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Formula: FeO(OH)
Hydroxide, diaspore group
Crystal System: Orthorhombic
Specific gravity: 4.27 to 4.29 measured, 4.18 calculated
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 Gascoyne River, Carnarvon Shire, Western Australia, Australia, goethite
pseudomorphs after marcasite
occur as nodules in white, fine-grained chalk along the river
(AJM 22.2.5-12).
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 Devil's Peak, Sai Kung District, New Territories, Hong Kong, China, the mineralisation occurred in
quartz veins in the contact zone between a
granite intrusion and acid volcanic rocks. The mine is now closed,
and inaccessible for collecting.
Goethite occurred as pseudomorphs after
pyrite
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)
The Ma On Shan Mine, Ma On Shan, Sha Tin District, New Territories, Hong Kong, China, is an abandoned
iron mine, with
both underground and open cast workings. The iron ores contain
magnetite as the ore mineral and occur predominantly as masses of all sizes
enclosed in a large skarn body formed by contact metasomatism of
dolomitic limestone at the
margins of a granite intrusion. In parts of the underground workings
magnetite is also found in
marble in contact with the
granite. The skarn rocks
consist mainly of tremolite,
actinolite, diopside and
garnet.
Goethite occurs as botryoidal and stalactitic forms along fissures in the
skarn
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)
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).
The San Valentín Mine, Sancti Spiritu, La Unión, Murcia, Spain is an open pit
lead - zinc -
iron mine. The deposit is a sub-volcanic, hydrothermal, late
Tertiary (66 million to 2.6 million years ago) replacement of
limestone.
(Mindat).
Some beautiful and unusual specimens of goethite-coated quartz
have been collected here. The otherwise routine clusters of quartz
crystal points are covered entirely by films of goethite which
render them vividly iridescent
(Minrec 55.1.111).
Image
The Sunnyside Deposit, Whitwell, Bolsover District, Derbyshire, England, UK, is hosted by late Permian (256 to 248
million years ago) dolostone that lies above a thick sequence of
Carboniferous (354 to 290 million years ago) Coal Measures sediments.
Goethite replacements of marcasite are ubiquitous in vein
baryte. The central cavities in the feeder veins are commonly lined
with abundant millimetre-size pseudomorphs. Sharp replacements
of goethite after marcasite are occasionally present in
cavities in replacement baryte. Rare cubic
pseudomorphs found in the same situation are almost certainly
after pyrite
(JRS 24.37-59).
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).
At Laverock Braes, Middleton Park, Aberdeen City, Scotland, UK, massive dark brown goethite is abundant.
Specimens commonly contain cavities lined by distinctive chocolate-brown botryoidal crusts, with a characteristic
radiating structure. Goethite forms along the edges of some iron- and
manganese-bearing veins, where it may be shattered and criss-crossed by
later manganite veinlets. Powdery brown
limonite, which may be recent in origin, coats some specimens found
loose in the till. Cavities in massive goethite are commonly infilled with white to red-stained
baryte
(JRS 22.19).
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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