Pentahydrite
Pentahydrite is an IMA-recognized mineral species in the sulfate class, with the chemical formula Mg(SO₄)·5H₂O.
It crystallizes in the triclinic crystal system (space group P-1). It is a member of the Chalcanthite structural group.
Key properties: specific gravity 1.896; optically biaxial.
IMA symbol Phy, grandfathered status — known before the IMA’s founding, named in 1951.
An efflorescence on shales (Grand Junction, Colorado, USA); an efflorescence on mine timbers (Virginia City, Nevada, USA); in acid sulfate soils (Basse-Casamance Valley, Senegal).
It is found associated with Alunogen, chalcanthite.
Its name comes from the Greek penta, for five, alluding to the amount of water in its composition.
Index entry. This entry combines official IMA/RRUFF species data with published physical-property data — a full written entry hasn’t been published yet.
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