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Black Fe3O4 is a mixed iron(II)—iron(III) oxide also obtained when iron is oxidized in a limited supply of air. It melts at 1538°, has a density of 5.2 and a heat of formation of -267.0 kcal mole-1. It occurs naturally as magnetite (lodes tone or magnetic iron ore), is strongly ferromagnetic and has a fairly high electrical conductivity.
Structure and conformation
Magnetite has the
inverse spinel structure189 in which the Fe2+ ions are in octahedral sites and the Fe3+ ions
are half in octahedral and half in tetrahedral sites in a cubic close-packed array of oxide ions. It is insoluble in aqueous acids ; it can be slowly oxidized to Fe2O3 by heating in air.
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