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

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).

Back to Minerals