Pyrite

pyrite

marcasite

sphalerite

chalcopyrite

Images

Formula: FeS2
Oxidation states: Fe2+S1-2 (AM 87.1692-1698)
Sulphide, the isometric paramorph of marcasite, which is orthorhombic

Varieties

Bravoite is a nickel-bearing variety of pyrite

Properties

Specific gravity: 4.8 to 5
Hardness: 6 to 6½
Streak: Greenish black
Colour: Pale brass-yellow
Solubility: Insoluble in hydrochloric acid and sulphuric acid; slightly soluble in nitric acid
Common impurities: Ni,Co,As,Cu,Zn,Ag,Au,Tl,Se,V
Environments:

Plutonic igneous environments
Pegmatites
Carbonatites
Sedimentary environments
Metamorphic environments
Hydrothermal environments
Basaltic cavities

Pyrite is the most common and widespread of the sulphide minerals. It forms at both high and low temperature, but the largest masses probably at high temperature. It is found in plutonic igneous environments including pegmatites and carbonatites; it is a common mineral in clastic and also chemical sedimentary rocks, being both primary and secondary. It occurs in contact metamorphic deposits, disseminated hydrothermal replacement deposits, hydrothermal replacement lodes and as a primary mineral in hypothermal (high temperature) and mesothermal (moderate temperature) hydrothermal veins.
Pyrite may be found in dolostone and limestone.
It is associated with many minerals but found most frequently with chalcopyrite and sphalerite.

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. Pyrite is a primary sulphide, and the commonest sulphide in the Two Mile deposit. It occurs as grains and small aggregates in magnetite together with quartz and chlorite, and generally associated with other sulphides, particularly chalcopyrite (AJM 22.1.37).

At the Mount Kelly deposit, Gunpowder District, Queensland, Australia, the deposit has been mined for oxide and supergene copper ores, predominantly malachite, azurite and chrysocolla. The ores overlie primary zone mineralisation consisting of quartz-dolomite-sulphide veins hosted in dolomite-bearing siltstone and graphitic schist.
Pyrite occurs in quartz-carbonate veins with chalcopyrite, and as finely disseminated grains throughout the siltstone, together with minor chalcopyrite and rare sphalerite. Weathered pyrite is associated with covellite, hematite, jarosite, goethite and brochantite. Paragenesis for the primary zone is dolomite followed by pyrite, then chalcopyrite and sphalerite, and lastly bornite (AJM 22.1.20 & 25).

At Mount Moliagul, Moliagul, Central Goldfields Shire, Victoria, Australia, pyrite crystals to 3 mm are found commonly with molybdenite in cavities in the quartz veins, in aplite and in granodiorite (AJM 21.1.44).

At the Mount Deverell variscite deposit, Milgun Station, Western Australia, crystals of pyrite have been replaced by goethite, alunite, variscite, crandallite and apatite. The variscite deposits are hosted by marine sedimentary rocks (AJM 20.2.27).

In Bulgaria pyrite pseudomorphs after chalcopyrite have been found (R&M 95.3.275).

At the Blue Points mine, Thunder Bay, Ontario, Canada, pyrite has been found with chalcopyrite and marcasite (R&M 94.4.326), and as inclusions in quartz variety amethyst (R&M 94.4.325, 333).

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

At Berg Aukas, Grootfontein, Otjozondjupa Region, Namibia, pyrite is disseminated in some of the ore bodies, but goethite pseudomorphs after pyrite are more prevalent than unaltered pyrite (R&M 96.2.132).
The paragenetic sequence for the sulphides is proposed to be pyrite (oldest) - bornite - chalcopyrite - tennantite - sphalerite - galena - enargite - germanite - renierite - tetrahedrite - jordanite (youngest) (R&M 96.2.113).

At the Huanzala mine, Peru, pyrite pseudomorphs after pyrrhotite have been found (KL p132).

At the Witwatersdand Goldfield, South Africa, pyrite occurs in two forms. One is so-called buckshot pyrite, well rounded smooth nodules that may be detrital. Also large well formed crystals of secondary pyrite are relatively common in some gold mines. They are usually simple cubes, up to 9 cm on edge. Some calcite specimens have attractive dustings of tiny pyrite crystals on the surfaces of the calcite. Associated minerals include quartz, calcite, sphalerite and galena (R&M 96.4.335-337).

