Ettringite

Ettringite
Ettringite, 3.3×2.6×2.5 cm. Kalahari manganese fields, Northern Cape Province, South Africa
General
Category	Sulfate minerals
Formula (repeating unit)	Ca6Al2(SO4)3(OH)12·26H2O
Strunz classification	7.DG.15
Crystal system	Trigonal
Crystal class	Ditrigonal pyramidal (3m) H-M symbol: (3m)
Space group	P31c
Unit cell	a = 11.23, c = 21.44 [Å]; Z = 2
Identification
Color	Colorless, pale yellow, milky white
Crystal habit	Acicular growth, striated prismatic crystals; fibrous to cottonlike
Cleavage	Perfect on {1010}
Mohs scale hardness	2-2.5
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to opaque
Specific gravity	1.77
Optical properties	Uniaxial (-)
Refractive index	nω = 1.491, nε = 1.470
Birefringence	δ = 0.021
Ultraviolet fluorescence	Non-fluorescent
Solubility	Partially soluble in water
Alters to	Partially dehydration on atmospheric exposure, becomes opaque
References

Ettringite is a hydrous calcium aluminium sulfate mineral with formula: Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O. It is a colorless to yellow mineral crystallizing in the trigonal system. The prismatic crystals are typically colorless, turning white on partial dehydration. It is part of the ettringite-group which includes other sulfates such as thaumasite and bentorite.

Ettringite was first described in 1874 by J.Lehmann, for an occurrence near the Ettringer Bellerberg volcano, Ettringen, Rheinland-Pfalz, Germany. It occurs within metamorphically altered limestone adjacent to igneous intrusive rocks or within xenoliths. It also occurs as weathering crusts on larnite in the Hatrurim Formation of Israel. It occurs associated with portlandite, afwillite and hydrocalumite at Scawt Hill, Ireland and with afwillite, hydrocalumite, mayenite and gypsum in the Hatrurim Formation. It has also been reported from the Zeilberg quarry, Maroldsweisach, Bavaria; at Boisséjour, near Clermont-Ferrand, Puy-de-Dôme, Auvergne, France; the N’Chwaning mine, Kuruman district, Cape Province, South Africa; in the US, occurrences were found in spurrite-merwinite-gehlenite skarn at the 910 level of the Commercial quarry, Crestmore, Riverside County, California and in the Lucky Cuss mine, Tombstone, Cochise County, Arizona.