Pyrrhotite

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Formula: Fe₇S₈
Sulphide, pyrrhotite group
Crystal System: Monoclinic
Specific gravity: 4.58 to 4.65 measured, 4.69 calculated
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).
Pyrrhotite from Mount Isa - Image

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).

The Ma On Shan Mine, Ma On Shan, Sha Tin District, New Territories, Hong Kong, China, is an abandoned iron mine, with both underground and open cast workings. The iron ores contain magnetite as the ore mineral and occur predominantly as masses of all sizes enclosed in a large skarn body formed by contact metasomatism of dolomitic limestone at the margins of a granite intrusion. In parts of the underground workings magnetite is also found in marble in contact with the granite. The skarn rocks consist mainly of tremolite, actinolite, diopside and garnet. Pyrrhotite occurs as granular to compact masses in the quartz veins associated with chalcopyrite and pyrite (Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)

At Chuen Lung, Tsuen Wan District, New Territories, Hong Kong, China, in fissure veins in granite rocks in a small stream near Chuen Lung, silver-bearing galena occurs associated with massive granular amber coloured sphalerite, chalcopyrite, pyrite and pyrrhotite (Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)

At the Xianghuapu Mine, Xianghualing Sn-polymetallic ore field, Linwu County, Chenzhou, Hunan, China, massive bronze-brown pyrrhotite has been found; it is decidedly magnetic (AESS).
Pyrrhotite from Xianghuapu - Image

The Bairendaba Ag-polymetallic deposit, Hexigten Banner, Chifeng City, Inner Mongolia, China, is a mesothermal magmatic-hydrothermal vein-type silver - lead - zinc deposit, hosted in Hercynian (about 419 to 299 million years ago) quartz diorite.
It is suggested that, with decreasing temperature, mineral compositions changed progressively from tungstate and oxide, to diatomic sulphide, to simple sulphide, to an antimony sulphosalt mineral, and finally to an antimonide.
Pyrrhotite has been found in magnificent, lustrous crystals up to 10 cm across, in a thick and broadly beveled habit. Associations can include lustrous, purple, cubic to cuboctahedral fluorite crystals and lustrous cubic galena (Minrec 53.347-359).
Pyrrhotite from Bairendaba - Image

At the Yaogangxian Mine, Yaogangxian W-Sn ore field, Yizhang county, Chenzhou, Hunan, China, pyrrhotite is often coated with small pyrite crystals (R&M 80.1.55).
Pyrrhotite from Yaogangxian - Image

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

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).
Pyrrhotite from Mponeng - Image

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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