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Formula: Fe2+Fe3+2O4
Multiple oxide, spinel subgroup
Crystal System: Isometric
Specific gravity: 5.175 measured, 5.2 calculated
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).
Magnetite from Mount Anakie - Image
At the Payún Matrú volcano, Altiplano de Payún Matrú, Agua Escondida District, Malargüe Department, Mendoza Province,
Argentina,
hematite
pseudomorphs
after magnetite have been found
(KL p138).
Magnetite from the Payún Matrú Volcano - Image
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).
Magnetite from Faraday - Image
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 Shengou tin mine, Xide County, Liangshan Yi, Sichuan, China, massive magnetite occurs
(AESS).
Magnetite from Shengou - Image
At Johanngeorgenstadt, Erzgebirgskreis, Saxony, Germany, the ore deposits chiefly consisted of
pyrite with a little pyrrhotite and
sporadic chalcopyrite,
arsenopyrite, sphalerite and
galena. Magnetite was often found and mined in large amounts. As a rule,
it was massive and granular to leafy, but pseudomorphs of magnetite
after hematite also were noted
(MinRec 55.5.599).
At the Raskoh mountains, Kharan, Balochistan, Pakistan, epidote
pseudomorphs
after magnetite have been found
(KL p226).
At Sapat Gali, Naran, Kaghan Valley, Mansehra District, Khyber Pakhtunkhwa Province, Pakistan, very rarely, sharp,
slightly pitted or striated, black magnetite crystals occur with
forsterite crystals, embedded in a white, chalky matrix of
palygorskite, magnesite and
talc created by the
serpentinization of
peridotite. The magnetite crystals are simple dodecahedrons
reaching 6 cm in size
(MinRec 51.6.785-801).
Magnetite from Sapat Gali - Image
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).
At the SK Star #2 Mining Claim, The Cove, Topaz Mountain, Thomas Range, Juab County, Utah, USA, magnetite forms
black octahedral crystals as a condensate from supersaturated volcanic gas
(MinRec 51.6.809-810).
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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