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Formula: Fe2O3 oxide
Hydrohematite is a variety of hematite with the formula Fe2-x(OH)xO3-x
Martiteis the name given to hematite pseudomorphs after
magnetite
Specularite is a variety of hematite characterized by aggregates of silvery, metallic, specular ("mirror-like")
hematite flakes or tabular, anhedral crystals.
Crystal System: Trigonal
Specific gravity: 5.26 measured, 5.255 calculated
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).
The Two Mile and Three Mile deposits, Paddy's River, Paddys River District, Australian Capital Territory, Australia,
are skarn deposits at the contact between
granodiorite and volcanic rocks.
Hematite is a primary oxide that occurs in granular
veins and intergrowths with magnetite and rarely as compact masses of
large blades in quartz
(AJM 22.1.38).
At the Mount Kelly deposit, Gunpowder District, Queensland, Australia, the copper ores
overlie primary zone mineralisation consisting of
quartz-dolomite-sulphide veins hosted
in siltstone and schist.
Hematite occurs as a red groundmass or as coatings on fractures associated with
goethite, as inclusions in quartz,
surrounding pyrite and associated with
brochantite
(AJM 22.1.21).
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).
At the PC Mine, Cataract Mining District, Jefferson county, Montana, USA, hematite was found primarily as
shiny platelets to 2 mm across included in quartz
(R&M 96.6.494).
At the Dafoe property, Pierrepont, St. Lawrence county, New York, USA, hematite occurs as coatings, sometimes
thick, and massive fillings in almost all areas of the property and on all specimens found there. It is present as
reddish-brown staining in and on calcite and
quartz crystals. Occasionally it is found as attractive red phantoms in clear
quartz. Other forms sometimes encountered are red to brown spherical masses and
splendent, bladed crystals to 1 cm. These are associated with calcite and
quartz crystals and in rare cases are found as attractive coatings on selective
faces of calcite crystals. Hematite rarely forms spectacular
pseudomorphs after large, complex, octahedral
magnetite crystals
(R&M 97.3.247-248).
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.
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