Magnetite is an oxide mineral with structural formula [Fe2+(Fe3+)2O4] having the spinel structure. Other minerals in the series include magnesioferrite [Mg2+(Fe3+)2O4], franklinite [Zn2+(Fe3+)2O4] and jacobsite [Mn2+(Fe3+)2O4]. The structure of magnetite consists of a closed-packed oxygen arrangement with the divalent Fe2+ ion in tetrahedral (four-fold) coordination and the trivalent Fe3+ ions in octahedral (six-fold) coordination with oxygens. The structure may also be seen as layered sheets of [Fe2+,Fe3+] between closed-packed sheets of oxygens. The octahedral sites are larger than the tetrahedral sites, and there are twice as many octahedral sites occupied as tetrahedral sites. Magnetite is by far the most common mineral of its series. It may contain significant amounts of Mg and sometimes form solid solutions with magnesioferrite. Very minor substitutions of other elements such as Co, Ni, Ti, V, and Cr may occur.
Magnetite minerals are reddish-black to brownish-black, opaque, with a metallic to submetallic luster. Magnetite and magnesioferrite are strongly magnetic, and were common components of lodestone. Most minerals of the magnetite series show octahedral crystal growth habit, with square or triangular sections, but they may also exist as anhedral granular forms. Magnetite may be associated with maghemite Fe2O3 and hematite which appears sometimes as lamellar intergrowths on magnetite crystals.
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Magnetite is one of the most ubiquitous of all minerals, occurring in a wide variety of igneous, metamorphic, and sedimentary rocks. It usually occurs as small disseminated crystals or grains composing not more than 1% of the rock in which it is found. Magnetite crystals may also be segregated to form large rock masses, but this is relatively uncommon. In mafic rocks, magnetite is usually titaniferous, and closely associated with pyroxenes, olivines, Ca-plagioclase, and apatite. Metamorphic rocks derived from ferruginous sediments in regional and contact metamorphism commonly yield magnetite with hematite and other iron-hydroxides, as occurs in the Mesabi Iron Range in northern Minnesota where it is the major component of the taconite ores mined there. Magnetite is also deposited in high-temperature hydrothermal sulfide veins. Grains of magnetite are major constituents of the opaque and heavy miineral fractions in sand and silt of detrital sediments and soils.
Magnetite occurs in many soils inherited from the parent materials and is usually found in the sand and silt size fractions. It is relatively stable under surface soil conditions, but may weather to different secondary minerals. There is some speculation, based mainly on studies of the magnetic properties of soils, that fine-grained magnetite may form in soils. Magnetotopic bacteria have been shown to produce magnetite under appropriate conditions. Magnetite can be readily separated from soil with the use of a hand magnet. The variation of the ratio of magnetite to other resistant minerals is used in soil genesis studies to determine provenance or similarity of sediment origins.
Wechsler, B.A., D.H. Lindsley, and C.T. Prewitt. 1984. Crystal structure and cation distribution in titanomagnetites (Fe3-xTixO4) MT100-1350. American Mineralogist 69:754-770.