Outstanding black crystals of the rare Binntal mineral fetiasite from the type locality in Gorb (Lercheltini Area, Binn valley, Wallis, Switzerland). Fetiasite is an arsenite. — Photo: David Hospital · CC BY-SA 4.0
Fetiasite
Fetiasite is an IMA-recognized mineral species in the oxides & hydroxide class, with the chemical formula (Fe²⁺,Fe³⁺,Ti⁴⁺)₃O₂As³⁺₂O₅.
It crystallizes in the monoclinic crystal system (space group P2_1/m).
Key properties: Mohs hardness 5; specific gravity 4.6; Metallic to semimetallic luster.
IMA symbol Fet, IMA status: approved, named in 1991.
Deposited from arsenic-bearing solutions in Alpine fissures in gneisses of the upper greenschist to lower amphibolite facies.
It is found associated with Asbecasite, cafarsite, cervandonite, anatase, chlorite, feldspar, mica, quartz.
The name refers to Fe, Ti, As in its composition.