Violarite ( Fe²⁺ Ni₂³⁺ S₄) is a

supergene A supergene is a chromosomal region encompassing multiple neighboring genes that are inherited together because of close genetic linkage, i.e. much less recombination than would normally be expected. This mode of inheritance can be due to genomic ...

sulfide mineral associated with the

weathering Weathering is the deterioration of rocks, soils and minerals (as well as wood and artificial materials) through contact with water, atmospheric gases, sunlight, and biological organisms. It occurs '' in situ'' (on-site, with little or no move ...

and

oxidation Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

of primary

pentlandite Pentlandite is an iron–nickel sulfide with the chemical formula . Pentlandite has a narrow variation range in nickel to iron ratios (Ni:Fe), but it is usually described as 1:1. In some cases, this ratio is skewed by the presence of pyrrhotite ...

nickel Nickel is a chemical element; it has symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive, but large pieces are slo ...

sulfide

ore Ore is natural rock or sediment that contains one or more valuable minerals, typically including metals, concentrated above background levels, and that is economically viable to mine and process. The grade of ore refers to the concentration ...

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 grey 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 Saprolite is a chemically weathered rock. Saprolites form in the lower zones of soil profiles and represent deep weathering of the bedrock surface. In most outcrops, its color comes from ferric compounds. Deeply weathered profiles are widespread ...

ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are usua ...

lithologies. Violarite has a characteristic violet colour, hence the name from the

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

'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

pyrrhotite Pyrrhotite (''Pyrrhus of Epirus, pyrrhos'' in Greek language, Greek meaning "flame-coloured"'')'' is an iron sulfide mineral with the formula Fe(1−x)S (x = 0 to 0.125). It is a nonstoichiometric compound, nonstoichiometric variant of FeS, th ...

pyrite The mineral pyrite ( ), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue ...

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 Goethite (, ) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the α- polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite has been well known since ancient t ...

and formation of a

gossan Gossan (eiserner hut or eisenhut) is intensely oxidized, weathered or decomposed rock, usually the upper and exposed part of an ore deposit or mineral vein. In the ''classic'' gossan or iron cap all that remains is iron oxides and quartz, often ...

iferous boxwork, with nickel tending to remain as impurities within the goethite or haematite, or rarely as carbonate minerals.

Occurrence

Violarite is reported widely from the oxidised

regolith Regolith () is a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestria ...

above primary nickel sulfide ore systems worldwide. It is of particular note from the Mount Keith

dunite Dunite (), also known as olivinite (not to be confused with the mineral olivenite), is an intrusive igneous rock of ultramafic composition and with phaneritic (coarse-grained) texture. The mineral assemblage is greater than 90% olivine, wit ...

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 Widgiemoolthalite is a rare mineral hydration, hydrated nickel(II) carbonate mineral with the chemical formula . Usually bluish-green in color, it is a brittle mineral formed during the weathering of nickel sulfide. Present on gaspéite surfaces, ...

and

hellyerite Hellyerite, NiCO3·6(H2O), is a hydrated nickel carbonate mineral. It is light blue to bright green in colour, has a hardness of 2.5, a vitreous luster, a white streak and crystallises in the monoclinic system. The crystal habit is as platy and ma ...

, 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 occupies 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 Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys. Metallurgy encompasses both the ...

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

* * 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. * Thorber, M. R. (1972) Pyrrhotite-the matrix of nickel sulphide mineralization. Newcastle Conference, Australasian Institute of Mining and Metallurgy, May–June, 1972, 51–58. * Thorber, 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. * Thorber, M. R. (1975b) Supergene alteration of sulphides, II. A chemical study of the Kambalda nickel deposits. Chemical Geology, 151 117–144. {{Refend Nickel minerals Iron(II) minerals Thiospinel group Economic geology Cubic minerals Minerals in space group 227