Magnetite is a mineral, one of the three common naturally occurring iron oxides and a member
of the spinel group. Magnetite is the most magnetic of all the naturally occurring minerals
on Earth. Naturally magnetized pieces of magnetite, called lodestone, will attract small pieces
of iron, and this was how ancient people first noticed the property of magnetism.
Small grains of magnetite occur in almost all igneous and metamorphic rocks. It is black
or brownish-black with a metallic luster, has a Mohs hardness of 5–6 and a black streak.
The chemical IUPAC name is iron(II,III) oxide and the common chemical name is ferrous-ferric
oxide.
Properties Lodestones were used as an early form of magnetic
compass. Magnetite typically carries the dominant magnetic signature in rocks, and so it has
been a critical tool in paleomagnetism, a science important in understanding plate tectonics
and as historic data for magnetohydrodynamics and other scientific fields. The relationships
between magnetite and other iron-rich oxide minerals such as ilmenite, hematite, and ulvospinel
have been much studied; the reactions between these minerals and oxygen influence how and
when magnetite preserves a record of the Earth's magnetic field.
Magnetite has been very important in understanding the conditions under which rocks form. Magnetite
reacts with oxygen to produce hematite, and the mineral pair forms a buffer that can control
oxygen fugacity. Commonly, igneous rocks contain grains of two solid solutions, one of magnetite
and ulvospinel and the other of ilmenite and hematite. Compositions of the mineral pairs
are used to calculate how oxidizing was the magma: a range of oxidizing conditions are
found in magmas and the oxidation state helps to determine how the magmas might evolve by
fractional crystallization. Magnetite also occurs in many sedimentary
rocks, including banded iron formations. In many igneous rocks, magnetite-rich and ilmenite-rich
grains occur that precipitated together in magma. Magnetite also is produced from peridotites
and dunites by serpentinization. The Curie temperature of magnetite is 858 K.
Distribution of deposits
Magnetite is sometimes found in large quantities in beach sand. Such black sands are found
in various places, such as California and the west coast of the North Island of New
Zealand. The magnetite is carried to the beach via rivers from erosion and is concentrated
via wave action and currents. Huge deposits have been found in banded iron
formations. These sedimentary rocks have been used to infer changes in the oxygen content
of the atmosphere of the Earth. Large deposits of magnetite are also found
in the Atacama region of Chile, Valentines region of Uruguay, Kiruna, Sweden, the Pilbara,
Midwest and Northern Goldfields regions in Western Australia, New South Wales in the
Tallawang Region, and in the Adirondack region of New York in the United States. Kediet ej
Jill, the highest mountain of Mauritania, is made entirely of the mineral. Deposits
are also found in Norway, Germany, Italy, Switzerland, South Africa, India, Indonesia,
Mexico, and in Oregon, New Jersey, Pennsylvania, North Carolina, Virginia, New Mexico, Utah,
and Colorado in the United States. In 2005, an exploration company, Cardero Resources,
discovered a vast deposit of magnetite-bearing sand dunes in Peru. The dune field covers
250 square kilometers, with the highest dune at over 2,000 meters above the desert floor.
The sand contains 10% magnetite. Transformation of ferrous hydroxide into magnetite
Under anaerobic conditions, the ferrous hydroxide2) can be oxidized by the protons of water to
form magnetite and molecular hydrogen. This process is described by the Schikorr reaction:
3 Fe(OH)2 → Fe3O4 + H2 + 2 H2O ferrous hydroxide → magnetite + hydrogen
+ water The well-crystallized magnetite is thermodynamically
more stable than the ferrous hydroxide2 ). Biological occurrences
Biomagnetism is usually related to the presence of biogenic crystals of magnetite, which occur
widely in organisms. These organisms range from bacteria to animals, where these crystals
are found in the brain. These crystals are involved in magnetoreception, the ability
to sense the polarity or the inclination of the Earth's magnetic field, and aid in navigation.
