Pyrrhotite

pyrrhotite

pentlandite

scheelite

bismuthinite

Images

Formula: Fe7S8 sulphide
Specific gravity: 4.6
Hardness: 4
Streak: Dark grey
Colour: Bronze
Solubility: Insoluble in water, nitric and sulphuric acid; soluble with decomposition in hydrochloric acid
Common impurities: Ni,Co,Cu
Environments:

Plutonic igneous environments
Pegmatites
Carbonatites
Metamorphic environments (typical)
Hydrothermal environments

Pyrrhotite is a primary mineral that may be found as a minor constituent of some igneous rocks. It is also found in pegmatites, contact metamorphic deposits, and in the enriched zone of hypothermal (high temperature) hydrothermal veins.
Pyrrhotite may be found in gabbro and norite (a gabbro where the main mafic mineral is orthopyroxene). It occurs in them as disseminated grains or as large masses associated with pentlandite, chalcopyrite and other sulphides.

Localities

The Two Mile and Three Mile deposits, Paddy's River, Paddys River District, Australian Capital Territory, Australia, are skarn deposits at the contact between granodiorite and volcanic rocks. Pyrrhotite is a primary sulphide that has been observed intergrown with pyrite in hedenbergite-rich skarn from the dump at the Two Mile adit. It shows alteration to marcasite (AJM 22.1.37).

At Desolation Prospect, Mount Isa, Australia, trace amounts of pyrrhotite occur as small inclusions in pyrite. Grains of galena are present in the pyrrhotite, which also contains minor cobalt and nickel (AJM 17.2.84).

At the Cobar Deposit, Australia, pyrrhotite is associated with early stage gold and with later stage chalcopyrite-cubanite. Where chalcopyrite-cubanite-pyrrhotite overprints early stage gold-bismuthinite, bismuth, gold, scheelite and pyrite apparently are recrystallised (AJM 11.2.67).

At Yaogangxian, China, pyrrhotite is often coated with small pyrite crystals (R&M 80.1.55).

At Santa Eulalia, Aquiles Serdán Municipality, Chihuahua, Mexico, pyrite and marcasite pseudomorphs after pyrrhotite have been found (R&M 95.3.275).

The Mponeng Mine, West Wits, Far West Rand, West Rand District Municipality, Gauteng, South Africa, has produced the finest pyrrhotite crystals known to date (June 2021) from any southern Africa locality. Crystals are up to 4 cm, tabular and hexagonal. They are associated with quartz and albite. Some specimens have an alteration coating of goethite and others have thin layers of finely crystallised sphalerite around the periphery of the pyrrhotite. Doubly terminated, barrel-shaped, hexagonal crystals tapering on edge have also been found (R&M 96.4.339-341).

At Barrasford Quarry, Chollerton, Northumberland, England, UK, pyrrhotite crystals up to 2 mm across have been seen, associated with galena and sphalerite in quartz-calcite veins (JRS 21.11).

At the Emmons pegmatite, Greenwood, Oxford county, Maine, USA, pyrrhotite has been found associated with molybdenite as millimetre sized grains. The Emmons pegmatite is an example of a highly evolved boron-lithium-cesium-tantalum enriched pegmatite (R&M 94.6.514).

Amity, Town of Warwick, Orange county, New York, USA, is an area of granite intrusions into marble and associated gneiss. The marble is mostly composed of white crystalline calcite that often has small flakes or spheres of graphite and phlogopite. Pyrrhotite occurs frequently in silver metallic masses to 6 cm that are strongly magnetic. Occasionally crude crystal faces are present (R&M 96.5.438).

Alteration

Oxidation of Pyrrhotite (JGE 84.65-76)

Sulphide oxidation generally occurs only in areas where dissolved or gaseous oxygen is present. Oxygen and ferric iron are important oxidants for pyrrhotite. When oxygen is the primary oxidant the reaction can be:
2Fe7S8 + 31O2 + 2H2O → 14Fe2+ + 16(SO4)2- + 4H+
The oxidation of ferrous iron produces ferric ions:
4Fe2+ + O2 + 8H+ → 4Fe3+ + 2H2O
If the pH is not too low (the environment is not too acid) the ferric ions can precipitate out of solution to form ferric hydroxide:
Fe3+ + 3H2O → Fe(OH)3(s) + 3H+
Ferric iron can in turn oxidise more pyrrhotite, generating more acidity in the system:
Fe7S8 + 62Fe3+ + 32H2O → 69Fe2+ + 8(SO4)2- + 64H+
There is evidence that the oxidation may not be complete, and instead generate elemental sulphur:
2Fe7S8 + 7O2 + 28H+ → 14Fe2+ + 16S0 + 14H2O
and
Fe7S8 + 14Fe3+ → 21Fe2+ + 8S0

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