Polydymite

Polydymite, Ni²⁺Ni₂³⁺S₄, is a supergene thiospinel sulfide mineral associated with the weathering of primary pentlandite nickel sulfide.

Polydymite
General
Category	Sulfide mineral Thiospinel group Spinel structural group
Formula (repeating unit)	Ni3S4
IMA symbol	Pld
Strunz classification	2.DA.05
Crystal system	Cubic
Crystal class	Hexoctahedral (m3m) H-M symbol: (4/m 3 2/m)
Space group	Fd3m
Unit cell	a = 9.48 Å; Z = 8
Identification
Color	Pale to steel-gray
Crystal habit	As octahedral crystals, massive, granular to compact
Twinning	Twinning on {111}
Cleavage	Indistinct on {001}
Fracture	Conchoidal to uneven
Mohs scale hardness	4.5 – 5.5
Luster	Metallic
Streak	Black gray
Diaphaneity	Opaque
Specific gravity	4.5 – 4.8
Alters to	Tarnishes to copper-red
References

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 verdigris and patina from associated copper and arsenic sulfides, and is typically in amorphous to massive infill of lower saprolite ultramafic lithologies.

Polydymite is the nickel equivalent of violarite and in many cases these two minerals are formed together, potentially in solid solution.

Common contaminants of polydymite are cobalt and iron. Polydymite forms a series with linnaeite, Co⁺²Co⁺³₂S₄.

Paragenesis

Polydymite 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.

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

Occurrence

Polydymite is reported widely from the oxidised regolith above primary nickel sulfide ore systems worldwide. It is less common than related violarite, due to the high iron content of most primary sulfides.

Economic importance

Polydymite 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 more valuable primary (hypogene) mineralisation.