked. The sheet says gypsum forms partly by hydration of anhydrite. From the two sheets’ densities, a mole of anhydrite occupies 45.68 cm³ and a mole of gypsum 74.30 cm³: 62.7% more solid volume, and 100 g of anhydrite becomes 126.5 g of gypsum.
- Crystals to 17 m. The sheet records huge crystals in a cave complex in the Naica Pb–Ag mine, Chihuahua, Mexico, and calls gypsum the most common sulfate mineral.
| Species | Formula | Hardness | Density (measured) | Best cleavage on the sheet | Tenacity | 20.00 g piece in water |
|---|---|---|---|---|---|---|
| Gypsum | CaSO4·2H2O | 1.5 to 2 | 2.317 | {010}, perfect | Flexible, inelastic | 11.37 g |
| Calcite | CaCO3 | 3 | 2.7102 | {10-11}, perfect | Brittle | 12.62 g |
| Glauberite | Na2Ca(SO4)2 | 2.5 to 3 | 2.75 to 2.85 | {001}, perfect | Brittle | 12.73 to 12.98 g |
| Aragonite | CaCO3 | 3.5 to 4 | 2.95 | {010}, distinct | Brittle | 13.22 g |
| Anhydrite | CaSO4 | 3 to 3.5 | 2.98 | {010}, perfect | Brittle | 13.29 g |
| Celestine | SrSO4 | 3 to 3.5 | 3.97 | {001}, perfect | Brittle | 14.96 g |
| Baryte | BaSO4 | 3 to 3.5 | 4.50 | {001}, perfect | Brittle | 15.56 g |
What gypsum is: calcium sulphate with water in the crystal
The formula carries two waters. The Handbook of Mineralogy writes CaSO4·2H2O; the September 2026 IMA-CNMNC Master List writes Ca(SO4)(H2O)2, status G, grandfathered, with no year and country unknown. Worked on standard atomic weights the formula mass is 172.16, and the water is 20.93% of it. Those figures reproduce the sheet’s ideal column; its Zaleschiki, Ukraine, analysis is within 0.5 percentage points of them: H2O 20.82, CaO 32.36, SO3 46.00.
Where it forms, per the sheet: a common constituent of sedimentary rocks, particularly marine salt deposits; in soils; by hydration of anhydrite; by reaction between sulphuric acid and carbonate rock in oxidising sulphide deposits; by volcanic gases acting on calcium-bearing rock; and as efflorescences in mines and speleothems in caves. Its associates are halite, celestine, calcite, aragonite, anhydrite, dolomite and sulphur.
The name is from the Greek for calcined gypsum used as plaster. Gypsum is the 2 on Mohs’ scale in the USGS listing, and its sheet gives 1.5 to 2, varies with direction. Questions about water and heat are on is selenite water soluble and are not repeated here.
Selenite, satin spar, alabaster and desert rose: what each name means
All four are gypsum names, by the International Gem Society’s definitions:
- Selenite: colourless, transparent gypsum crystals. The IGS derives the name from the Greek for moon, for the pearly lustre on cleavage surfaces; the Handbook records gypsum as pearly on {010} cleavages.
- Satin spar: the massive, fibrous variety, named for its satiny lustre. The Handbook lists a fibrous habit and a lustre silky if fibrous, and a fracture splintery parallel [001].
- Alabaster: the massive, granular variety, carved for thousands of years. The IGS notes that the word is also used for a form of opalescent glass.
- Desert rose: in desert regions, minerals such as barite and gypsum forming tabular crystals in rose-like shapes, with trapped sand. A desert rose label is therefore not proof of gypsum; baryte weighs 15.56 g in water for 20.00 g against gypsum’s 11.37 g.
None of the four is a species. We searched the IMA list for selenite, satin, alabaster and desert rose as row names and found none; the label should read gypsum (selenite) and so on. Selenite questions are on why is my selenite turning yellow.
How to tell gypsum from calcite, anhydrite and other white minerals
The rule: under 12 g in water for a 20.00 g piece, among the seven in the table, is gypsum. Method: the hardness, density, cleavage and tenacity fields of the seven Handbook sheets were extracted by script, and the in-water weight worked as 20 − 20/D. Gypsum gives 11.37 g, then calcite 12.62 g and glauberite 12.73 to 12.98 g. The method is on how to measure specific gravity at home. The rule covers these seven; it is not a survey of every white mineral.
Hardness and tenacity agree. Gypsum is the only one of the seven at 2 or below, and the only one whose sheet says flexible: a thin cleavage flake bends and stays bent. The other six are brittle. Calcite at 3 scratches gypsum. The IGS separates the two by specific gravity, 2.71 to 2.94 for calcite against 2.30 to 2.33 for gypsum.
Against anhydrite, its water-free relative: anhydrite is 3 to 3.5, 2.98, brittle, and cleaves into pseudocubic fragments, against gypsum’s 1.5 to 2 and flexible flakes. Aragonite and celestine are heavier still. Hardness method: how to test mineral hardness properly.
Gypsum and anhydrite: the hydration arithmetic
The two sheets give the inputs. Anhydrite, CaSO4, has a formula mass of 136.13 and a measured density of 2.98. Gypsum, CaSO4·2H2O, has 172.16 and 2.317. One mole therefore occupies 136.13 ÷ 2.98 = 45.68 cm³ as anhydrite and 172.16 ÷ 2.317 = 74.30 cm³ as gypsum.
Result: hydrating anhydrite to gypsum gives 62.7% more solid, and the mass rises by the ratio 172.16 ÷ 136.13, so 100 g of anhydrite becomes 126.5 g of gypsum. Run backwards, gypsum that loses all its water to anhydrite keeps 61.5% of its volume. These are ideal-formula figures from two measured densities, not a field measurement, and the sheets give no rate for either reaction.
Why it matters on a label. Both sheets name the same evaporite districts: Aussee, Styria, and Hall, Tirol, appear on both. A white sulphate from those mines can be either, and the 20.00 g in-water weight separates them by 1.92 g: 11.37 g against 13.29 g.
Gypsum twins and giant crystals: where fine gypsum comes from
Twinning, from the rendered sheet: very common by contact on {100}, forming cruciform and V-shaped twins; butterfly or heart-shaped twins by contact on {-101}; rare on {-209}. Crystals are coarsely striated parallel to [001], lenticular in rosettes, may be curved, bent. Twin laws in general are on mineral twinning explained.
The Distribution field, for especially fine or large specimens: Racalmuto, Girgenti and Cianciana, Sicily; the Eisleben–Mansfeld–Sangerhausen district and Königslutter, Germany; Bex, Switzerland; Aussee and Hall, Austria; Zaragoza, Spain; Montmartre, Paris; large crystals from Tarnobrzeg, Poland; Meskerabad, Iran; and Cloncurry, Queensland. In the Americas it names Naica and the San Antonio mine, Santa Eulalia, Chihuahua; the El Teniente mine, Chile; South Wash, Wayne Co., Utah; Great Salt Plains, Oklahoma; and speleothems in Lechuguilla Cave, New Mexico.
No British locality is on the gypsum sheet, and we add none. The rest of the reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.