• Chemistry: Ca3Si(CO3)(SO4)(OH)6 - 12H2O, Hydrated Calcium Silicon Carbonate Sulfate Hydroxide.
  • Class: Sulfates
  • Group: Ettringite
  • Uses: Only as mineral specimens.
  • Specimens

Thaumasite is a fairly rare mineral. Its chemistry and classification are interesting. It is sometimes classified as a silicate due to its silicon content and there is an argument to be made for its classification as a carbonate as well. However it does belong to a predominantly sulfate mineral group, the Ettringite Group and its properties seem more in line with its inclusion into the sulfate class.

Four out of every five atoms in this mineral is either a part of a water molecule or an hydroxide. It's almost all water! This fact is reflected in its very low specific gravity of only 1.9, that's less than twice the specific gravity of water.


  • Color is colorless or white.
  • Luster is vitreous.
  • Transparency: Crystals are translucent.
  • Crystal System is trigonal; bar 3 2/m.
  • Crystal Habits include acicular crystals and massive forms.
  • Cleavage is poor and rarely seen.
  • Fracture is uneven.
  • Hardness is 3.5
  • Specific Gravity is approximately 1.9 (well below average)
  • Streak is white.
  • Associated Mineral is commonly spurrite.
  • Notable Occurrences include Crestmore, Riverside Co., California and Paterson, New Jersey, USA and Langban, Sweden.
  • Best Field Indicators are density, hardness and association.
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