Hematite

oxide

ilmenite

Formula: Fe2O3 oxide
Specific gravity: 5.2 to 5.3
Hardness: 6½
Streak: Reddish brown
Colour: Reddish brown, grey, black
Solubility: Slightly soluble in hydrochloric acid
Environments:

Plutonic igneous environments
Pegmatites
Carbonatites
Sedimentary environments
Metamorphic environments (typical)
Volcanic sublimates and hot spring deposits
Hydrothermal environments

Hematite occurs as microscopic grains in almost all rocks, especially metamorphic rocks.
It is found in plutonic igneous environments as an accessory mineral in feldspar-rich igneous rocks such as granite, and in pegmatites and carbonatites.
Large ore bodies of hematite are usually of sedimentary origin. Hematite is also found in red sandstone as the cementing material that binds the quartz grains together.
Hematite occurs both in contact and regional metamorphic deposits, where it may have originated from the oxidation of limonite, siderite or magnetite.
It occurs in disseminated hydrothermal replacement deposits and in hydrothermal replacement lodes, as well as in the oxidation zone of epithermal (low temperature) and mesothermal (moderate temperature) hydrothermal veins.
It may also occur as a sublimation due to volcanic activity.

Hematite is a common constituent of marl.

At the Blue Points mine and at the Thunder Bay Amethyst mine, Thunder Bay, Ontario, Canada, microscopic spherulites of hematite occur as inclusions in quartz variety amethyst, often imparting a characteristic red coloration (R&M 94.4.320 and 332-333).

At the Llynclys quarry, near Oswestry, Shropshire, England, UK, hematite occurs on dolomite (RES p294).

Alteration

Hematite may form as an alteration product of ilmenite (AJM 18.2.26).

aegirine, epidote and CO2 to albite, hematite, quartz, calcite and H2O
4NaFe3+Si2O6 + 2Ca2(Al2Fe3+ [Si2O7](SiO4)O(OH) + 4CO2 → 4Na(AlSi3O8) + 3Fe2O3 + 2SiO2 + 4CaCO3 + H2O
(DHZ 2A p511)

calcite, hematite and quartz to andradite and CO2
3CaCO3 + Fe2O3 + 3SiO2 → Ca3Fe3+2Si3O12 + 3CO2

fayalite, oxygen and H2O to hematite and silicic acid
2Fe2SiO4 + O2 + 4H2O → 2Fe2O3 + 2H4SiO4
On prolonged exposure to the air Fe2+ compounds are oxidised to Fe3+ compounds according to reactions such as the one above (KB p334).

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).

hematite, wüstite, quartz and calcite to andradite, hedenbergite, magnetite and CO2
2Fe2O3 + 2FeO + 5SiO2 + 4CaCO3 → Ca3Fe3+2(SiO4)3 + CaFe2+Si2O6 +Fe2+Fe3+2O4 +4CO2

magnetite to hematite
2Fe3O4 + ½O2 ⇌ 3Fe2O3
Equilibrium is to one side or the other depending on temperature and pressure.

siderite, oxygen and H2O to hematite and silicic acid
2Fe2CO3 + O2 + 4H2O → 2Fe2O3 + 2H2CO3
On prolonged exposure to the air Fe2+ compounds are oxidised to Fe3+ compounds according to reactions such as the one above (KB p334).

Common impurities: Ti,Al,Mn,H2O

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