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Formula: Ca6Al2(SO4)3(OH)12.26H2O
Hydrated sulphate containing hydroxyl, ettringite group. Since
it is so highly hydrated it tends to be unstable and dehydrate over
time, alter and change colour (BC).
Crystal System: Trigonal
Specific gravity: 1.77 measured, 1.76 calculated
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.
Environments:
Metamorphic environments
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).
Very attractive, brilliant yellow ettringite crystals, some doubly terminated and some associated with ice-white
drusy calcite have been found. The ettringite occurs as crystals up to
6 cm in size, sometimes associated with pale blue brucite
(AJM 11.1.39-41, 10.1.38).
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).
The ettringite occurs as semi-transparent yellow crystals up to 4 cm long, some associated with small tabular
manganite crystals
(AJM 10.1.38).
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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