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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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