Magnetite

Compound · Fe₃O₄

Elements

Crystal structure

One unit cell, the smallest block that repeats in every direction to build the whole crystal. Atoms on its faces, edges and corners are shared with the neighboring cells. Farther atoms are drawn fainter.
Cell: a = b = c = 8.3941 Å, all angles 90°
Space group: F d -3 m :2

Atom positions from the Crystallography Open Database entry 9007644 (Fleet, M. E., 1981).

Where it's found

With hematite in banded iron formations, and on its own in a few huge igneous ore bodies, above all Kiruna in northern Sweden. The taconite of Minnesota's Mesabi Range is magnetite scattered through hard chert. It is also the black sand on many beaches and a trace in nearly every igneous rock. Lodestone is magnetite that has been magnetized, probably by lightning.

How it's mined

Kiruna is mined underground by sublevel caving, the ore drawn off from a level 1,365 m down; the taconite pits of the Mesabi are open pit. Because magnetite is strongly magnetic the ore, often only 25 to 30% iron, is crushed, ground fine and run past magnetic drums that pull the magnetite out of the waste. The concentrate is rolled into pellets and fired for blast furnaces and direct reduction.

Notes

Lodestone was the first compass needle, and the word magnet comes from Magnesia in Greece, where it was found. Each formula unit has one iron(II) and two iron(III). One iron(III) sits in a small tetrahedral hole in the oxygen packing; the other and the iron(II) share the larger octahedral holes, so electrons hop between them and the mineral conducts, which is why it is black and metallic rather than red like hematite.