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

No twinning known for this mineral species.

No fakes listed for this mineral species.