Goethite

goethite

cuprite

serpentine

chalcopyrite

Images

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
Common impurities: Mn
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.

Localities

At the Mount Kelly deposit, Gunpowder District, Queensland, Australia, the deposit has been mined for oxide and supergene copper ores, predominantly malachite, azurite and chrysocolla. The ores overlie primary zone mineralisation consisting of quartz-dolomite-sulphide veins hosted in dolomite-bearing siltstone and graphitic schist.
Goethite is common, coating fractures together with hematite and secondary copper minerals. Iridescent goethite occurs along the walls in the oxide zone (AJM 22.1.21).

At the Marron Volcanics of the Olalla Area, South-Central British Columbia, Canada, goethite has been found as two types: as limonitised pseudomorphs after pyrite and as radial sprays of golden brown acicular crystals to 3 mm on brewsterite and heulandite. The pseudomorphs and the unaltered pyrite are associated with one another. Goethite was found primarily near the pumphouse along the Yellow Lake road cut (R&M 96.6.522).

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 Chessy, Lyon, Rhone-Alpes, France, goethite pseudomorphs after cuprite have been found (KL p142).

At Chihuahua, Mexico, goethite pseudomorphs after gypsum have been found (KL p143).

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 Croft Quarry, Croft, Blaby, Leicestershire, England, UK, microscopic spheroids of goethite have been found on quartz in association with calcite. A specimen has been found where rusty spots of goethite replacing pyrite were associated with a small cubo-octahedral galena crystal, with some cerussite alteration, on analcime (JRS 20.17).

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

In Teller county, Colorado, USA, goethite pseudomorphs after siderite have been found with microcline (KL p145).

At Pelican Point, Utah, USA, goethite pseudomorphs after pyrite have been found (KL p144).

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)

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