Sphalerite

galena

wurtzite

Formula: ZnS
Sulphide, polymorph of wurtzite. Wurtzite is the high temperature polymorph of sphalerite; the transition for pure ZnS occurs at 1020oC, and this transition temperature is lowered by the presence of iron in solid solution ( AM 46.1382).
Specific gravity: 3.9 to 4.2
Hardness: 3½ to 4
Streak: White, yellow to brown if iron is present
Colour: Yellow, brown, red, green, black, rarely colourless
Solubility: Moderately soluble in hydrochloric acid; slightly soluble in nitric acid; insooluble in water
Environments:

Igneous environments
Carbonatites
Sedimentary environments
Metamorphic environments
Hydrothermal environments

Sphalerite is extremely common. Its occurrence and mode of origin are similar to those of galena, with which it is commonly found. Sphalerite occurs in granite, gabbro, sedimentary deposits and contact metamorphic deposits. It is found as a primary mineral in hypothermal (high temperature) hydrothermal veins associated with arsenopyrite, galena and quartz, and in replacement deposits associated with chalcopyrite, galena and pyrite.
Sphalerite with only minor galena occurs associated with pyrrhotite, pyrite and magnetite.
It also may be found in gabbro and granite.
In the oxidised zone of epithermal veins, sphalerite (primary) alters to secondary hemimorphite, smithsonite and manganese-bearing willemite.

At the Blue Points mine, Thunder Bay, Ontario, Canada, sphalerite is associated with calcite and minor chalcopyrite in silicified mudstone breccia (R&M 94.4.330).

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 (RES pps 49-50).

At the Gregory mine, Ashover, Derbyshire, England, UK, sphalerite is associated with fluorite (RES p102, 103).

At the Odin mine, Castleton, Derbyshire, England, UK, sphalerite is associated with fluorite and baryte (RES p130).

At Millclose mine, Darley Dale, Derbyshire, England, UK, sphalerite is associated with pyrite, fluorite and calcite (RES p93, 95).

At Eyam, Derbyshire, England, UK, sphalerite is associated with calcite, fluorite and galena (RES p117).

At Ladywash mine, Eyam, Derbyshire, England, UK, sphalerite is associated with fluorite and pyrite (RES p120).

At Cloud Hill quarry, Breedon on the Hill, Leicestershire, England, UK, sphalerite is associated with calcite, baryte and chalcopyrite in dolomitised limestone (RES p207).

At Lord Ferrer's mine, Staunton Harold, Ashby-de-la-Zouch, Leicestershire, England, UK, sphalerite is associated with galena, chalcopyrite, calcite, baryte and dolomite (RES p222).

At the Callow Hill quarry, Pontesbury, Shropshire, England, UK, sphalerite has been found in quartz veinstone associated with galena (RES p291).

At Judkins quarry, Nuneaton, Warwickshire, England, UK, sphalerite is associated with baryte and calcite (RES p325).

At Cookes Peak mining district, Luna county, New Mexico, USA, sphalerite was the principal source for zinc in secondary smithsonite. Unoxidised masses occur most often with pyrite and galena in replacement pods in dolomite (R&M 94.3.236).

Alteration

Sphalerite is a dimorph of wurtzite. The transition from sphalerite to wurtzite occurs around 1020oC.

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 sulfate is a strong oxidizing agent; it attacks sphalerite as below:

sphalerite, ferric sulphate and water to zinc sulphate, ferrous sulphate and sulphuric acid
ZnS + 4Fe2(SO4)3 + 4H2O → ZnSO4 + 8FeSO4 + 4H2SO4
(AMU/b3-3-7.htm)

sphalerite to covellite: Because covellite is less soluble than sphalerite, supergene covellite may form below the zone of oxidation when dissolved copper ions Cu2+ replace zinc ions Zn2+ from sphalerite.
Cu2+ + sphalerite → covellite + Zn2+
Cu2+ + ZnS → CuS + Zn2+
(KB p527)

Common impurities: Mn,Cd,Hg,In,Tl,Ga,Ge,Sb,Sn,Pb,Ag

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