Fetiasite

fetiasite

asbecasite

cafarsite

cervandonite-(Ce)

Images

Formula: (Fe2+,Fe3+,Ti4+)3O2As3+2O5
Arsenite, titanium-bearing mineral
Crystal system: Monoclinic
Specific gravity: 4.6 measured on altered material, 4.74 to 4.80 calculated
Hardness: ~5
Colour: Brown to black, red-brown when altered; creamy white in reflected light
Environments

Metamorphic environments
Hydrothermal environments

Localities

There are two co-type localities, Monte Cervandone area, Devero Alp, Baceno, Verbano-Cusio-Ossola Province, Piedmont, Italy and Gorb, Lärchultini, Binn, Goms, Valais, Switzerland.

At the Monte Cervandone area, Devero Alp, Baceno, Verbano-Cusio-Ossola Province, Piedmont, Italy, fetiasite was first found in the Devero Valley, on the Italian side of Cervandone, in 1985. When fresh, fetiasite crystals are dark brown or black with a well-developed basal cleavage and a somewhat metallic lustre. Upon weathering they commonly develop a reddish surface coating. Typically, fetiasite forms tabular crystals with a diamond-shaped outline, but more complex crystal aggregates also occur. Single crystals may reach 3 cm in length. In 1991, major finds of fetiasite occurred in old, long-ago worked-out anatase-bearing clefts in the Lärcheltini Zone; specimens showing wedge-shaped and lozenge-shaped fetiasite crystals in radial groups to 3 cm, as well as individual crystals to 2 cm, were found (MinRec 56.3.312).
Fetiasite is deposited from arsenic-bearing solutions in Alpine fissures in gneisses of the upper greenschist to lower amphibolite facies. Associated minerals include asbecasite, cafarsite, cervandonite, anatase, chlorite, feldspar, mica and quartz (HOM).

Gorb, Lärchultini, Binn, Goms, Valais, Switzerland, is the second co-type locality.
Fetiasite from Gorb - Image

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