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"Limonite" is a term for unidentified massive hydroxides and oxides of iron, with no visible crystals, and a
yellow-brown streak. It is most commonly the mineral species goethite,
but it can also consist of varying proportions of other minerals, including
jarosite group minerals and
hematite.
Localities
At the Clargillhead vein, Garrigill, Alston Moor, Eden, Cumbria, England, UK, limonite replaces 10to 40
micron diameter pentagonal dodecahedral pyrite crystals,locally forming
aggregates up to 200 microns across. Banded limonite also replaces
chalcopyrite and pale-coloured
secondary copper sulphides
(JRS 23.51).
Formation
Both ferrous and ferric sulphate (products of the weathering of pyrite near
the surface) react further with water to form ferric
hydroxide, which is insoluble in water and so is deposited where it is formed.
6FeSO4 (ferrous sulphate) + 3O + 3H2O → Fe2(SO4)3 +
2Fe(OH)3
(ferric hydroxide)
Fe2(SO4)3 (ferric sulphate) + 6H2O → 2Fe(OH)3 (ferric hydroxide)
+ 3H2SO4
The ferric hydroxide alters to hematite and
goethite,
and forms the ever present limonite that characterizes all oxidised zones.
2Fe(OH)3 → Fe2O3 (hematite) +
3H2O
Fe(OH)3 → FeO(OH) (goethite) + H2O
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