Ettringite

Rare hydrous calcium aluminium sulfate

Ettringit is mostly yellow or colourless. Milky white crystals are rare.

ettringit ettringite
Ettringite from the N´Chwaning mines, South Africa

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Origin of name: after the type locality at Ettringen, Rhineland-Palatinate, Germany.

Synonyms and trade names: Woodfordite, Chalkomorphite

Can be confused with: other yellow or colourless minerals. Identification is quite straightforward due to the very low values for hardness, density and refractive index.

Localities: in more than 100 deposits worldwide. Most deposits are in Europe (some in Austria), some in the Middle East, Russia, Japan, the USA and South Africa. One deposit is reported from Amatuku Atoll in Tuvalu.

However, only ettringite from the N´Chwaning mines in the Kalahari manganese field, Northern Cape Province, South Africa, is of gemmological interest.

Handling: due to the very low hardness, perfect cleavage and high sensitivity to acids ettringite is entirely unsuitable for jewellery use!

Worth knowing: ettringit contains up to 46% (by weight) of water making it one of the most hydrous minerals

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Gemmological Properties of Ettringite

Formula:
Ca6Al2(SO4)3(OH)12·26H2O
Crystal system:
trigonal
Mohs hardness:
2 - 2.5
Specific gravity:
1.77
Refractive index:
1.470 - 1.491
Max. Birefringence:
0.021
Luminiscence:
none
Lustre:
vitreous
Cleavage:
perfect
Fracture:
uneven
Colour:
pale yellow, colourless, milky white