Magnetite

magnetite

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

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. Magnetite is a primary oxide, and one of the main primary minerals here. Rare large dodecahedral crystals to 2.2 cm have been found in quartz (AJM 22.1.38).

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 lots 10 and 11 of concession 1, Bathurst Township, Lanark County, Ontario, Canada (DeWitts corner), the deposit is located in the Grenville Geological Province, which consists mostly of marble, gneiss, and quartzite. Syenite-migmatite was also reported in the area where the vein-dikes are located. Characteristic features of the vein-dikes include the fact that perfectly formed euhedral crystals of different minerals can often be found floating in calcite with no points of contact with the walls. Sometimes these crystals have inclusions of calcite, irregular or rounded in shape. It has been argued that at least some of the vein-dikes were formed as a result of melting of Grenville marble.
Magnetite occurs as small octahedral crystals, usually less than 2 mm, on spinel (R&M 97.6.556-564).

At San Shek Wan, Lantau Island, Islands District, New Territories, Hong Kong, China, it is necessary to visit the site at low tide, when the outcrop on the beach is exposed. The outcrop is a sequence of metamorphosed sandstone, metamorphosed siltstone and granite, and a small skarn body was also identified with abundant magnetite and quartz veins. Little red crystals of garnet were found in the magnetite (Geological Society of Hong Kong newsletter 16.1.2, Mineralogy Society of Hong Kong field trip).

At Sha Lo Wan, Lantau Island, Islands District, New Territories, Hong Kong, China, the exposed skarn zone is about 5 m wide, and is composed mainly of garnet, vesuvianite, diopside and epidote, with scattered magnetite (Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)

The Ma On Shan Mine, Ma On Shan, Sha Tin District, New Territories, Hong Kong, China, is an abandoned iron mine, with both underground and open cast workings. The iron ores contain magnetite as the ore mineral and occur predominantly as masses of all sizes enclosed in a large skarn body formed by contact metasomatism of dolomitic limestone at the margins of a granite intrusion. In parts of the underground workings magnetite is also found as clusters of minute octahedra scattered in marble in contact with the granite. The skarn rocks consist mainly of tremolite, actinolite, diopside and garnet (Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)

At the Shijiang Shan-Shalonggou mining area, Inner Mongolia, China, the mineral deposits occur predominantly in veins of hydrothermal origin in skarn. Magnetite is a common accessory mineral found on many of the borate specimens. Large attractive crystals with octahedral edges reaching 35 mm, and sometimes with an attached cluster of olshanskyite, have been observed (R&M 96.5.402).

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 enstatite-ferrosilite
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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