Silver

native element

calcite

Formula: Ag
Native element
Specific gravity: 9.6 - 12
Hardness: 2½ to 3
Streak: Silver white
Colour: Silver white
Solubility: Insoluble in hydrochloric acid; slightly soluble in sulphuric acid; moderately soluble in nitric acid
Environments:

Plutonic igneous environments
Placers
Hydrothermal environments

Native silver is found in the enriched zone of hypothermal (high temperature) hydrothermal veins, and sometimes also as a primary mineral, either in epithermal (low temperature) veins associated with sulphides, zeolites, calcite, baryte, fluorite and quartz, or in hypothermal (high temperature) veins associated with uraninite, arsenides and sulphides of cobalt, nickel and silver and native bismuth.

At Tsumeb, Namibia, native silver has been found associated with chalcocite, and as cementation on native copper (R&M 93.6.548).

At the New Cliffe Hill quarry, Stanton under Bardon, Leicestershire, England, UK, native silver has been found with azurite and cuprite on diorite (RES p198).

Alteration

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; it attacks silver according to the reaction:

silver + ferric sulphate → silver sulphate + ferrous sulphate
2Ag + Fe2(SO4)3 → Ag2SO4 + 2FeSO4
(AMU b3-3.7)

Common impurities: Au,Hg,Cu,Sb,Bi

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