Goethite

oxide

chalcopyrite

Formula: FeO(OH)
Oxide containing hydroxyl
Specific gravity: 4.3
Hardness: 5 to 5½
Streak: Brown to brownish yellow
Colour: Yellow, brown to dark brown and reddish brown
Solubility: Slightly soluble in hydrochloric acid
Environments:

Pegmatites
Carbonatites
Sedimentary environments
Hydrothermal environments

Goethite is a very common mineral, typically formed by the oxidation of iron-bearing minerals. It also forms as a direct inorganic or biogenic precipitate from water and it is widespread as a deposit in bogs and springs. Large quantities of goethite have resulted from the weathering of serpentine. It also occurs in vesicles in volcanic rocks, and in the oxidation zone of hypothermal (high temperature) hydrothermal veins.

At the Blue Points mine, Thunder Bay, Ontario, Canada, goethite has been observed occasionally as microscopic acicular needles in quartz variety amethyst (R&M 94.4.318).

At Old Ham mine, Clearwell, Gloucestershire, England, UK, goethite occurs with calcite in dolomitised rock (RES p153).

At Iron Acton, Gloucestershire, England, UK, goethite occurs with calcite in sandstone (RES p168).

At Cloud Hill quarry, Breedon on the Hill, Leicestershire, England, UK, goethite is associated with hematite, quartz and calcite (RES p206).

Alteration

chalcopyrite to goethite
chalcopyrite + oxygen + water ⇌ cupric sulphate + goethite + sulphuric acid
4CuFeS2(solid) + 17O2(gaseous) + 6H2O(liquid) ⇌ 4Cu2+SO4(aqueous) + 4FeO(OH)(solid) + 4H2SO4(aqueous)
(JRS 18.12)

hematite and H2O to goethite
Fe2O3 + H2O ⇌ 2FeO(OH)
Both forward and reverse reactions are slow, but equilibrium in most natural environments is displaced to the left, favouring the formation of hematite (KB p362).

marcasite to goethite
Pseudomorphs of goethite after marcasite are fairly common in the Northern Pennine orefield, England, although unaltered marcasite is very rare there. The alteration takes place in three stages:
First each sulphur atom in the marcasite combines with four oxygen atoms to form sulphate groups (SO4)2-, resulting in ferrous (divalent iron) sulphate and sulphuric acid, as for the oxidation of pyrite, noted above:
marcasite + water + oxygen → ferrous sulphate + sulphuric acid
2FeS2 + 2H2O + 7O2 → 2Fe2+SO4 + 2H2SO4
Next the ferrous sulphate is oxidised to ferric (trivalent iron) sulphate:
ferrous sulphate + sulphuric acid + oxygen → ferric sulphate + water
4Fe2+SO4 + 2H2SO4 + O2 → 2Fe3+2(SO4)3 + 2H2O
Lastly ferric sulphate in solution is stable only at very low pH (very acid solution) and usually rapidly breaks down to form hydroxy iron oxides such as goethite, and sulphuric acid:
ferric sulphate + water → goethite + sulphuric acid
Fe3+2(SO4)3 + 4H2O → 2Fe3+O(OH) + 3H2SO4
(JRS 14.69-76)

Common impurities: Mn

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