Magnetite (Fe3O4) is a hard, black, strongly ferromagnetic (see magnet and ferromagnetism) form of iron oxide that is one of the major ores of iron, second only to hematite. Magnetite crystallizes in the cubic system, as octahedral and dodecahedral crystals, and as granular masses. It is a mixed iron(II)-iron(III) oxide, also called ferrosoferric oxide, and belongs to the spinel group of minerals. It is widely distributed and occurs as an accessory mineral in many igneous and metamorphic rocks. The largest deposits are found in northern Sweden. Permanently magnetized deposits of magnetite were used in the ancient world as material for compasses, under the name lodestone. Hardness 6; relative density 5.2.
Magnetite and life
Tiny crystals of magnetite are produced by some terrestrial bacteria, known as magnetotatic bacteria, and used for orientation. The discovery of similar crystals in some Mars meteorites, including ALH84001, has been taken as evidence of past martian life, though this claim is hotly disputed.