THAUMASITE

    Class : Sulfates, chromates, molybdates
    Subclass : Hydrated sulfates
    Crystal system : Hexagonal
    Chemistry : Ca3Si(SO4)(CO3)(OH)6 12H2O
    Rarity : Uncommon


Thaumasite is a mineral whose place in the classification remains debated : complex sulfates for some, silicates admitting SO4 and CO3 groups for others. The position of the silicon atoms at the center of the octahedron and not in the framework of SiO4 tetrahedra argues in favor of belonging to the sulfates ; it is then integrated into the ettringite group. It is a late mineral present in sulfide deposits, in contact metamorphic rocks, and also a reaction product of geothermal or marine waters on basalts and tuffs. Its name comes from the Greek thaumazein (to be surprised), because of its very particular chemical composition. Thaumasite rarely forms acicular prismatic crystals grouped in fibroradiate aggregates, it occurs more readily in powdery masses. It is colorless, white or slightly yellowish. It is sometimes cut as a fine gemstone.

Main photo : Thaumasite from N'Chwaning II Mine, South Africa © Chris Emproto

Terlinguaite from Mariposa Mine, Texas, USA © Alex Earl
Terlinguaite from McDermitt Mine, Nevada, USA © Doug Merson
Terlinguaite from McDermitt Mine, Nevada, USA © Michael Cox
Terlinguaite and quartz from McDermitt Mine, Nevada, USA © Stephan Wolfsried

Thaumasite in the World

Splendid crystals of thaumasite come from the manganese mines of Hotazel (South Africa): more precisely from N'Chwaning 2 Mine, which yielded pale yellow gemmy crystals of 5 cm constituting the best known specimens, and from Wessel (gemmy crystals of 15 mm). Imposing crystalline masses and large anhydrite crystals pseudomorphs come from the basalts of Paterson (New Jersey). Thaumasite is also known from Crestmore (California), with spurrite and ettringite and in large crystalline aggregates associated with prehnite at Centreville (Virginia). It is also present in many volcanoes of the Eifel (Germany) and in the Swedish deposits of Langban and Bjelke.

Thaumasite in France

In France, thaumasite is known in the basalts of Espalion (Aveyron) as well as in the pyrometamorphic rocks of the mine of St-Maime (Alpes-de-Haute-Provence).

Twinning

No twinning known for this mineral species.

Fakes and treatments

No fakes listed for this mineral species.



Hardness : 3.5
Density : 1.877
Fracture : Sub-conchoidal
Streak : White


TP : Translucent to transparent
RI : 1.468 to 1.507
Birefringence : 0.039
Optical character : Uniaxial -
Pleochroism : None
Fluorescence : None


Solubility : -

Magnetism : NoneRadioactivity : None