ettringite

{{Infobox mineral | name = Ettringite | boxwidth = | boxbgcolor = #bfbf50 | image = Ettringite-Oyelite-Ettringite-Kalahari-Namibia-136mm 0762closeup.jpg | imagesize = 260 | alt = | caption = Ettringite, Kalahari manganese fields, Northern Cape Province, South Africa | category = Sulfate minerals | formula = | IMAsymbol = Ett | molweight = | strunz = 7.DG.15 | dana = | system = Trigonal | class = Ditrigonal pyramidal (3m) H-M symbol: (3m) | symmetry = P31c | unit cell = a = 11.23, c = 21.44 [Å]; Z = 2 | color = Colorless, pale yellow, milky white | colour = | habit = Acicular growth, striated prismatic crystals; fibrous to cottonlike | twinning = | cleavage = Perfect on {100} | fracture = | tenacity = | mohs = 2–2.5 | luster = Vitreous | streak = White | diaphaneity = Transparent to opaque | gravity = 1.77 | density = | polish = | opticalprop = Uniaxial (−) | refractive = nω = 1.491, nε = 1.470 | birefringence = δ = 0.021 | pleochroism = | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence = Non-fluorescent | absorption = | melt = | fusibility = | diagnostic = | solubility = Partially soluble in water | impurities = | alteration = Partially dehydration on atmospheric exposure, becomes opaque | other = | prop1 = | prop1text = | references = }} Ettringite is a hydrous calcium aluminium sulfate mineral with formula: . 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.

==Discovery and occurrence== thumb|left|170px|Ettringite, 6.5 × 3.2 cm. N'Chwaning Mines, Kalahari manganese fields, Northern Cape Province, South Africa Ettringite was first described in 1874 by , 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, Arizona.