Violarite
Encyclopedia
Violarite 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.
-pyrrhotite
-pyrite
assemblage.
Violarite is produced at the expense of both pentlandite and pyrrhotite, via the following basic reaction;
Pentlandite + Pyrrhotite --> Violarite + Acid9S8 + Fe(1-x)S + O
2 → 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 gossan
ous boxwork, with nickel tending to remain as impurities within the goethite or hematite, or rarely as carbonate minerals.
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.
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.
Supergene (geology)
In ore deposit geology, supergene processes or enrichment occur relatively near the surface. Supergene processes include the predominance of meteoric water circulation with concomitant oxidation and chemical weathering. The descending meteoric waters oxidize the primary sulfide ore minerals and...
sulfide mineral associated with the weathering
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
and oxidation of primary pentlandite
Pentlandite
Pentlandite is an iron-nickel sulfide, 9S8. Pentlandite usually has a Ni:Fe ratio of close to 1:1. It also contains minor cobalt.Pentlandite forms isometric crystals, but is normally found in massive granular aggregates. It is brittle with a hardness of 3.5 - 4 and specific gravity of 4.6 - 5.0 and...
nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
sulfide ore
Ore
An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element....
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
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...
ultramafic lithologies
Lithology
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples or with low magnification microscopy, such as colour, texture, grain size, or composition. It may be either a detailed description of these characteristics or be a summary of...
.
Violarite has a characteristic violet colour, hence the name from the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
'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 pentlanditePentlandite
Pentlandite is an iron-nickel sulfide, 9S8. Pentlandite usually has a Ni:Fe ratio of close to 1:1. It also contains minor cobalt.Pentlandite forms isometric crystals, but is normally found in massive granular aggregates. It is brittle with a hardness of 3.5 - 4 and specific gravity of 4.6 - 5.0 and...
-pyrrhotite
Pyrrhotite
Pyrrhotite is an unusual iron sulfide mineral with a variable iron content: FeS . The FeS endmember is known as troilite. Pyrrhotite is also called magnetic pyrite because the color is similar to pyrite and it is weakly magnetic...
-pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...
assemblage.
Violarite is produced at the expense of both pentlandite and pyrrhotite, via the following basic reaction;
Pentlandite + Pyrrhotite --> Violarite + Acid9S8 + Fe(1-x)S + O
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
2 → 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
Goethite , named after the German polymath Johann Wolfgang von Goethe, is an iron bearing oxide mineral found in soil and other low-temperature environments. Goethite has been well known since prehistoric times for its use as a pigment. Evidence has been found of its use in paint pigment samples...
and formation of a gossan
Gossan
Gossan 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 in the form of boxworks, quartz lined cavities retaining the shape of the dissolved...
ous 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 regolithRegolith
Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...
above primary nickel sulfide ore systems worldwide. It is of particular note from the Mount Keith dunite
Dunite
Dunite is an igneous, plutonic rock, of ultramafic composition, with coarse-grained or phaneritic texture. The mineral assemblage is greater than 90% olivine, with minor amounts of other minerals such as pyroxene, chromite and pyrope. Dunite is the olivine-rich end-member of the peridotite group...
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
Polydymite
Polydymite, Ni2+Ni23+S4, is a supergene thiospinel sulfide mineral associated with the weathering of primary pentlandite nickel sulfide.Polydymite 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...
, gaspeite
Gaspeite
Gaspeite is an extremely rare nickel carbonate mineral named for the place it was first described, in the Gaspé Peninsula, Canada.Gaspeite's formula is CO3 and it is a bright green mineral. It forms massive to reniform pappillary aggregates in fractures, bottryoidal concretions in laterite or...
, widgiemoolthalite and hellyerite
Hellyerite
Hellyerite, NiCO3·6, is an 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...
, 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
Metallurgy
Metallurgy is a domain of materials science that studies the physical and chemical behavior of metallic elements, their intermetallic compounds, and their mixtures, which are called alloys. It is also the technology of metals: the way in which science is applied to their practical use...
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.