Images
Formula: (Cu,Fe,Ag)9Se8
Selenide,
pentlandite group
Crystal System: Isometric
Specific gravity: 5.39 calculated
Hardness: 2½
Colour: Bronze-brown
Environments
Localities
At the type locality, the Chaméane Uranium Deposit, Le Vernet-Chaméane, Issoire, Puy-de-Dôme, Auvergne-Rhône-Alpes,
France, geffroyite occurs in a complex quartzitic vein cutting
granite. Three stages of mineralisation, separated by
brecciation, are recognised. In the first stage
pitchblende was deposited with copious
baryte gangue; in the second stage
quartz with löllingite,
arsenopyrite, pyrite,
chalcopyrite, tetrahedrite,
tennantite and clausthalite
were deposited, with partial replacements of baryte by
quartz, pitchblende by
chalcopyrite, and
chalcopyrite by
clausthalite. In the third stage the
selenides bukovite,
athabascaite, umangite,
eucairite, eskebornite, more
clausthalite, geffroyite,
chameanite and giraudite occur
with an ankerite gangue, followed
by a second generation of chalcopyrite.
Geffroyite occurs in very fine-grained intergrowth with
eskebornite, clausthalite,
chameanite and giraudite
(AM 67.1074-1082).
Geffroyite from Chaméane - Image
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