Thaumasite is a rare calcium sulfate-silicate hydroxide of the Ettringite Group. It forms colorless, white to pale yellow, transparent, hexagonal, elongated prismatic crystals sometimes associated with calcite. Thaumasite was first described by Nordenskiold in 1878 from the type locality: Bjelke mines, Areskutan, Are, Jamtland, Sweden, and was named from the Greek word, thaumazein (to be surprised) which refers to its unusual composition with carbonate, sulfate and hexahydroxysilicate anions. Other notable localities include: Langban, Filipstad, Varmland, Sweden; Tsumeb mine, Tsumeb, Otjikoto Region, Namibia; and as exceptional, gemmy crystals that are the best-of-species from the Wessels mine, Kalahari Manganese Fields, Northern Cape Province, South Africa. Thaumasite occurences in the USA include: as minute crystals and large crystalline aggregates associated with prehnite near Centreville, Fairfax Co., Virginia; as fine crystalline masses at Patterson and West Paterson, Passaic Co., New Jersey; and with spurrite and ettringite at Crestmore, Riverside Co., California.
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