Violarite

Violarite (Fe²⁺Ni₂³⁺S₄) is a supergene sulfide mineral associated with the weathering and oxidation of primary pentlandite nickel sulfide ore minerals.

Violarite crystallises in the isometric system, with a hardness of 4.5 to 5.5 and a specific gravity of about 4, is dark violet gray to copper-red, often with verdigris and patina from associated copper and arsenic sulfides, and is typically in amorphous to massive infill of lower saprolite ultramafic lithologies.

Violarite has a characteristic violet colour, hence the name from the Latin 'violaris' alluding to its colour especially when viewed in polished section under a microscope.

Paragenesis

Violarite is formed by oxidisation of primary sulfide assemblages in nickel sulfide mineralisation. The process of formation involves oxidation of Ni²⁺ and Fe²⁺ which is contained within the primary pentlandite-pyrrhotite-pyrite assemblage.

Violarite is produced at the expense of both pentlandite and pyrrhotite, via the following basic reaction;

Pentlandite + Pyrrhotite --> Violarite + Acid:(Fe,Ni)₉S₈ + Fe_(1-x)S + O₂ → Fe²⁺Ni₂³⁺S₄ + H₂SO₃

Violarite is also reported to be produced in low-temperature metamorphism of primary sulfides, though this is an unusual paragenetic indicator for the mineral.

Continued oxidation of violarite leads to replacement by goethite and formation of a gossanous boxwork, with nickel tending to remain as impurities within the goethite or hematite, or rarely as carbonate minerals.

Occurrence

Violarite is reported widely from the oxidised regolith above primary nickel sulfide ore systems worldwide. It is of particular note from the Mount Keith dunite body, Western Australia, where it forms an important ore mineral.

It is also reported from open cast mines around the Kambalda Dome, and Widgiemooltha Dome, in association with polydymite, gaspeite, widgiemoolthalite and hellyerite, among other supergene nickel minerals.

Economic importance

Violarite is an important transitional ore in many nickel sulfide mines, as it has increased nickel tenor (Ni% as a total of sulfide)and occupes a position within the mineralised profile where it must be extracted to pay for development down to the most valuable fresh mineralisation.

Violarite mineralisation requires different metallurgy to primary nickel sulfides, due to the different nature of its gangue and its flotation properties. This may require additional treatment and processing, so in some cases low-grade violarite mineralisation is considered refractory ore.

References

* Dunn, J.G., & V.L. Howes (1996): The oxidation of violarite: Thermochimica Acta: 282/283: 305-3
* Grguric, B.A. (2002), Hypogene violarite of exsolution origin from Mount Keith, Western Australia: field evidence for a stable pentlandite-violarite tie line: Mineralogical Magazine: 66: 313-326.
* Tenailleau, C., Pring A., Tschmann B., Brugger J., Grguric B., and Putnis A. (2006), Transformation of pentlandite to violarite under mild hydrothermal conditions: American Mineralogist: 91: 706-709.
* THORNBER, M. R. (1972) Pyrrhotite-the matrix of nickel sulphide mineralization. Newcastle Conference, Australasian Institute of Mining and Metallurgy, May-June, 1972, 51-58.
* THORNBER, M. R. (1975a) Supergene alteration of sulphides, I. A chemical model based on massive nickel sulphide deposits at Kambalda, Western Australia. Chemical Geology, 15, 1-14.
* THORNBER, M. R. (1975b) Supergene alteration of sulphides, II. A chemical study of the Kambalda nickel deposits. Chemical Geology, 151 117-144.