Chitons have teeth made of magnetite on their radula.
Synthetic magnetite
Magnetite can be prepared in the laboratory as a ferrofluid in the Massart method by mixing
iron(II) chloride and iron(III) chloride in the presence of sodium hydroxide. Magnetite
can also be prepared by the chemical co-precipitation in presence of ammonia, which consist in a
mixture of a solution 0.1 M of FeCl3·6H2O and FeCl2·4H2O with mechanic agitation of
about 2000 rpm. The molar ratio of FeCl3:FeCl2 can be 2:1; heating this solution at 70 °C,
and immediately the speed is elevated to 7500 rpm and adding quickly a solution of NH4OH,
immediately a dark precipitate will be formed, which consists of nanoparticles of magnetite.
In both cases, the precipitation reaction rely on a quick transformation of acidic hydrolyzed
iron ions into the spinel iron oxide structure, by hydrolysis at elevated pH values.
Considerable efforts has been devoted towards controlling the particle formation process
of magnetite nanoparticles due to the challenging and complex chemistry reactions involved in
the phase transformations prior to the formation of the magnetite spinel structure. Magnetite
particles are of interests in bioscience applications such as in magnetic resonance imaging since
iron oxide magnetite nanoparticles represent a non-toxic alternative to currently employed
gadolinium-based contrast agents. However, due to lack of control over the specific transformations
involved in the formation of the particles, truly superparamagnetic particles have not
yet been prepared from magnetite, i.e. magnetite nanoparticles that completely lose their permanent
magnetic characteristic in the absence of an external magnetic field. The smallest values
currently reported for nanosized magnetite particles is Hc = 8.5 A m−1, whereas the
largest reported magnetization value is 87 Am2 kg−1 for synthetic magnetite.
Applications Magnetic recording
Magnetic iron oxides are often used in magnetic storage, for example in the magnetic layer
of hard disks, floppy disks and cassette tapes. These consist of a thin sheet of plastic material,
with embedded magnetic particles. The particles can be magnetized to represent binary or analog
data. Magnetic ink character recognition also uses magnetic particles suspended in an ink
which can be read by special scanning hardware. Most newly generated information, such as
text, photographs, and audiovisual recordings, is now stored in magnetic media, and much
of the world's legacy of information in other media has been transcribed to magnetic form,
because it is cheap, compact, and computer-accessible. Catalysis
Magnetite is the catalyst for the industrial synthesis of ammonia.
As a sorbent Magnetite powder efficiently removes arsenic(III)
and arsenic(V) from water, the efficiency of which increases ~200 times when the magnetite
particle size decreases from 300 to 12 nm. Arsenic-contaminated drinking water is a major
problem around the world, which can be solved using magnetite as a sorbent.
Other Because of its stability at high temperatures,
it is used for coating industrial watertube steam boilers. The magnetite layer is formed
after a chemical treatment. Gallery of magnetite mineral specimens
See also Bluing, a process in which steel is partially
protected against rust by a layer of magnetite Buena Vista Iron Ore District
Corrosion product Ferrite
Greigite Maghemite
Magnesia Magnetotactic bacteria
Mill scale Mineral redox buffer
References
Further reading Lowenstam, Heinz A.; Weiner, Stephen. On Biomineralization.
USA: Oxford University Press. ISBN 0-19-504977-2. Chang, Shih-Bin Robin; Kirschvink, Joseph
Lynn. "Magnetofossils, the Magnetization of Sediments, and the Evolution of Magnetite
Biomineralization". Annual Review of Earth and Planetary Sciences 17: 169–195. Bibcode:1989AREPS..17..169C.
doi:10.1146/annurev.ea.17.050189.001125. External
links Mineral galleries
Bio-magnetics History of Magnetite Mining in the NJ Highlands
Magnetite mining in New Zealand Accessed 25-Mar-09 Mineral Survey of Santa Cruz County - Black
Sand Peruvian sand dunes