Magnetite is a
ferrimagnetic mineral with chemical formula
Fe3O4, one of several
iron oxides and a member of the
spinel group. The chemical
IUPAC name is
iron(II,III) oxide and the common chemical name
ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe
2O
3, which is one part
wüstite (FeO) and one part
hematite (Fe
2O
3). This refers to the different oxidation states of the iron in one structure, not a
solid solution. The
Curie temperature of magnetite is 858 K.
Properties
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 man first discovered the property of
magnetism. Lodestone was 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 discovering and 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, as the complicated
reactions between these minerals and
oxygen influence how and when magnetite preserves records of the Earth's magnetic field.
Magnetite has been very important in understanding the conditions under which rocks form and evolve. 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 between magnetite and ulvospinel and the other between ilmenite and hematite. Compositions of the mineral pairs are used to calculate how oxidizing was the
magma (i.e., the
oxygen fugacity of 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.
Small grains of magnetite occur in almost all igneous rocks and
metamorphic rocks. 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 from magma. Magnetite also is produced from
peridotites and
dunites by
serpentinization.
Magnetite is a valuable source of
iron ore. It dissolves slowly in
hydrochloric acid.
Distribution of deposits
thumb|left|A fine textured sample, ~5cm across
Magnetite and other heavy minerals (dark) in a quartz
beach sand (
Chennai, India).
Magnetite is sometimes found in large quantities in beach sand. Such
black sands (mineral sands or iron sands) are found in various places such as
California and the west coast 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,
Kiruna,
Sweden, the
Pilbara, Midwest and Northern Goldfields regions in
Western Australia, and in the
Adirondack region of
New York in the
United States. Deposits are also found in
Norway,
Germany,
Italy,
Switzerland,
South Africa,
India,
Mexico, and in
Oregon,
New Jersey,
Pennsylvania,
North Carolina,
Virginia,
New Mexico,
Utah, and
Colorado in the
United States. Recently, in June 2005, an exploration company, Cardero Resources, discovered a vast deposit of magnetite-bearing sand dunes in
Peru. The dune field covers 250 square kilometers (100 sq mi), with the highest dune at over 2,000 meters (6,560 ft) above the desert floor. The sand contains 10% magnetite.
Biological occurrences
Crystals of magnetite have been found in some
bacteria (e.g.,
Magnetospirillum magnetotacticum) and in the brains of
bees, of
termites, fish, some
birds (e.g., the
pigeon) and humans. These crystals are thought to be involved in
magnetoreception, the ability to sense the
polarity or the
inclination of the Earth's
magnetic field, and to be involved in
navigation. Also,
chitons have teeth made of magnetite on their
radula making them unique among animals. This means they have an exceptionally abrasive tongue with which to scrape food from rocks.
The study of
biomagnetism began with the discoveries of
Caltech paleoecologist Heinz Lowenstam in the 1960s.
Preparation as a ferrofluid
Crystal structure of 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 also can be prepared by chemical co-precipitation, which consist in a mixture of a solution 0.1 M of FeCl
3·6H
2O and FeCl
2·4H
2O with mechanic agitation of about 2000 rpm. The molar ratio of FeCl
3:FeCl
2 can be 2:1; heating this solution at 70 °C, and immediately the rpm is elevated to 7500 rpm and adding quickly a solution of NH
4OH (10 volume %), immediately a dark precipitate will be formed, which consist of nanoparticles of magnetite.
Application as a sorbent
Magnetite powder efficiently removes As(III) and As(V) from water, and the efficiency of the removal increases ~200 times when the magnetite particle size decreases from 300 to 12 nm. Arsenic(As)-contaminated drinking water is a major problem around the world, which can be solved using magnetite as a sorbent.
Jewelry
Magnetite ring
Magnetite is commonly used as the primary material in jewelry worn by adherents of the discredited
of
magnet therapy. When polished and made into jewelry magnetite has a dark, shiny finish with a smooth surface.
See also
- Bluing (steel) is a process in which steel is partially protected against rust by a layer of magnetite.
- Magnesia (in natural mixtures with magnetite)
