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Anhydrite

CaSO4

Anhydrite is a sulfate mineral prized by collectors for its exceptional color range, with notable Chinese occurrences.

Elemental Composition (by mass)
ElementMass %Visual
O Oxygen47.01%
Ca Calcium29.44%
S Sulfur23.55%
Computed from simplified end-member formula. Solid-solution series, water content, and trace substitutions cause real-world variation.
IMA Abbreviation (Whitney-Evans 2010)
Anh
→ Anhydrite
Anhydrous Ca sulfate
Standard symbol from American Mineralogist (Whitney & Evans, 2010). Used in thin-section labeling, phase diagrams, and IMA-style species records.
Pseudomorph Relationships
Replaced by — this mineral commonly becomes:
→ Gypsum hydration
Anhydrite (CaSO₄) hydrates to gypsum (CaSO₄·2H₂O) with ~63% volume increase — important in salt cap rock.
Salt dome cap rocks.
Replaces — this mineral is often a pseudomorph after:
← Gypsum dehydration
Reverse — dehydration of gypsum under burial.
Deep sedimentary basins.
A pseudomorph (Greek "false form") is a mineral with the external shape of another species — the chemistry has changed but the crystal habit is inherited.
PolymorphsShares the formula CaSO4 with: Gypsum — same chemistry, different crystal structure.
Mohs 3–3.5
Vickers (~) 170 HV
Knoop (~) 185 HK
Geological setting
SedimentaryEvaporite
Element composition by mass

Formula: CaSO₄ · molar mass: 136.13 g/mol

O 47.01%
Ca 29.44%
S 23.55%

Computed from atomic weights (IUPAC 2021). Site-occupancy groups (Fe,Mn) split equally.

Mohs Hardness 3–3.5

Anhydrite sits at 3–3.5 on the Mohs scale — can be scratched by a steel knife.

Colors:
Streak
White
Crystal system
Orthorhombic
SulfatesSulfates
TL;DR · 1 min read
Anhydrite (CaSO₄) is the anhydrous calcium sulfate, the dehydration counterpart of gypsum (CaSO₄·2H₂O). It forms in evaporite basins by precipitation from saturated brines and as a primary phase in marine evaporite sequences.

Anhydrite (CaSO₄) is the anhydrous calcium sulfate, the dehydration counterpart of gypsum (CaSO₄·2H₂O). It forms in evaporite basins by precipitation from saturated brines and as a primary phase in marine evaporite sequences. Anhydrite hydrates to gypsum at the surface, expanding by ~63% — a problem in salt-mining and tunneling.

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