Anhydrite
Anhydrite is CaSO4, calcium sulphate without water: orthorhombic, hardness 3 to 3.5, density 2.98, three cleavages at right angles. Angelite is anhydrite.
Anhydrite is calcium sulphate without water, Ca(SO4) on the September 2026 IMA list: orthorhombic, hardness 3 to 3.5, density 2.98, with three cleavages at right angles that break it into near-cubes. It holds no water against gypsum’s 20.93%, and 29.44% calcium against gypsum’s 23.28%. Angelite is a trade name for blue-grey anhydrite (International Gem Society).
In short
- The row and the sheet agree. The September 2026 IMA-CNMNC Master List lists anhydrite as Ca(SO4), status G, year 1804, country Austria. The Handbook of Mineralogy anhydrite sheet writes CaSO4 and derives the name from the Greek for without water, in contrast to the hydrous calcium sulphates.
- No water, more calcium. Worked on standard atomic weights, CaSO4 (136.13) is 41.19% CaO and 58.81% SO3, the sheet’s ideal column, and 29.44% calcium. Gypsum, CaSO4·2H2O, is 23.28% calcium. The water count across anhydrite, bassanite and gypsum is the rule on is gypsum a mineral and is not repeated here.
- The unequal-faces rule. Anhydrite and halite both break into near-cubes, and their sheets share four evaporite localities. Anhydrite’s three cleavages differ in quality and lustre: perfect and pearly on {010}, nearly perfect and vitreous on {100}, good to imperfect and vitreous to greasy on {001}. Halite’s one cleavage form, {001}, gives three identical directions. Anhydrite is also harder, 3 to 3.5 against 2 to 2.5.
- Gypsum’s partner. The anhydrite sheet says it commonly forms by dehydration of gypsum; the gypsum sheet says gypsum forms by hydration of anhydrite. A 20.00 g piece reads 13.29 g in water as anhydrite and 11.37 g as gypsum, the figures on the gypsum page. Telling the two apart is on is anhydrite gypsum.
- Angelite is anhydrite. The International Gem Society calls angelite a translucent blue-grey anhydrite variety, discovered in Peru and introduced to gem markets in the late 1980s. A script finds no angelite row on the IMA list: the label should read anhydrite.
| Species | IMA formula | System | Cleavage on the sheet | Hardness | Density | 20.00 g piece in water | Optics |
|---|---|---|---|---|---|---|---|
| Anhydrite | Ca(SO4) | Orthorhombic | Three, unequal: {010} perfect, {100} nearly perfect, {001} good to imperfect; pseudocubic fragments | 3–3.5 | 2.98 | 13.29 g | Biaxial (+), 1.567–1.618 |
| Halite | NaCl | Cubic | {001} perfect: three equal directions | 2–2.5 | 2.168 | Not weighed (soluble) | Isotropic, n = 1.5443 |
| Gypsum | Ca(SO4)(H2O)2 | Monoclinic | {010} perfect, {100} distinct; flexible flakes | 1.5–2 | 2.317 | 11.37 g | Biaxial (+), 1.521–1.530 |
| Calcite | Ca(CO3) | Hexagonal | {10-11} perfect | 3 | 2.7102 | 12.62 g | Uniaxial (−) |
| Celestine | Sr(SO4) | Orthorhombic | One perfect, one good, one poor | 3–3.5 | 3.97 | 14.96 g | Biaxial (+) |
| Baryte | Ba(SO4) | Orthorhombic | One perfect, one less perfect, one imperfect | 3–3.5 | 4.50 | 15.56 g | Biaxial (+) |
What is anhydrite? The IMA row and the sheet
The row. The September 2026 IMA-CNMNC Master List gives Ca(SO4), status G for grandfathered, year 1804, country Austria, with a second reference in Canadian Mineralogist 13 (1975), 289. The country column records where a mineral was first described; it is not a type-locality field. The sheet gives type material at the Mining Academy, Freiberg, Germany, number 16538.
On the sheet. Orthorhombic. Crystals tabular on {010}, {100} or {001}, or equant with large pinacoidal faces, elongated along [100] or [001], to 15 cm, with about 40 forms recorded; typically granular, nodular, parallel or divergent fibrous, or massive. Twinning simple or repeated on {011}, common. Fracture uneven to splintery; brittle. Hardness 3 to 3.5; density 2.98 measured, 2.95 calculated. Colourless to pale blue or violet if transparent; white, mauve, rose, pale brown or grey from included impurities. Streak white to pale grey. Biaxial (+), α = 1.567 to 1.574, β = 1.574 to 1.579, γ = 1.609 to 1.618; violet varieties are pleochroic.
Where it forms. The Occurrence field: a major component in sedimentary evaporite deposits and in the cap rocks above salt domes, commonly formed by dehydration of gypsum; in igneous rocks, fumarolic deposits and seafloor hydrothermal chimneys; also an alteration product in hydrothermal deposits. Associates: gypsum, halite, sylvite, polyhalite, dolomite, calcite, magnesite, celestine and sulphur.
How much water and calcium is in anhydrite? The calcium check
Worked. CaSO4 has a formula mass of 136.13 on standard atomic weights. Calcium, 40.078 of it, is 29.44%; sulphur 23.55%; as oxides, CaO 41.19 and SO3 58.81, the ideal column the sheet prints. The sheet’s Yonaibata mine, Japan, analysis reads SO3 58.37 and CaO 41.13, which is 29.40% calcium and 23.38% sulphur, and corresponds, after deducting CO2 as calcite, to Ca0.99S1.00O4.
The calcium check: anhydrite is 29.44% calcium and gypsum 23.28%, so a gram of anhydrite carries 1.265 times the calcium of a gram of gypsum. The International Gem Society says anhydrite exceeds gypsum in calcium content; the arithmetic confirms it, and the whole difference is the water. Gypsum, CaSO4·2H2O, is 20.93% water; anhydrite none.
What this page does not repeat. The three calcium sulphate rows on the IMA list, with two, half and no water, are the subject of is gypsum a mineral; the solubility figures for gypsum and anhydrite are on is selenite water soluble. This page gives no recipe for keeping either dry.
Anhydrite or halite? The unequal-faces rule
The unequal-faces rule: a white or colourless evaporite fragment bounded by three pairs of faces at right angles is anhydrite if the three pairs differ in quality and lustre, and halite if they are alike. The anhydrite sheet gives {010} perfect with a pearly lustre, {100} nearly perfect and vitreous, {001} good to imperfect and vitreous to greasy, yielding pseudocubic fragments. The halite sheet gives one cleavage form, {001}, perfect; in a cubic mineral that one form is three equivalent directions, so every face of a halite cleavage cube is the same kind of face.
Why the question arises. The two sheets name the same evaporite districts. A script over the Distribution fields finds Hall in Tirol, Stassfurt-Leopoldshall in Saxony-Anhalt, the Salt Range in Punjab and the Carlsbad potash district in New Mexico on both. The gem society says anhydrite’s cleavages meet at right angles and reveal cube-like forms, which gave it the name cube spar.
The second and third numbers. Hardness: anhydrite 3 to 3.5, halite 2 to 2.5, so anhydrite is a full step harder at the closest values; the method is on how to test mineral hardness properly. Density: 2.98 against 2.168, so a cube of anhydrite weighs 1.37 times a halite cube of the same size. Halite is not weighed in water here, because its sheet says it is soluble; its storage is covered on halite. Optics: halite is isotropic, anhydrite birefringent, 0.042 to 0.044, worked from the sheet’s indices.
Anhydrite and gypsum: one turns into the other
Both directions are on the sheets. The anhydrite sheet says anhydrite commonly forms by dehydration of gypsum; the gypsum sheet says gypsum forms in soils and salt deposits directly by evaporation or later by hydration of anhydrite. The International Gem Society says anhydrite exposed to water over time alters into the much more common gypsum. Neither sheet gives a rate, and this page gives none.
The numbers agree with the gypsum page. Hardness 3 to 3.5 against 1.5 to 2; density 2.98 against 2.317; in water, 13.29 g against 11.37 g for a 20.00 g piece. The volume arithmetic of the change is on gypsum. How to tell the two apart, and where the tests fail, is on is anhydrite gypsum.
Angelite, cube spar and the other names
Angelite. The International Gem Society anhydrite article (Arem, updated February 2022) calls angelite, or angeline, a translucent blue-grey anhydrite variety, discovered in Peru and introduced to worldwide gem markets in the late 1980s, used for beads, spheres and carvings. A script over the September 2026 IMA list finds no angelite row. Angelite is anhydrite; the trade name is not a species.
Blue Egyptian anhydrite. The same article says the ancient Egyptians used white anhydrite and a blue-tinted variety, and that the ancient source of the blue material has never been found. It says blue and violet colours in anhydrite disappear when heat treated and are restored by gamma irradiation, and that the colours therefore possibly arise from natural radiation in the earth. That is the gem society’s hedged inference, printed as such.
Where the sources differ. The Handbook sheet records no fluorescence; the gem society’s panel gives red under longwave UV, for German material. The sheet lists three cleavages; the panel lists two. Both are printed here. Why minerals glow at all is on fluorescent minerals explained.
Where anhydrite is found
The sheet’s Distribution field. Numerous occurrences worldwide. In Austria, Hall in Tirol, Ischl and Hallein in Salzburg, and Aussee in Styria. In Germany, Stassfurt-Leopoldshall and Douglashall near Westeregeln, Saxony-Anhalt, and Wathlingen near Celle, Lower Saxony. Gemmy crystals from the Simplon Tunnel, Valais, Switzerland. In Italy, the Campiano mine, Boccheggiano district, Tuscany, and Vesuvius. The Faraday mine, Bancroft, Ontario. In the USA, Paterson, New Jersey; the Fairfax quarry, Centreville, Virginia; large deposits in the Carlsbad potash district, New Mexico; the Boiling Salt Dome, Texas. Also Naica, Chihuahua, Mexico; the Salt Range, Punjab, India; Mt. Pinatubo, Philippines; and Morococha, Peru.
On this site. No British locality is on the sheet, and this page adds none. The species pages are listed in the species index, including celestine and baryte, the strontium and barium sulphates. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.