THE ETTRINGITE GROUP OF MINERALS

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The Ettringite Group is composed of hydrated calcium sulfate hydroxide minerals with a common trigonal structure. It is named for its most common member, ettringite, which is actually a rather uncommon mineral. The general formula for this group is Ca6X2(SO4)3(O, OH)12 - 26H2O. The X can be either aluminum, chromium, iron, manganese or silicon.

The SO4 ion group can be partially replaced by the rare B(OH)4 ion group as in a couple of the members of this group sturmanite and charlesite. The SO4 ion group can also be partially replaced by carbonate ions as in thaumasite and jouravskite. In ettringite and bentorite, these types of substitutions are minimal. Thaumasite is sometimes classified as a silicate because of its silicon content, but it lacks the SiO4 tetrahedrons required for inclusion in that class.