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Hematite is composed of $\ce{Fe2O3}$, and is paramagnetic, whereas magnetite is $\ce{Fe3O4}$ and is diamagnetic. Magnetite's nature is due to the presence of both $\ce{Fe^{2+}}$ and $\ce{Fe^{3+}}$ (Wikipedia even goes so far as to call it $\ce{FeO}\cdot \ce{Fe2O3}$, but explains that this is not a "solid solution").

I can surmise that in the crystalline solid, there are divalent and trivalent cations dispersed within. I can also assume that there might be some sharing of electrons between the oxygens and each of these cations. One would assume this also happens within the hematite crystal, so what is it about "tossing" the divalent cation into the mix of the crystalline structure that makes the magnetite diamagnetic?

Does the charge differential between the two cations cause a permanent dipole, if so, why don't all of the small dipoles simply cancel each other out?

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Magnetite has a spinel structure with two types of $\ce{Fe}$ sites: octahedral and tetrahedral, respectively in green and orange below.

enter image description here

(in this structure, oxygen atoms are the small red balls).

This structure is called a spinel, with $\ce{Fe^{2+}}$ ion in tetrahedral coordination and $\ce{Fe^{3+}}$> ions in octahedral sites. The coupling between atoms from these two lattices are superexchange interactions, which result in an antiparallel alignment of spins. However, because the magnetic moments of the two lattices are not equal, their addition results in a net magnetic moment, even though they are of opposition alignment. This is called ferrimagnetism, and in practice most of its macroscopic properties are similar to ferromagnetism, though they stem from a completely different microscopic magnetic ordering.

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  • $\begingroup$ Nice with the spinning diagram. Thanks for clarifying, I've obviously got to read more about it! $\endgroup$ – jonsca May 16 '12 at 12:50
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Magnetite is not diamagnetic. It is ferrimagnetic

Ferrimagnetism requires layers of unequal but opposite magnetic moments in the crystal lattice. The two iron ions have different magnetic moments, and they probably are arranged nicely in the lattice, so the ferrimagnetic property comes from there.

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  • $\begingroup$ Thanks I had no idea that was a classification. Could you explain a bit more about "arranged nicely"? $\endgroup$ – jonsca May 16 '12 at 12:11
  • $\begingroup$ @jonsca read the Wikipedia pagr. Or F'xs awesome answer ;-) $\endgroup$ – ManishEarth May 16 '12 at 14:41

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