Ettringite - N'Chwaning Mines, Kalahari manganese fields, Northern Cape Province, South Africa (6.5x3.2 cm) — Photo: Didier Descouens · CC BY-SA 3.0
Ettringite
A hydrous calcium-aluminium sulfate forming colorless trigonal prisms that whiten as they dehydrate — important in cement chemistry.
Intro adapted from Wikipedia (CC BY-SA).
Ettringite is an IMA-recognized mineral species in the sulfate class, with the chemical formula Ca₆Al₂(SO₄)₃(OH)₁₂·27H₂O.
It crystallizes in the hexagonal crystal system (space group P31c). It is a member of the Ettringite structural group.
Key properties: Mohs hardness 2–2.5; specific gravity 1.77; Vitreous luster; refractive index 1.481; optically uniaxial.
IMA symbol Ett, IMA status: approved, named in 1874.
Found in metamorphosed limestone near igneous contacts or in xenoliths; as weathering crusts on larnite rocks (Hatrurim Formation, Israel).
It is found associated with Portlandite, afwillite, hydrocalumite (Scawt Hill, Ireland); afwillite, hydrocalumite, mayenite, gypsum (Hatrurim Formation, Israel).
The name refers to the occurrence near Ettringen, Germany.