"Don Bruder" wrote in message
...
magnetite, hematite,

But not these two, which I know in both cases to be forms of iron, not
aluminum.

These things were listed rather ambiguously, but I know what was meant:
Hematite, corundum, chromium oxide and I think a number of rare earth
oxides, all take the same crystal structure, and have similar atomic
dimensions so are somewhat replacable by each other.

Magnetite, in particular, is definitely ferrous -
....
and spinel.

Eh... Might be, but not sure, and not interested enough to find out.

The other interesting aspect of the above-mentioned trio is they form a nice
stable crystal structure with divalent metals, known as the spinel
structure. The divalent metals include (let's see if I can get them all
from memory):
magnesium, manganese, iron, nickel, zinc, and in a related vein, lead.
(Note that iron appears in both lists because it is di- and tri-valent!)
Thus, what you get are 15 combinations of these elements, known as spinels,
of the form XO.Y2O3 = XY2O4. For example, the namesake mineral spinel is
magnesium aluminate, MgAl2O4. Chromite, the chromium ore, is ferrous
chromite (not chromate), FeCr2O4.

A consequence of the dual behavior of iron is magnetite, FeO.Fe2O3 = FeFe2O4
= Fe3O4. Magnetite is very stable: at high temperatures, it can be reduced
to ferrous oxide (FeO), but between 300 and 560°C, FeO disproportionates to
iron (metal) and magnetite! Likewise, it can be oxidized (most easily by
weathering at low temperature), but Fe2O3 actually decomposes into Fe3O4
with a release of oxygen at 1451°C. Magnetite also has a higher melting
point than FeO, slightly higher than the melting point of steel (though you
can't observe this directly because any oxide attached to that melting piece
of steel will be reduced by the metal, causing the slag to literally drip
off the iron, which looks very interesting).

Industrially, chromite refractories such as MgCr2O4, FeCr2O4 and MgAl2O4 are
often used to buffer between cheap, acidic, silica-rich firebricks and more
expensive, basic dolomite and magnesia bricks. The basic bricks are used
because, in addition to being highly refractory (MP 2400°C), they absorb
sulfur and phosphorous from the steel contained within. These basic
refractories gave rise to the terms "basic oxygen process" and "basic open
hearth process", neither of which are mechanically "basic".

Perhaps being classed in the "Hematite group" indicates they all have
similar crystal structures? They definitely don't have the same
molecular structures!

That's because they aren't in molecules, they're in crystals. ;o)

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms