Is anhydrite gypsum?
No. Anhydrite is CaSO4, hardness 3 to 3.5; gypsum is CaSO4·2H2O, hardness 1.5 to 2. A refractometer decides at the 1.55 line; a mixed specimen fools the scale.
No. Anhydrite is calcium sulphate without water, CaSO4, hardness 3 to 3.5, density 2.98; gypsum is CaSO4·2H2O, 20.93% water, hardness 1.5 to 2, density 2.317. They are separate IMA species that turn into each other. A refractometer decides: every gypsum index is 1.530 or less, every anhydrite index 1.567 or more.
In short
- Two rows, one calcium sulphate. The September 2026 IMA-CNMNC Master List lists anhydrite as Ca(SO4), G, 1804, Austria, and gypsum as Ca(SO4)(H2O)2, G, no year. The anhydrite sheet says it commonly forms by dehydration of gypsum; the gypsum sheet says gypsum forms by hydration of anhydrite.
- The 1.55 line. On their Handbook sheets, gypsum’s indices are 1.521, 1.523 and 1.530; anhydrite’s run from 1.567 to 1.618. A reading below 1.55 is not anhydrite; a reading of 1.567 or above is not gypsum. The gap is 0.037. This is a refractometer decision, not a home test.
- In the hand. Anhydrite is 3 to 3.5 and brittle, with three cleavages giving near-cubes; gypsum is 1.5 to 2, with one perfect cleavage giving flexible flakes. A 20.00 g piece reads 13.29 g in water as anhydrite and 11.37 g as gypsum; that separation belongs to the gypsum page and is linked, not repeated.
- The mixed-piece trap. Because each turns into the other, one specimen can be both. Worked on the two densities, a piece half gypsum and half anhydrite by mass reads 12.33 g, which is neither mineral’s figure and sits 0.12 g below quartz. A mixed piece fails the weight test; it does not fail the 1.55 line, which reads each grain.
- Same mines. A script over the two Distribution fields finds Hall in Tirol, Aussee in Styria and Naica in Chihuahua on both. A label from those places does not decide the species. Angelite is anhydrite and selenite is gypsum.
| Property | Anhydrite | Gypsum | Decides? |
|---|---|---|---|
| IMA formula | Ca(SO4) | Ca(SO4)(H2O)2 | On paper only |
| Water by mass, worked | 0% | 20.93% | Not by eye |
| Crystal system | Orthorhombic | Monoclinic | Needs crystals |
| Hardness | 3–3.5 | 1.5–2 | Yes, on clean material |
| Cleavage | {010} perfect, {100} nearly perfect, {001} good to imperfect: pseudocubic fragments | {010} perfect, {100} distinct, {011} | Usually |
| Tenacity | Brittle | Flexible, inelastic | Yes, on a thin flake |
| Density | 2.98 | 2.317 | Yes, unless mixed |
| 20.00 g piece in water | 13.29 g | 11.37 g | Yes, unless mixed |
| Refractive indices | α 1.567–1.574, β 1.574–1.579, γ 1.609–1.618 | α 1.521, β 1.523, γ 1.530 | Yes: the 1.55 line |
| Birefringence, worked | 0.042–0.044 | 0.009 | Yes, with optics |
| Lustre | Pearly on {010}, vitreous to greasy on {001}, vitreous on {100} | Subvitreous, pearly on {010}, silky if fibrous | No |
| Colour | Colourless to pale blue or violet; white, mauve, rose, pale brown, grey | Colourless, white; yellow, tan, blue, pink, brown, grey, black if impure | No |
| UV on the sheet | Nothing recorded | Nothing recorded | No |
| Shared localities | Hall, Aussee, Naica | Hall, Aussee, Naica | No |
Is anhydrite the same as gypsum? Two species that swap
No. They are two species of calcium sulphate, one dry and one with two waters in the crystal. The September 2026 IMA-CNMNC Master List gives anhydrite, Ca(SO4), status G, 1804, Austria, and gypsum, Ca(SO4)(H2O)2, status G with no year. Worked on standard atomic weights, gypsum is 20.93% water and anhydrite none. The third calcium sulphate row, bassanite, and the water-count rule are on is gypsum a mineral.
Why they are confused. The sheets describe each forming from the other: anhydrite commonly by dehydration of gypsum, gypsum by hydration of anhydrite. Both are white evaporite minerals of salt deposits, both cleave, and both sheets list the other as an associate. The International Gem Society calls gypsum anhydrite’s much more abundant alter ego and says anhydrite exposed to water over time alters into it.
How do you tell anhydrite from gypsum? The 1.55 line
The 1.55 line: every refractive index on the gypsum sheet is 1.530 or less, and every index on the anhydrite sheet is 1.567 or more, so a reading below 1.55 rules out anhydrite and a reading at 1.567 or above rules out gypsum. The gap between gypsum’s highest index and anhydrite’s lowest is 0.037. The gem society’s anhydrite panel, 1.570 to 1.614, sits inside the sheet’s range.
Birefringence says the same. Worked from the sheets, anhydrite’s γ − α is 0.042 to 0.044 and gypsum’s 0.009: about five times greater. The International Gem Society gives 0.044 for anhydrite. Both numbers need a refractometer or a polarising microscope; this page offers no home method for either.
Where the line fails. Bassanite, the half-water calcium sulphate, has α 1.550 to 1.559 and γ 1.577 to 1.584 on its Handbook sheet, so it reads inside the gap and overlaps anhydrite; its sheet describes needles to 0.1 mm, not hand specimens. halite, isotropic at n = 1.5443, also reads below the line.
In the hand: hardness, cleavage and weight
Hardness. Anhydrite 3 to 3.5; gypsum 1.5 to 2, varying with direction. At the closest values anhydrite is a full Mohs step harder; calcite, at 3, matches the bottom of anhydrite’s range and is harder than gypsum. The method is on how to test mineral hardness properly.
Cleavage and tenacity. Anhydrite has three cleavages, perfect on {010}, nearly perfect on {100}, good to imperfect on {001}, and breaks into pseudocubic fragments; it is brittle. Gypsum has one perfect cleavage, on {010}, and its tenacity is flexible, inelastic: a thin flake bends and stays bent. Why anhydrite’s near-cubes are not halite is on anhydrite.
Weight. A 20.00 g piece reads 13.29 g in water as anhydrite and 11.37 g as gypsum. That separation, and the volume change between the two, are worked on the gypsum species page and are not this page’s asset. The method is on how to measure specific gravity at home; questions about water and selenite are on is selenite water soluble.
When the tests fail: the mixed-piece trap
The mixed-piece trap: a specimen that is part gypsum and part anhydrite reads between the two in water, and at about 35 to 44% gypsum by mass it reads like calcite or quartz. Worked by adding volumes from the two densities: a 20.00 g piece that is a quarter gypsum reads 12.81 g, half gypsum 12.33 g, three quarters 11.85 g. At 34.8% gypsum it reads calcite’s 12.62 g; at 43.5% it reads quartz’s 12.45 g. The sheets describe each forming from the other, so a partly converted piece is consistent with them.
What still works. The 1.55 line reads a single grain or face, so it identifies each part of a mixed piece. Hardness, too, is a test of the spot scratched, not of the whole specimen. Only the bulk weight averages the two.
Locality does not decide. A script over the two Distribution fields finds three places named on both sheets: Hall in Tirol, Aussee in Styria and Naica in Chihuahua. A fourth word, Carlsbad, appears on both but names different sites, the potash district on the anhydrite sheet and Carlsbad Caverns National Park on the gypsum sheet.
Angelite and selenite: the trade names
Angelite is anhydrite. The International Gem Society calls angelite a translucent blue-grey anhydrite variety, discovered in Peru. Selenite is gypsum. The same society, as quoted on the gypsum page, calls colourless, transparent gypsum crystals selenite. A script over the September 2026 IMA list finds no row for either name.
What that means on a label. A blue-grey carving labelled angelite is anhydrite, hardness 3 to 3.5; a clear blade labelled selenite is gypsum, 1.5 to 2. Yellowing selenite is discussed on why is my selenite turning yellow. The identification guides are in the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.