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Formula: FeNi2S4
Sulphide, linnaeite subgroup,
thiospinel group,
spinel supergroup,
forms a series with
siegenite
Crystal System: Isometric
Specific gravity: 4.79 calculated
Hardness: 4½ to 5½
Streak: Black
Colour: Violet grey, copper red, light grey or steel grey
Common impurities: Co,Cu
Environments
Violarite is of hydrothermal origin. Associated minerals include pyrrhotite,
millerite, chalcopyrite and
pentlandite
(HOM).
It can be found in abundance in the supergene alteration zones of
numerous nickel sulphide deposits, where it replaces
primary nickel
sulphide minerals such as pentlandite. Violarite can also form as a
primary phase through exsolution during the cooling of
pentlandite
(KL 9-10).
Localities
The type locality is the Vermilion mine, Denison Township, Sudbury District, Ontario, Canada.
In west central Illinois, USA, violarite has been found in intimate association with
millerite
(Minrec 36.5.428-429).
Alteration
The transformation of pentlandite to violarite occurs at 80oC
within a pH range of 3 to 5 (quite acid). The reaction can be expressed as
pentlandite, oxygen (gaseous) and hydrogen sulphide (aqueous) to violarite,
pyrite and water
4(Ni,Fe)9S8 + 11O2 + 22H2S = 9FeNi2S4 + 9FeS2
+ 22H2O
It is assumed that the excess iron is removed in the solution:
pentlandite, oxygen (gaseous), hydrogen sulphide (aqueous) and H+ to
violarite, Fe2+ and water
8(Ni,Fe)9S8 + 13O2 + 8H2S + 36H+ = 18FeNi2S4
+ 18Fe2+ + 26H2O
In an experiment where Fe3+ was added, the reaction became:
pentlandite, Fe3+ and hydrogen sulphide (aqueous) to H+,
violarite and Fe2+
4(Ni,Fe)9S8 + 26Fe3+ + 4H2S = 8H+ + 9FeNi2S4
+ 35Fe2+
(KL 9-10).
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