Magnetite exposed on the ground. The mineral is black and irregularly smooth. Individual chunks jut at angles characteristic of the crystal habit.
Magnetite and pyrite from Piedmont Italy
Category Oxide minerals – Spinel group
Chemical formula iron(II,III) oxide, Fe3O4
Strunz classification 04.BB.05
Crystal symmetry Isometric 4/m 3 2/m
Unit cell a = 8.397 Å; Z=8
Color Black, gray with brownish tint in reflected light
Crystal habit Octahedral, fine granular to massive
Crystal system Isometric Hexoctahedral
Twinning On {Ill} as both twin and composition plane, the spinel law, as contact twins
Cleavage Indistinct, parting on {Ill}, very good
Fracture Uneven
Tenacity Brittle
Mohs scale hardness 5.5–6.5
Luster Metallic
Streak Black[1]
Diaphaneity Opaque
Specific gravity 5.17–5.18
Solubility Dissolves slowly in hydrochloric acid
References [2][3][4][5]
Major varieties
Lodestone Magnetic with definite north and south poles

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 is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part wüstite (FeO) and one part hematite (Fe2O3). 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 (585 °C; 1,085 °F). It is black or brownish-black with a metallic luster, has a Mohs hardness of 5–6 and a black streak.



Magnetite is the most magnetic of all the naturally occurring minerals on Earth.[6] 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. 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 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. 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 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.

Distribution of deposits

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, 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. 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. 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 (100 sq mi), with the highest dune at over 2,000 meters (6,560 ft) above the desert floor. The sand contains 10% magnetite.[7]

Biological occurrences

Crystals of magnetite have been found in some bacteria (e.g., Magnetospirillum magnetotacticum) and in the brains of bees, termites, fish, some birds (e.g., the pigeon) and humans.[8] 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.[9]

Magnetite also can be prepared by chemical co-precipitation, 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 (10 volume %), immediately a dark precipitate will be formed, which consists of nanoparticles of magnetite.[citation needed]

Transformation of ferrous hydroxide into magnetite

Under anaerobic conditions, the ferrous hydroxide (Fe(OH)2 ) can be oxidized by the protons of water to form magnetite and molecular hydrogen.[citation needed] 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 (Fe3O4) is thermodynamically more stable than the ferrous hydroxide (Fe(OH)2 ).

This process also occurs during the anaerobic corrosion of iron and steel in oxygen-free groundwater and in reducing soils below the water table.

Application 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.[10] Arsenic-contaminated drinking water is a major problem around the world, which can be solved using magnetite as a sorbent.


Because of its stability at high temperatures, it is used for coating industrial water tube steam boilers. The magnetite layer is formed after a chemical treatment (e.g. by using hydrazine).

Magnetite is also used as a catalyst for various industrial chemical processes, such as: Fischer-Tropsch process, the Haber-Bosch process and the water gas shift reaction.

Gallery of magnetite mineral specimens

See also


  1. ^
  2. ^ Handbook of Mineralogy
  3. ^
  4. ^ Webmineral data
  5. ^ Hurlbut, Cornelius S.; Klein, Cornelis (1985). Manual of Mineralogy (20th ed.). Wiley. ISBN 0471805807. 
  6. ^ Harrison, R. J.; Dunin-Borkowski, RE; Putnis, A (2002). "Direct imaging of nanoscale magnetic interactions in minerals" (free-download pdf). Proceedings of the National Academy of Sciences 99 (26): 16556–16561. doi:10.1073/pnas.262514499. PMC 139182. PMID 12482930. 
  7. ^ Ferrous Nonsnotus
  8. ^ Baker, R R; J G Mather, J H Kennaugh (1983-01-06). "Magnetic bones in human sinuses". Nature 301 (5895): 79–80. doi:10.1038/301078a0. PMID 6823284. 
  9. ^ Massart, R. “Preparation of aqueous magnetic liquids in alkaline and acidic media” IEEE transactions on magnetics, 17, 2, 1981. 1247–1248
  10. ^ J.T. Mayo et al. (2007). "The effect of nanocrystalline magnetite size on arsenic removal" (free download). Sci. Technol. Adv. Mater. 8: 71. doi:10.1016/j.stam.2006.10.005. 

Further reading

External links

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