Brown crystals of the rare mineral hydrocalumite on a contrasting matrix. From: Kopeisk, Chelyabinsk Oblast', Urals Region, Russian Federation. — Photo: David Hospital · CC BY-SA 4.0
Hydrocalumite
Hydrocalumite is an IMA-recognized mineral species in the carbonate class, with the chemical formula Ca₄Al₂(OH)₁₂(Cl,CO₃,OH)₂·4H₂O.
It crystallizes in the monoclinic crystal system (space group P2/c). It is a member of the Hydrotalcite structural group.
Key properties: Mohs hardness 3; specific gravity 2.09–2.15; Vitreous, pearly on cleavages luster; optically biaxial.
IMA symbol Hcl, grandfathered status — known before the IMA’s founding, named in 1934.
A rare late-stage hydrothermal mineral in skarns formed from contact metamorphism of limestone or in xenoliths in lava.
It is found associated with Afwillite, portlandite, ettringite, larnite, plombièrite, hydrogrossular, calcite.
The name refers to a HYDROus mineral containing CAlcium and ALUMinum.