At Alderley Edge, Cheshire, England, UK, copper mineralised solutions percolated through porous sandstone and deposited barium, cobalt, copper, lead, vanadium and zinc minerals between the sand grains. Anhydrite formed as cement in permeable rocks, then baryte was deposited, followed by pyrite, chalcopyrite, sphalerite and galena. Subsequently a second generation of baryte and iron-rich calcite followed. These minerals crystallised from highly saline, sulphate-rich brines, at a temperature of 50 to 60oC. About 65 million years ago the deposit was uplifted, and oxygenated ground water oxidised original sulphide minerals; pyrite was oxidised to goethite (RES pps 49-50).

At the Hampstead Farm quarry, Chipping Sodbury, South Gloucestershire, England, UK, pyrite occurs with baryte and calcite (RES pps 173).

At Croft Quarry, Croft, Blaby, Leicestershire, England, UK, small amounts of pyrite have been found associated with magnetite and molybdenite. Pyrite also occurs as groups of dull brown cubes to 1 mm with calcite, minor chalcopyrite and marcasite on analcime (JRS 20.23-24).

At Granitethorpe quarry, Sapcote, Blaby, Leicestershire, England, UK, pyrite occurs with epidote and some large crystals of pink feldspar; it seems likely that this is an occurrence associated with a pegmatite. Pyrite has been found forming the cores of nodular masses consisting of pyrite, quartz and epidote, completely enclosed within the formerly quarried tonalite which were presumably xenoliths (JRS 20.24).

At Earl Ferrers mine, Staunton Harold, Leicestershire, England, UK, pyrite is associated with sphalerite and galena (RES pps 216).

At Coalfield North opencast, Heather, Leicestershire, England, UK, pyrite is associated with calcite (RES pps 227).

At the Wotherton mine, Chirbury, Shropshire, England, UK, pyrite is associated with calcite and chalcopyrite (RES pps 285).

At Llynclys quarry, near Oswestry, Shropshire, England, UK, pyrite occurs in a mudstone - clay matrix (RES pps 295).

At the Magma mine, Pioneer District, Pinal county, Arizona, USA, pyrite is sometimes associated with quartz (R&M 95.1.86-87).

At Willow Springs, Pinal county, Arizona, USA, limonite pseudorphs after pyrite occur in quartz veins R&M 94.2.166).

At the Fat Jack mine, Yavapai county, Arizona, USA, limonite pseudorphs after pyrite are not uncommon R&M 94.2.165).

At the Emmons pegmatite, Greenwood, Oxford county, Maine, USA, pyrite is associated with sphalerite in rhodochrosite masses with lithiophilite. Pyrite replaced by goethite occurs in cavities of the albite replacement mass. The Emmons pegmatite is an example of a highly evolved boron-lithium-cesium-tantalum enriched pegmatite (R&M 94.6.514).

At the Little Gem amethyst mine, Jefferson county, Montana, USA, pyrite occurs in scattered small concentrations, mostly altered to limonite. Unaltered pyrite occurs only as small crystals included in the quartz variety amethyst (R&M 93.6.512).

At the PC Mine, Cataract Mining District, Jefferson county, Montana, USA, pyrite occurred as simple heavily striated cubes to 3 cm in association with quartz, but few pockets contained pyrite (R&M 96.6.494).

At Cookes Peak mining district, Luna county, New Mexico, USA, pyrite is almost always associated with primary galena and sphalerite as massive replacement lenses in the un-oxidised portions of mines (R&M 94.3.233).

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. Pyrite is occasionally found in striated cubes with modified faces. Many of these crystals are dark with a surface alteration to goethite (R&M 96.5.439).

At the Pyrites Mica mine, St Lawrence county, New York, USA, pyrite is associated with meionite (R&M 93.4.343).

