Black Fencooperite in colourless sanbornite. From: Trumbull Peak Ba-silicate deposit, Trumbull Peak, Clearing House, Clearing House Mining District, East Belt, Mariposa Co., California, USA. — Photo: Leon Hupperichs · CC BY-SA 3.0
Fencooperite
Fencooperite is an IMA-recognized mineral species in the silicate class, with the chemical formula Ba₆Fe³⁺₃Si₈O₂₃(CO₃)₂Cl₃·H₂O.
It crystallizes in the hexagonal crystal system (space group P3m1).
Key properties: Mohs hardness 4.5–5; specific gravity 4.338; Grayish black streak; Vitreous to adamantine luster; optically uniaxial.
IMA symbol Fco, IMA status: approved, named in 2000.
Found in gillespite-rich zones in sanbornite-quartz lenses in low-grade metasedimentary rocks.
It is found associated with Titantaramellite, anandite, kinoshitalite, celsian, alforsite, barite, diopside, pyrrhotite.
The mineral was named to honor mineral collector Joseph Fenimore (“Fen”) Cooper, Jr. (b. 1937), of Santa Cruz, California, USA, who helped collect the samples in which the new phase was identified.