Violarite

violarite

pentlandite

millerite

siegenite

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Formula: FeNi2S4
Sulphide, linnaeite group, forms a series with siegenite
Specific gravity:
Hardness: 4½ to 5½
Streak: Black
Colour: Violet grey, copper red, light grey or steel grey
Common impurities: Co,Cu
Environments

Hydrothermal 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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