Editing Pentlandite (section)

== Paragenesis ==
Pentlandite is the most common terrestrial nickel sulfide. It typically forms during cooling of a sulfide melt. These sulfide melts, in turn, are typically formed during the evolution of a silicate melt. Because nickel is a [[chalcophile]] element, it has preference for (i.e. it "partitions into") sulfide phases.<ref name="Mansur-2021">{{Cite journal |last1=Mansur |first1=Eduardo T. |last2=Barnes |first2=Sarah-Jane |last3=Duran |first3=Charley J. |date=2021-01-01 |title=An overview of chalcophile element contents of pyrrhotite, pentlandite, chalcopyrite, and pyrite from magmatic Ni-Cu-PGE sulfide deposits |url=https://doi.org/10.1007/s00126-020-01014-3 |journal=Mineralium Deposita |language=en |volume=56 |issue=1 |pages=179–204 |doi=10.1007/s00126-020-01014-3 |bibcode=2021MinDe..56..179M |s2cid=221674533 |issn=1432-1866}}</ref> In sulfide undersaturated melts, nickel substitutes for other [[transition metal]]s within [[Mafic|ferromagnesian]] minerals, the most common being [[olivine]], as well as nickeliferous varieties of [[amphibole]], [[biotite]], [[pyroxene]] and [[spinel]]. Nickel substitutes most readily for Fe<sup>2+</sup> and Co<sup>2+</sup> because or their similarity in size and charge.<ref>{{Cite journal |last1=Rajamani |first1=V. |last2=Naldrett |first2=A. J. |date=1978-02-01 |title=Partitioning of Fe, Co, Ni, and Cu between sulfide liquid and basaltic melts and the composition of Ni-Cu sulfide deposits |url=https://doi.org/10.2113/gsecongeo.73.1.82 |journal=Economic Geology |volume=73 |issue=1 |pages=82–93 |doi=10.2113/gsecongeo.73.1.82 |bibcode=1978EcGeo..73...82R |issn=1554-0774}}</ref>

In sulfide saturated melts, nickel behaves as a chalcophile element and [[partition coefficient|partitions]] strongly into the sulfide phase. Because most nickel behaves as a compatible element in [[igneous differentiation]] processes, the formation of nickel-bearing sulfides is essentially restricted to sulfide saturated mafic and ultramafic melts. Minor amounts of nickel sulfides are found in mantle [[peridotite]]s.<ref name="Mansur-2021" />

The behaviour of sulfide melts is complex and is affected by copper, nickel, iron, and sulfur ratios. Typically, above 1100&nbsp;°C, only one sulfide melt exists. Upon cooling to 1000&nbsp;°C, a solid containing mostly Fe and minor amounts of Ni and Cu is formed. This phase is called monosulfide solid solution (MSS), and is unstable at low temperatures decomposing to mixtures of pentlandite and [[pyrrhotite]], and (rarely)  [[pyrite]].  It is only upon cooling past ~{{convert|550|°C}} (dependent on composition) that the MSS undergoes [[exsolution]]. A separate phase, usually a copper-rich sulfide liquid may also form, giving rise to [[chalcopyrite]] upon cooling.<ref>{{Cite journal |last1=Shewman |first1=R. W. |last2=Clark |first2=L. A. |date=1970-02-01 |title=Pentlandite phase relations in the Fe–Ni–S system and notes on the monosulfide solid solution |url=http://www.nrcresearchpress.com/doi/10.1139/e70-005 |journal=Canadian Journal of Earth Sciences |language=en |volume=7 |issue=1 |pages=67–85 |doi=10.1139/e70-005 |bibcode=1970CaJES...7...67S |issn=0008-4077}}</ref>

These phases typically form [[aphanitic]] equigranular massive sulfides, or are present as disseminated sulfides within rocks composed mostly of silicates. Pristine magmatic massive sulfide are rarely preserved as most deposits of nickeliferous sulfide have been metamorphosed.

Metamorphism at a grade equal to, or higher than, [[greenschist]] [[facies]] will cause solid massive sulfides to deform in a ductile fashion and to travel some distance into the [[country rock (geology)|country rock]] and along structures.<ref>{{Cite journal |last1=Frost |first1=B. R. |last2=Mavrogenes |first2=J. A. |last3=Tomkins |first3=A. G. |title=Partial Melting of Sulfide Ore Deposits During Medium- and High-Grade Metamorphism |date=2002-02-01 |url=https://doi.org/10.2113/gscanmin.40.1.1 |journal=The Canadian Mineralogist |volume=40 |issue=1 |pages=1–18 |doi=10.2113/gscanmin.40.1.1 |issn=0008-4476}}</ref> Upon cessation of metamorphism, the sulfides may inherit a [[Foliation (geology)|foliated]] or [[Shear (geology)|sheared]] texture, and typically develop bright, equigranular to globular aggregates of [[porphyroblast]]ic pentlandite crystals known colloquially as "fish scales".<ref>{{Cite journal |last=McQueen |first=K. G. |date=1987-05-01 |title=Deformation and remobilization in some Western Australian nickel ores |url=https://dx.doi.org/10.1016/0169-1368%2887%2990032-1 |journal=Ore Geology Reviews |language=en |volume=2 |issue=1 |pages=269–286 |doi=10.1016/0169-1368(87)90032-1 |bibcode=1987OGRv....2..269M |issn=0169-1368}}</ref>

Metamorphism may also alter the concentration of nickel 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 be associated with [[metasomatism]], and it is particularly common for [[arsenic]] to react with pre-existing sulfides, producing [[nickeline]], [[gersdorffite]] and other Ni–Co arsenides.<ref>{{Cite journal |last1=Piña |first1=R. |last2=Gervilla |first2=F. |last3=Barnes |first3=S.-J. |last4=Ortega |first4=L. |last5=Lunar |first5=R. |date=2015-03-01 |title=Liquid immiscibility between arsenide and sulfide melts: evidence from a LA-ICP-MS study in magmatic deposits at Serranía de Ronda (Spain) |url=https://doi.org/10.1007/s00126-014-0534-3 |journal=Mineralium Deposita |language=en |volume=50 |issue=3 |pages=265–279 |doi=10.1007/s00126-014-0534-3 |bibcode=2015MinDe..50..265P |s2cid=140179760 |issn=1432-1866}}</ref>