"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
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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