Formula: Ca6Al2(SO4)3(OH)12.26H2O
Hydrated sulphate containing hydroxyl. Since it is so highly hydrated it tends to be unstable and dehydrate over time, alter and change colour (BC).
Specific gravity: 1.77
Hardness: 2 to 2½
Streak: White
Colour: Colourless to white, yellow to light brown, colourless in transmitted light
Solubility: Partially decomposed by water, giving an alkaline solution. Easily soluble in dilute acids.

Hydrothermal environments

Ettringite occurs in metamorphosed limestone near igneous contacts or in xenoliths (HOM).

At the Hatrurim Formation, Negev, Israel, ettringite occurs as weathering crusts on larnite rocks associated with afwillite, hydrocalumite, mayenite and gypsum (HOM).

There are two co-type localities, both in the Ettringer Feld, Southern lava flow, Bellerberg volcano, Ettringen, Mayen, Eifel Volcanic Fields, Germany. Here ettringite occurs in cavities in metamorphosed limestone inclusions in basaltic rocks rich in leucite and nepheline (Dana, Mindat).

At the Wessels mine, Hotazel, Kalahari manganese field, Northern Cape, South Africa, ettringite is associated with the extremely rare calcium copper sulphate vonbezingite (R&M 94.3.250-253).

At the N'Chwaning II Mine, Kuruman, Kalahari manganese field, Northern Cape, South Africa, ettringite occurs as a secondary alteration mineral associated with manganite, calcite and hematite. It has also been found here on brucite (BC).

At the Lucky Cuss Mine, Arizona, USA, ettringite is an alteration product of calcium-aluminium silicates (Dana).

At the Commercial quarry, Crestmore, California, USA, ettringite occurs as a vein filling in spurrite-gehlenite-merwinite skarn (AM 48.1394-1396) associated with calcite and afwillite (AM 45.1275-1278). Many of the ettringite crystals are covered by oriented overgrowths of thaumasite, occasionally enclosed by a second generation of ettringite (AM 48.1394-1396).

At Scawt Hill, Larne, County Antrim, Northern Ireland, UK, ettringite is associated with portlandite, afwillite and hydrocalumite (HOM).

At Franklin, New Jersey, USA, ettringite occurs with andradite and manganese-rich biotite (Dana).

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