Pentlandite is an iron-nickel sulfide, (Fe,Ni)9S8. Pentlandite usually has a Ni:Fe ratio of close to 1:1. It also contains minor cobalt. It is named after the Irish scientist Joseph Barclay Pentland (1797-1873), who first noted the mineral.
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 is non-magnetic. It has a yellowish bronze color.
Pentlandite is the most common terrestrial nickel sulfide mineral formed from immiscible sulfide-silicate melts under normal mantle and crustal conditions.
Nickel, being a chalcophile element, prefers to inhabit a sulfide phase versus a silicate or oxide phase within most terrestrial lithochemical systems (a few exceptions exist in unusual compositions). This behaviour is seen only when the particular rock is molten and sulfur saturated.
In sulfur undersaturated melts, nickel will substitute for other transition metals within ferromagnesian minerals, the most usual being olivine, although nickeliferous varieties of amphibole, biotite, pyroxene and spinels are known. Ni substitutes most readily for Fe2+ and Mg2+.
In sulfur saturated melts, nickel behaves as a chalcophile element and partitions strongly into the sulfide phase. Because most nickel exists in ultramafic rocks and behaves as a compatible element in igneous differentiation processes, the formation of nickel-bearing sulfides is essentially restricted to sulfur saturated mafic and ultramafic melts.
The sulfide melt, being at or above 1000°C, is in the form of monosulfide solid solution (MSS), an amalgam of compositonal "mineral" components of pentlandite, pyrite and pyrrhotite, and usually containing a small percentage of chalcopyrite (Cu being chalcophile), all of which are in an amorphous form. It is only upon cooling past ~550°C (dependant on composition) that the MSS undergoes exsolution into its component sulfide phases.
These phases are typically formed in an aphanitic equigranular granoblastic massive sulfide phase, or as matrix ore or disseminated sulfides held within the overlying silicate rock matrix. Intact magmatic massive sulfide is rarely preserved as, aside from the Norilsk deposit, most deposits of nickeliferous sulfide have been metamorphosed.
Metamorphism, especially if it is of at least middle greenschist facies, will cause the solid massive sulfide to revert to MSS. During deformation the MSS will act in a ductile fashion, and it is often considered to have the consistency of toothpaste, able to travel great distances into the country rock and along structures. Upon cessation of metamorphism, the MSS solution reverts again to the component sulfides, but it usually inherits a foliated or sheared texture, and typically sees growth of bright, equigranular to globular aggregates of porphyroblastic pentlandite crystals known colloquially as "fish scales".
Metamorphism may also reconstitute the MSS and sulfide composition, which may alter the concentration of Ni and the Ni:Fe ratio and Ni:S ratio of the sulfides (see sulfide tenor). In this case, pentlandite may be replaced by millerite, and rarely heazlewoodite. Metamorphism may also see the introduction of aggressive metasomatism, and it is particularly common for arsenic to enter the MSS, producing niccolite, gersdorffite and other Ni-Co arsenides.
Pentlandite is found within the lower margins of mineralised ultramafic to mafic layered intrusions, the prime example being the Bushveld igneous complex, South Africa, the Voiseys Bay troctolite intrusive complex in Canada, the Duluth gabbro, in North America, and various other localities throughout the world. In these locations it forms an important nickel ore.
Pentlandite is also the principal ore mineral won from Kambalda type komatiitic nickel ore deposits, the type examples of which are in the Yilgarn Craton of Western Australia. Similar deposits exist at Nkomati, Namibia, in the Thompson Nickel Belt, Canada, and a few examples from Brazil.
Pentlandite, but primarily chalcopyrite and PGEs, are won from the supergiant Norilsk nickel deposit, in trans-Siberian Russia.
The Sudbury deposit in Ontario, Canada, is associated with a meteorite impact crater. Pentlandite-pyrite-pyrrhotite ore in this locattion was formed from an extensive melt sheet formed by melting of rock post impact, which became sulfur saturated and formed extensive sheetlike ore deposits.