Thaumasite Locality: N'Chwaning II Mine, N'Chwaning Mines, Kuruman , Kalahari manganese fields, Northern Cape Province , South Africa ( Locality at mindat.org ) Size: miniature, 3.1 x 2.1 x 1.9 cm Thaumasite A fine specimen with gem crystals, on matrix, from a surprising and small find of late 2005. These are the best seen in 20 years, of this rare gem species related to ettringite and sturmanite. And, they happen to be beautiful. In person, there is more lustre. — Photo: Robert M. Lavinsky · CC BY-SA 3.0
Thaumasite
A rare calcium silicate-carbonate-sulfate with octahedral silicon — notorious for corroding concrete.
Intro adapted from Wikipedia (CC BY-SA).
Thaumasite is an IMA-recognized mineral species in the silicate class, with the chemical formula Ca₃Si(OH)₆(CO₃)(SO₄)·12H₂O.
It crystallizes in the hexagonal crystal system (space group P6_3/m). It is a member of the Ettringite structural group.
Key properties: Mohs hardness 3.5; specific gravity 1.877; Vitreous to dull, greasy, silky luster; refractive index 1.483; optically uniaxial.
IMA symbol Tma, grandfathered status — known before the IMA’s founding, named in 1878.
A very late-stage mineral in some sulfide ore deposits. In contact metamorphic zones and from the reaction of geothermal waters or seawaters with basalts and tuffs.
It is found associated with Zeolites, apophyllite, analcime, calcite, gypsum, pyrite.
Its name comes from the Greek to be surprised, because of its remarkable composition, without parallel at the time of its discovery.