Pentlandite

pyrrhotite

Formula: (Ni,Fe)9S8
Sulphide, forms a series with cobaltpentlandite
Specific gravity: 4.6 to 5
Hardness: 3½ to 4
Streak: Pale bronze-brown
Colour: Pale bronze-yellow; bronze; brown; reddish brown when argentian
Solubility: Insoluble in 1:1 hydrochloric acid Dana
Environments

Plutonic igneous environments

Pentlandite is the most abundant nickel mineral. It forms through magmatic segregation in mafic and ultra-mafic plutonic igneous rocks (AES, Webmin) with iron and nickel sulphides and arsenides, almost always associated with pyrrhotite (Dana). It is also found in mantle xenoliths and undersea “blacksmoker” deposits, and rarely in stony meteorites (HOM).
Associated minerals include pyrrhotite, troilite, chalcopyrite, cubanite, mackinawite and magnetite (HOM).

There are two co-type localities, the Espedalen mines, Gausdal, Oppland, Norway, and the Craignure Mine, Inveraray, Argyll and Bute, Scotland, UK (Mindat).

In Botswana there are several nickel deposits, most of which contain pentlandite, pyrrhotite, chalcopyrite and pyrite (BC).

At Saint-Pierre-de-Broughton, Quebec, Canada, in the Alpine-type fissures and talc deposit, pentlandite is the only nickel mineral found thus far, occuring as inclusions in foliated masses of talc in the talc-carbonate rocks (R&M 85.6.505).

At the Hunting Hill quarry, Rockville, Maryland, USA, pentlandite is found in serpentine with minor magnetite and talc (Minrec 36.5.444), but this reference does not mention pyrrhotite from this locality.

In Zimbabwe pentlandite usually occurs with other nickel minerals, notably millerite, at several nickel deposits in ultramafic rocks (BC).

Alteration

The transformation of pentlandite to violarite has been investigated under mild hydrothermal conditions, at constant values of pH (3 to 5, quite acid). At 80oC, 20% by weight of the pentlandite transforms to violarite in 33 days (AM 91.706-709).

Common impurities: Co,Ag,Cu

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