Magnesio-foitite tourmaline from Madagascar. A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical compo — Photo: James St. John · CC BY 2.0
Foitite
A rare tourmaline whose X site is mostly empty — the vacancy-dominant member of the group.
Intro adapted from Wikipedia (CC BY-SA).
Foitite is an IMA-recognized mineral species in the silicate class, with the chemical formula (Fe²⁺₂Al)Al₆(Si₆O₁₈)(BO₃)₃(OH)₃(OH).
It crystallizes in the hexagonal crystal system (space group R3m). It is a member of the Tourmaline structural group.
Key properties: Mohs hardness 7.5; specific gravity 3.17; typically black or brown or blue; Grayish white streak; Vitreous luster; optically uniaxial.
IMA symbol Foi, IMA status: approved, named in 1992.
Probably in granite pegmatites.
It is found associated with The original specimens are loose crystals without matrix.
The mineral was named to honor Franklin F. Foit, Jr. (1942{), of Washington State University, Pullman, Washington, USA, for his work on tourmaline group minerals.
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