VIOLARITE
Violarite – Fe2+Ni3+2S4 – is a rarer sulfide, typical of nickeliferous magmatic sulfide deposits. It is an important transitional ore in nickeliferous sulfide mines, where its position in the mineralized profile requires its extraction to pay for mine development to richer mineralization located at greater depths. As it requires different beneficiation than primary nickeliferous sulfides, in some cases it is considered a refractory ore (difficult to exploit). When it integrates saprolites on ultramafic rocks, it can also be an important ore mineral.
It is classified in the Linnaeite Subgroup, Thiospinel Group, Spinel Supergroup. It forms a series with polydymite, another series with greigite, and a third series with siegenite.
It may contain Co and Cu. The varieties recorded for violarite are only mixtures of other minerals, such as violarite replacing millerite.
1. Characteristics
Crystal system: Cubic hexaoctahedral.
Color: Violet gray, copper red, light gray to steel gray. Tarnishes quickly.
Habit: Massive or forming nodules up to 5 mm in diameter.
Cleavage: {001} perfect.
Tenacity: Brittle.
Twinning: No.
Fracture: No information available.
Mohs Hardness: 4.5 – 5.5
Parting: No.
Streak: Black.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 4.79 (calculated).
2. Geology and Deposits
Violarite is one of several Ni sulfides found in basic-ultrabasic intrusions, such as Sudbury (Canada). In these deposits, it can be primary or a product of alteration (oxidation) of other sulfides, mainly from the pentlandite/pyrrhotite/pyrite association: (Fe,Ni)9S8 + Fe(1-x)S + O2 → Fe2+Ni23+S4 + H2SO3
It can form as a product of low-temperature metamorphism on primary sulfides, but this is an uncommon situation.
It is typical for saprolites developed on ultramafic lithologies (e.g., dunites), as the continuous oxidation of violarite leads to its replacement by goethite and the formation of a gossan boxwork, with Ni tending to remain as an impurity in goethite or hematite, more rarely in carbonate minerals.
3. Mineral Associations
It is associated with common gangue minerals such as quartz and carbonates (calcite, dolomite).
In magmatic sulfide deposits (e.g., Sudbury, Canada), it is associated with sulfides characteristic of this paragenesis such as pyrite, pyrrhotite, chalcopyrite, pentlandite, cubanite, millerite, bravoite (pyrite-Ni), and siegenite. In this same paragenesis, Fe oxides such as magnetite, ilmenite, and hematite occur.
Also with polydymite, galena, millerite, sperrylite, niquelite, gersdorffite, and cobaltite.
In saprolites with goethite, rutile, zircon, gaspeite, widgiemoolthalite, and hellyerite, among others.
4. Transmitted Light Microscopy
This does not apply, as violarite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Violarite exhibits a relatively high hardness. Upon polishing, its hardness is much greater than that of arsenopyrite, greater than that of chalcopyrite and sphalerite, approximately equal to that of pentlandite, and less than that of bravoite, linnaeite, and pyrrhotite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Grayish-brown with a violet or pink tint.
It can be white with a violet, yellowish, or brownish tint.
Compared to the color of pentlandite, the color of violarite has a distinct violet tint. Pentlandite, in contrast, is yellow.
Compared to the color of pyrrhotite, the color of violarite does not show the violet tint.
Compared to the color of bravoite, the color of violarite is generally more violet.
Compared to the color of millerite, the color of violarite is darker, violet with pinkish tints.
Compared to the color of chalcopyrite, the color of violarite is pinkish cream.
Pleochroism: No.
Reflectivity: 44.03%. If with Co, 48.9%.
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Isotropic.
Internal reflections: No.
May be confused with: Few other minerals, considering paragenesis.
Bravoite is very similar, but has greater hardness and less developed cleavage.
Pyrite does not exhibit violet or pink hues in PPL.
General Characteristics:
Grain shape: generally occurs as a porous alteration product along intergranular boundaries and fractures of pentlandite, pyrrhotite, chalcopyrite, and millerite. It can also form equidimensional anhedral grains or occur as finely lamellar intergrowths. Incipient alteration of pentlandite to violarite, along the pentlandite parting, highlights this parting. Violarite can also form along fractures or even breakaway figures in pentlandite. The literature refers to a porous or “crackly” texture that violarite typically exhibits. Violarite can occur alongside bravoite.
Cleavage: Cubic (100) cleavage is frequently visible. Octahedral (111) cleavage may also be visible; in some cases it is very well developed.
Twinning does not occur.
Zoning does not occur.
Deformations: Cataclasis-related deformations may occur.
Exsolution lamellae of millerite can occur in violarite.
Alteration of pentlandite can be to violarite, bravoite (pyrite-Ni), and linnaeite.
Substitutions 1: Violarite can replace pentlandite, pyrite, and gersdorffite. Rarely replaces millerite.
Substitutions 2: Violarite is replaced by chalcopyrite.
Pseudomorphs of violarite on pentlandite and millerite can occur.
