Magnetite

minerals

epidote

spinel

corundum

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Formula: Fe2+Fe3+2O4
Multiple oxide, spinel group
Specific gravity: 5.175
Hardness: 5½ to 6½
Streak: Black
Colour: Black
Solubility: Slightly soluble in hydrochloric acid
Common impurities: Mg,Zn,Mn,Ni,Cr,Ti,V,Al
Environments:

Plutonic igneous environments
Volcanic igneous environments
Pegmatites
Carbonatites
Sedimentary environments
Placer deposits
Metamorphic environments (typical)
Hydrothermal environments

Magnetite is a primary and secondary mineral found in igneous environments, carbonatites, sedimentary environments including placers, regional metamorphic environments, massive hydrothermal replacement deposits and hydrothermal replacement lodes. It is a common constituent of sedimentary and metamorphic banded iron formations, and in such occurrences it is of a chemical sedimentary origin. It is found in black sands often associated with corundum, forming emery. In metamorphic environments it may be associated with serpentine. In some rocks magnetite may be one of the chief constituents and form large ore bodies.
It may be found in andesite, basalt, gabbro, granite, kimberlite, rhyolite, syenite,

Localities

At Mount Anakie, Anakie, City of Greater Geelong, Victoria, Australia, titanium-bearing magnetite crystals are very common in vesicles, especially associated with pseudobrookite and enstatite (AJM 21.1.32).

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

At the Faraday mine, Faraday Township, Hastings county, Ontario, Canada, ilmenite to 30 kg occurs with magnetite of similar size (R&M 94.5.413).

At the Raskoh mountains, Kharan, Balochistan, Pakistan, epidote pseudomorphs after magnetite have been found (KL p226).

At Croft Quarry, Croft, Blaby, Leicestershire, England, UK, magnetite occurs with molybdenite and a very small amount of pyrite. The magnetite and molybdenite are thought to be early in the paragenesis (JRS 20.20).

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 Black Rock mine, Beaver county, Utah, USA, sphalerite pseudomorphs after magnetite have been found (KL p130).

Alteration

aenigmatite, anorthite and O2 to hedenbergite, albite, ilmenite and magnetite
½Na4[Fe2+10Ti2]O4[Si12O36] + CaAl2Si2O8 + ½O2 = CaFe2+Si2O6 + 2NaAlSi3O8 + Fe2+Ti4+O3 + Fe2+Fe3+2O4
(DHZ 2A p651).

albite, diopside and magnetite to aegirine, Si2O6, garnet and quartz
2Na(AlSi3O8) + CaMgSi2O6 + Fe2+Fe3+2O4 ⇌ 2NaFe3+Si2O6 + Si2O6 + CaMgFe2+Al2(SiO4)3 + SiO2
This reaction may occur in blueschist facies rocks in Japan (DHZ 2A p512).

fayalite and H2O to magnetite, SiO2 and H2
3Fe2+2(SiO4) + 2H2O &38594; Fe2+Fe3+2O4 + 3SiO2 + 2H2
This reaction is highly exothermic (Wiki Serpentinite).

fayalite, H2O and O2 to cronstedtite and magnetite
6Fe2+2(SiO4) + 6H2O + ½O2 = 3Fe3Si2O5(OH)4 + Fe2+Fe3+2O4

forsterite, fayalite, H2O and CO2 to serpentine, magnetite and methane
18 Mg2SiO4 + 6Fe2SiO4 + 26H2O + CO2 → 12Mg3Si2O5(OH)4 + 4Fe3O4 + CH4

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
If wüstite, FeO, is also introduced hedenbergite and magnetite may form in addition to andradite:

titanomagnetite (ilmenite combined with magnetite), quartz, and aegirine-hedenbergite to aenigmatite, hedenbergite, magnetite and O2
6(Fe2+Ti4+O3 + Fe2+Fe3+2O4) + 12SiO2 + 12(NaFe3+Si2O6 + CaFe2+Si2O6) ⇌ 3Na4[Fe2+10Ti2]O4[Si12O36] + 12CaFe2+Si2O6 + 2Fe2+Fe3+2O4 + 5O2
(DHZ 2A p652)

jadeite, diopside, magnetite and quartz to aegirine, kushiroite (pyroxene) and hypersthene
2NaAlSi2O6 + CaMgSi2O6 + Fe2+Fe3+2O4 + SiO2 ⇌ 2NaFe3+Si2O6 + CaAlAlSiO6 + MgFeSi2O6
Aegirine in blueschist facies rocks may be formed by the above reaction (DHZ 2A 512).

magnetite to hematite
Magnetite may convert to hematite, and vice versa, depending on the pressure and temperature, according to the equation:
magnetite + oxygen ⇌ hematite
2Fe3O4 + ½O2 ⇌ 3Fe2O3

olivine and H2O to serpentine, magnetite and H2
6(Mg1.5Fe0.5)SiO4 + 7H2O → 3Mg3Si2O5(OH)4 + Fe2+Fe3+2O4 + H2
The iron Fe in olivine does not enter into the serpentine, but recrystallises as magnetite (R&M 90.6.521).

staurolite, annite and O2 to hercynite, magnetite, muscovite,corundum, SiO2 and H2O
2Fe2+2Al9Si4O23(OH) + KFe2+3 (AlSi3O10)(OH)2 +2O2 → 4Fe2+Al2O4 + Fe2+Fe3+2O4 + KAl2 (AlSi3O10)OH)2 + 4Al2O3 + 8SiO2 + 2H2O
(DHZ 1A p860).

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