Anglesite

sulphate

galena

Formula: Pb(SO4)
Sulphate, lead mineral
Specific gravity: 6.37 to 6.39
Hardness: 2½ to 3
Streak: White
Colour: Colourless, white, yellow, green; colourless in transmitted light.
Solubility: Anglesite is not very soluble in water.
Environments:

Hydrothermal environments

Anglesite is a common high temperature secondary mineral in the oxidation zone of hydrothermal replacement deposits rich in lead.
Lead will generally precipitate as primary galena from ore fluids rich in sulphur and lead. Removal of sulphur by precipitation of sulphides, however, may lead ultimately to an ore fluid from which galena cannot be precipitated, even with a high concentration of lead in solution. In these circumstances, anglesite, as well as cerussite and pyromorphite, could be precipitated as a primary mineral. (Strens (1963), MM 33.722-3).
Anglesite is commonly associated with galena, sphalerite, smithsonite, hemimorphite and iron oxides.

At Tsumeb, Namibia, anglesite has been found associated with native copper (R&M 93.6.539).

At Whitwell quarry, Derbyshire, England, UK, anglesite occurson an oxidised galena - baryte matrix (RES p136, 137).

Alteration

In the oxidation zone, oxidation of pyrite forms ferrous (divalent) sulphate and sulphuric acid:
pyrite + oxygen + water → 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 + water → ferric sulphate + ferric hydroxide
6FeSO4 + 3O + 3H2O → 2Fe2(SO4)3 + 2Fe(OH)3
Ferric sulfate is a strong oxidizing agent; it attacks galena as below.

Anglesite and cerussite do not usually occur together. Generally anglesite is stable in lower pH (more acid) environments and cerussite in higher pH (more alkaline) environments. Seawater has a pH of approximately 8.3 (somewhat alkaline) so cerussite is the stable lead supergene mineral in contact with seawater (JRS 18.9,11).
Hydrocerussite requires an alkaline environment, and it cannot co-exist with anglesite (JRS 18.11).

galena, ferric sulphate, water and oxygen to anglesite, ferrous sulphate and sulphuric acid
PbS + Fe2(SO4)3 + H2O + 3O → PbSO4 + 2FeSO4 + H2SO4
Galena is oxidised to anglesite and ferric iron is reduced to ferrous iron (AMU b3-.

galena and oxygen to anglesite
In air, at outcrops of galena,
PbS + 2O2 → PbSO4
At ordinary temperatures the equilibrium is displaced far to the right, and the apparent stability of galena is a result of the slowness of the oxidation (KB).

galena may also dissolve in carbonic acid from percolating rainwater to form hydrogen sulphide, which is then oxidised to form anglesite. (KB).
galena and carbonic acid to Pb2+, hydrogen sulphide and HCO3-
PbS + 2H2CO3 → Pb2+ + H2S + 2HCO3>- (KB).

hydrogen sulphide, oxygen, Pb2+ and HCO3>- to anglesite and carbonic acid
H2S + 2O2 + Pb2+ + 2HCO3- → PbSO4 + 2H2CO3 (KB)

Common impurities: Ba, Cu

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