At the Suever Stone Company quarry, Delphos, Van Wert county, Ohio, USA, pyrite is found throughout the quarry. Crystals occurred in pockets and within the dolostone matrix. Pyrite can be found that preceded fluorite or grew simultaneously with it or followed it, showing that the pyrite had a longer period of deposition than the fluorite.
In some pockets pyrite crystals are found intergrown with fluorite. The pyrite crystals have a “root” extending into the fluorite more or less perpendicular to the fluorite face from which they protrude. These interesting crystals demonstrate that the fluorite and pyrite were growing at the same time.
Some pyrite crystals have little dimples on their surface that look like tiny droplets of water, but they are composed entirely of pyrite; Circular rings are also found, and sometimes the area between crystals appears to have a puddle of water filling the gap, but it is all pyrite, and the pyrite that fills these features is crystallographically continuous with the surrounding crystal. Some calcite and fluorite crystals from this quarry display similarly uncommon features, and all of these unusual examples have one thing in common, they were found in oil-saturated pockets; the mechanism for their formation, however, is not known (R&M 95.6.509-513).

At Pelican Point, Utah, USA, goethite pseudomorphs after pyrite have been found (KL p144).

Alteration

Marcasite and pyrite are paramorphs. Marcasite is a mineral of low-temperature, near-surface, environments, forming from acid solutions. Pyrite is the more stable form of FeS2, and forms in higher temperatures and lower acidity or alkaline environments.

Oxidation of pyrite forms ferrous (divalent) sulphate and sulphuric acid:
pyrite + oxygen + H2O → ferric sulphate + sulphuric acid
FeS2 + 7O + H2O → FeSO4 + H2SO4
The ferrous (divalent) sulphate readily oxidizes to ferric (trivalent) sulphate and ferric hydroxide:
ferrous sulphate + oxygen + H2O → ferric sulphate + ferric hydroxide
6FeSO4 + 3O + 3H2O → 2Fe2(SO4)3 + 2Fe(OH)3
Ferric sulphate is a strong oxidizing agent, and it attacks sulphide minerals in hydrothermal veins, to form soluble salts which trickle downwards through the deposit to be redeposited in the enrichment zone (AMU b3-3.7).

Cu2+, pyrite and H2O to chalcocite, Fe2+, (SO4)2- and H+
14Cu2+ + 5FeS2 + 12H2O → 7Cu2S + 5Fe2+ + 3(SO4)2- +24H+
Because chalcocite is less soluble than pyrite, supergene chalcocite may form below the zone of oxidation when dissolved copper ions Cu2+ replace ferrous ions Fe2+ from pyrite.
(KB p527)

chalcopyrite, arsenopyrite and pyrite to Fe-tennantite and troilite
10CuFeS2 + 4FeAsS + FeS2 → Cu10Fe2As4S13 + 13FeS
(CM 28.725-738)

chalcopyrite, arsenopyrite and sulphur to Fe-tennantite and pyrite
10CuFeS2 + 4FeAsS + 13/2S2 → Cu10Fe2As4S13 + 12FeS2
This reaction occurs at a comparatively low temperature (CM 28.725-738).

chalcopyrite, stibnite and sulphur to Fe-tetrahedrite and pyrite
10 CuFeS2 + 2 Sb2S3 + 3/2 S2 → Cu10Fe2As4S13 + 8FeS2
(CM 28.725-738)

enargite and pyrite to Fe-tennantite, chalcopyrite and sulphur
4Cu3AsS4 + 4FeS2 → Cu10Fe2As4S13 + 2CuFeS2 + 7/2S2
(CM 28.725-738)

pyrite (primary) O and H2O to secondary melanterite and sulphuric acid.
FeS2 + 7O + 8H2O → FeSO4.7H2O + H2SO4
Melanterite indicates the presence of sulphuric acid, and it should be handled with care.

stibnite and pyrite to berthierite and sulphur
Sb2S3 + FeS2 → FeSb2S4 + l/2S2
(CM 28.725-738)

The diagram below is a Pourbaix diagram for Cu-Fe-S-H2O (IJNM 07(02).9.23). It shows the relationship between copper Cu, chalcopyrite CuFeS2, tenorite CuO, covellite CuS, cuprite Cu2O, chalcocite Cu2S, pyrite FeS2 and hematite Fe2O3.

Pourbaix Cu-Fe-S-H<sub>2</sub>O.jpg
























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