Ettringite as mineral was first described in 1874 by J Lehmann for a specimen collected at the the Ettringer Bellerberg Volcano, Ettringen, Rheinland-Pfalz in Germany. Ettringite as mineral also occurs in Bavaria, in the Puy-de-Dôme in France, and at the N’Chwaning and Wessels mines, in the Northern Cape, South Africa, as well as in California, Arizona and New Jersey in the US.
Ettringite is a hydrous calcium aluminium sulphate mineral with formula: Ca6Al2(SO4)3(OH)12·26H2O. It is a colourless, yellow to milky white mineral crystallising in the trigonal system. The prismatic crystals are typically colourless, turning white on partial dehydration. It is part of the ettringite-group which includes other sulfates such as thaumasite and bentorite.
Ettringite is rare and is sought after by collectors. The largest and most beautiful ettringite crystals ever found came from the N’Chwaning manganese mine. In 2004, 2 to 3 cm thick, vibrant yellow ettringite crystals were found in a pocket at the Wessels mine, which appears to have numbered several dozen specimens.
Ettringite occurs as a natural mineral and is technically important as a primary constituent of hydration of Portland cement concrete and super sulphated cements, and in its use as satin white for coating paper. Ettringite is known to dehydrate and its crystal structure changes when heated.
Ettringite, Ca6Al2(SO4)3(OH)12.26H2O, or C6A¯S3H32 as it is known in cement chemistry notation, is a major phase of interest in cement science as a hydration product and in polluted soil treatment since its structure can accommodate with many hazardous cations.
Ettringite also plays an important role in sulphate removal from water, in dental cements in polluted soils treatment and more generally as a phase for waste ions immobilisation.
References:
Thiery V, Trincal V, Davy CA (2017) Journal of Microscopy, pg 10-17
Cairncross B, Beukes NJ (2013) The Kalahari Manganese Field – The adventure continues…
Http://www.en.wikipedia.org/wiki/ettringite
https://www.le-comptoir-geologique.com/ettringite-encyclopedie