Hematite

minerals

ilmenite

wustite

hedenbergite

Images
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
Common impurities: Ti,Al,Mn,H2O
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.

Localities

At the Payun volcano, Argentina, hematite pseudomorphs after magnetite have been found (KL p138).

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).

In the White Desert on the border between Egypt and Libya hematite pseudomorphs after marcasite have been found (KL p139).

At Croft Quarry, Croft, Blaby, Leicestershire, England, UK, there appear to be at least four generations of hematite. It occurs as inclusions in analcime, as coatings on analcime and on analcime epimorphs after laumontite, and associated with or coating calcite (JRS 20.17).

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

At Coed-y-Brenin deposit, Ganllwyd, Gwynedd, Wales, UK, magnetite forms scattered crystals to 1 mm in size, associated with isolated specular hematite rosettes to 2.5 mm, both phases occurring embedded in or perched on chlorite (JRS 21.115).

At the Magma mine, Pioneer District, Pinal county, Arizona, USA, hematite is the most common gangue mineral, and crystals have been found to 2.5 cm, some with a dusting of malachite, in association with calcite crystals (R&M 95.1.86).

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).

The diagram below is a Pourbaix diagram for Cu-Fe-S-H2O (IJNM 07(02).9.23). It shows the relationship between copper Cu, chalcopyrite CuFeS2, tenorite CuO, covellite CuS, cuprite Cu2O, chalcocite Cu2S, pyrite FeS2 and hematite Fe2O3.

Pourbaix Cu-Fe-S-H<sub>2</sub>O.jpg
























Back to Minerals