Why is Magnetite Magnetic and Hematite Nonmagnetic?

During the recent GSM field trip to the Mesabi and Vermilion Iron Ranges, this difference in magnetism was found to be central in understanding some challenges associated with recovering iron from ores in northern Minnesota.

Magnetic fields and magnetic material properties arise from electrons in motion.  Electrons are always spinning and usually orbit atomic nuclei or travel linearly through space.  Electrons spinning and revolving in tiny orbits around atomic nuclei generate magnetic dipoles—fields with both north and south poles like those that would emanate from atomic-sized bar magnets.  In most materials, electrons orbit around the nucleus within configurations that have particular ranges, or bands, of energy.  These bands can be subdivided into orbitals, which are usually evenly filled with electrons that pair together such that the paired orbits are alike in orientation, but exactly opposite in direction.  When this occurs, the two magnetic dipoles that are generated point oppositely, cancel each other, and no large-scale, or bulk, magnetic properties are exhibited.  In those few materials that do exhibit bulk magnetic properties, electron orbitals nearer the nucleus are not evenly filled, so those electrons are not completely paired and through an interaction between adjacent atomic dipoles, a coupling occurs that tends to align the orbits of the electrons involved.  In this manner, alignment of great numbers of atomic dipoles in a material will produce bulk magnetism.

Magnetite, (Fe₃O₄), has 4/3 or 1 and 1/3 atoms of oxygen (O) for every atom of iron (Fe).  Also called lodestone, magnetite is a natural ferrite.  All ferrites exhibit strong magnetic properties and are hard, brittle ceramic-like materials.

Magnetite is composed of iron atoms in two different states, one atom with a valence of +2 (Fe⁺⁺ or ferrous iron) and two atoms with a valence of +3 (Fe⁺⁺⁺ or ferric iron).  A valence number is usually the number of outer electrons, which are those in the outermost and highest energy band.  Further, the valence indicates the number of negatively charged electrons that are available for ideal sharing (i.e. bonding) with other atoms and can be either – for a surplus or + for a deficiency.  In magnetite, the magnetic dipoles of the two Fe⁺⁺⁺ atoms are pointed oppositely and cancel each other.  However, the magnetic dipole of the Fe⁺⁺ atom tends to align with many adjacent dipoles of other Fe⁺⁺ atoms throughout the mineral by the same interaction mentioned previously, and this parallel alignment of many atomic dipoles produces a bulk magnetism called ferrimagnetism.  Since magnetite is strongly ferrimagnetic, ordinarily strong production magnets are sufficient to separate magnetite from a finely ground mixture of taconite ore, which includes magnetite and its host rock.  Typical taconite ore contains about 20-25% iron by weight.  After such a removal of iron-rich magnetite, the waste rock is called taconite tailings.

Hematite, (Fe₂O₃), has 3/2 or 1˝ atoms of oxygen for every atom of iron.  Thus, it is a more oxidized iron mineral than magnetite and may be considered an iron rust, because as metallic iron oxidizes, hematite can be formed.  Hematite is actually very weakly magnetic as is explained below.

Over time, randomly mobile hematite molecules will statistically self-organize to form a crystal structure, or crystal lattice, whose regular pattern of molecules in three dimensions can be regarded as produced by repeated translations in space of a unit cell of atoms.  The hematite crystal is notable in that the geometry of its packing distorts its lattice such that iron atoms within the unit cell are paired with slightly altered lattice spacings and although their dipoles would be expected to point oppositely, an interaction between the paired atoms tilts the two dipoles so they do not completely cancel, making bulk hematite very weakly magnetic.  Thus, exotically powerful and expensive magnets would be required to separate hematite from a finely ground mixture of hematite and host rock.

When processing taconite ore to concentrate iron-rich material for iron smelting, small pellets of wet magnetite that are held together with a binding agent are heated in an air furnace to about 1288°C (2350°F).  During this process, the magnetite is further oxidized to hematite and transforms from a strongly magnetic to a very weakly magnetic material.  This is why taconite pellets are not attracted by an ordinary magnet.