Pyrite

sulphide

chalcopyrite

Formula: FeS2
Sulphide
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
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.

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 Alderley Edge, Cheshire, England, UK, copper mineralised solutions percolated through porous sandstones 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 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 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 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 amethyst (variety of quartz) (R&M 93.6.512).

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).

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

Alteration

Marcasite and pyrite are polymorphs (same formula, different structure). 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).

pyrite to chalcocite: 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.
Cu2+ + pyrite + H2O to chalcocite + Fe2+ + (SO4)2- + H+
14Cu2+ + 5FeS2 + 12H2O → 7Cu2S + 5Fe2+ + 3(SO4)2- +24H+
(KB p527)

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.

Common impurities: Ni,Co,As,Cu,Zn,Ag,Au,Tl,Se,V

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