Thaumasite is a calcium silicate mineral, containing Si atoms in unusual octahedral configuration, with chemical formula Ca3Si(OH)6(CO3)(SO4)·12H2O, also sometimes more simply written as CaSiO3·CaCO3·CaSO4·15H2O.
Thaumasite (white) with prehnite (green) from Fairfax Quarry, Virginia
Group of thaumasite prisms from the Kuruman, Kalahari manganese fields, Northern Cape Province, South Africa (size: 4.2 × 3.1 × 1.2 cm)
Ettringite is a hydrous calcium aluminium sulfate mineral with formula: Ca6Al2(SO4)3(OH)12·26H2O. It is a colorless to yellow mineral crystallizing in the trigonal system. The prismatic crystals are typically colorless, turning white on partial dehydration. It is part of the ettringite-group which includes other sulfates such as thaumasite and bentorite.
Ettringite, Kalahari manganese fields, Northern Cape Province, South Africa
Ettringite, 6.5 × 3.2 cm. N'Chwaning Mines, Kalahari manganese fields, Northern Cape Province, South Africa
SEM image of fractured hardened cement paste, showing thin hexagonal plates of portlandite (calcium hydroxide) and needles of ettringite (micron scale)
Fibroradiated ettringite needles on slag (Concordia smelter, Eschweiler, Aachen, Germany)
