Pentlandite

Pentlandite
PentlanditeUSGOV.jpg
General
Category Mineral
Chemical formula iron nickel sulfide:(Fe,Ni)9S8
Identification
Color Yellowish bronze
Crystal habit Hexoctahedral rare; massive to granular
Crystal system Isometric
Cleavage absent - octahedral parting
Fracture Conchoidal
Mohs Scale hardness 3.5 - 4
Luster metallic
Refractive index opaque
Streak light bronze-brown
Specific gravity 4.6 - 5.0
Fusibility 1.5 - 2
Other Characteristics becomes magnetic on heating

Pentlandite is a mineral consisting mainly of iron-nickel sulfide, in which the ratio of nickel to iron is usually close to 1:1. Its chemical formula is written as (Fe,Ni)9S8. It also contains minor proportions of cobalt. It has a yellowish bronze color. It was named after the Irish scientist Joseph Barclay Pentland (1797-1873), who first noted the mineral.

Contents

This mineral is the most common terrestrial nickel sulfide mineral. It is therefore an important ore of nickel, which in turn has various applications. Also, pentlandite is the name given to a group of minerals with similar structures, as described below.

Occurrence

Pentlandite is found within the lower margins of mineralized ultramafic to mafic layered intrusions present in various localities throughout the world. For example, it is found in the Bushveld igneous complex, South Africa; the Voiseys Bay troctolite intrusive complex in Canada; and the Duluth gabbro in the U.S. state of Minnesota. In these locations, it forms an important nickel ore.

Pentlandite is also the principal ore mineral obtained 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 platinum group elements, are derived 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 location was formed from an extensive melt sheet formed by melting of rock post impact, which became sulfur saturated and formed extensive, sheet-like ore deposits.

Formation

Pentlandite is 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 behavior 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 nickeliferrous 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 compositional "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 about 550°C (dependent 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. 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.

Characteristics

Pentlandite forms isometric crystals, but is normally found in massive granular aggregates. It is brittle with a hardness of 3.5 to 4 on the Mohs scale, and its specific gravity is 4.6 to 5.0. It is nonmagnetic.

Pentlandite group of minerals

Pentlandite is also the name given to a group of minerals that are similar in structure and chemistry. Most members of the group, except pentlandite itself, are rare in occurrence. Their general chemical formula can be written as XY8(S, Se)8. The X position may correspond to a cation of manganese, cadmium, lead, or silver; the Y position may correspond to copper. Also, iron, cobalt, and nickel can be found in either X or Y position. Thus, for example, silver iron nickel sulfide is called argentopentlandite; cobalt iron nickel sulfide is called cobalt pentlandite. Shadlunite is the name given to iron copper lead cadmium sulfide.

See also

References

  • Farndon, John. 2006. The Practical Encyclopedia of Rocks & Minerals: How to Find, Identify, Collect and Maintain the World's best Specimens, with over 1000 Photographs and Artworks. London: Lorenz Books. ISBN 0754815412
  • Klein, Cornelis, and Barbara Dutrow. 2007. Manual of Mineral Science. 23rd ed. New York: John Wiley. ISBN 978-0471721574
  • Marston, R.J., D.I. Groves, D.R. Hudson, and J.R. Ross. 1981. Nickel Sulfide Deposits in Western Australia: a review. Economic Geology 76:1330-63.
  • Mineral Gallery. 2006. Pentlandite Amethyst Galleries. Retrieved October 14, 2007.
  • Pellant, Chris. 2002. Rocks and Minerals. Smithsonian Handbooks. New York: Dorling Kindersley. ISBN 0789491060
  • Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. 2001. Rocks, Gems and Minerals. Rev. ed. New York: St. Martin's Press. ISBN 1582381321

External links

All links retrieved April 13, 2015.

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