Formula: Ca3Si(OH)6(CO3)(SO4).12H2O
Compound sulphate, ettringite group
Crystal System: Hexagonal
Specific gravity: 1.877 measured, 1.90 calculated
Hardness: 3½
Streak: White
Colour: Colourless, white
Common impurities: Al,Fe,Mg
Environments:
Metamorphic environments
Hydrothermal environments
Basaltic cavities
Thaumasite is a very late-stage mineral in some sulphide ore deposits, formed in
contact metamorphic zones and from the reaction of
geothermal waters or seawaters with basalt and
tuff. Associated minerals include
zeolites,
apophyllite, analcime,
calcite, gypsum and
pyrite
(HOM).
At Eddy Creek quarry, Weld river, southern Tasmania, Australia, thaumasite has been found in a
skarn deposit
(AJM 18.1.65).
At the Francon quarry, Quebec, Canada, thaumasite has been found on fracture surfaces in a highly weathered
serpentine-bearing dike
(MinRec 37.1.47).
In the Deccan traps of India, thaumasite has been found as a fibrous aggregate on
scolecite
(MinRec 34.1.63).
At Crestmore, Riverside county, California, USA, thaumasite occurs with spurrite,
ettringite or monticellite
(Dana) in spurrite - merwinite -
gehlenite skarn
(AM 48.1394-1396).
At the Braen quarry, Passaic county, New Jersey, USA, thaumasite occurs in both
datolite
and quartz pockets in the pillow
basalt. On some
specimens thaumasite can be seen alongside unaltered gypsum
(MinRec 40.6.520).
At West Paterson and great Notch, Passaic county, New Jersey, USA, thaumasite is sometimes
pseudomorphous after
anhydrite
(Dana).
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