Purple crystal aggregates of the very rare mineral bentorite from the type locality and one of the only two known localities worldwide: Hatrurim Formation, Negev Desert, Israel. — Photo: David Hospital · CC BY-SA 4.0
Bentorite
A violet calcium-chromium sulfate hydrate forming transparent hexagonal crystals.
Intro adapted from Wikipedia (CC BY-SA).
Bentorite is an IMA-recognized mineral species in the sulfate class, with the chemical formula Ca₆Cr₂(SO₄)₃(OH)₁₂·26H₂O.
It crystallizes in the hexagonal crystal system (space group P6_3/mmc). It is a member of the Ettringite structural group.
Key properties: Mohs hardness 2; specific gravity 2.025; Very pale violet streak; Vitreous luster; optically uniaxial.
IMA symbol Bto, IMA status: approved, named in 1979.
Found in low-temperature hydrothermal veins in black calcite–spurrite marble.
It is found associated with Calcite, thaumasite, truscottite, vaterite, jennite, tobermorite, brownmillerite, mayenite, melnikovite, “chlorite”.
The mineral was named to honor Professor Y.K. Bentor, University of California, San Diego, California, USA, for his contributions to geology and mineralogy in Israel.