Hematite and Corundum have about as simple structures as any mineral. Oxygen atoms are hexagonally close packed, and the cations occupy 2/3 of the octahedral voids. Hematite and corundum are isostructural, but the cations are different enough in size that there's no solid solution. The diagram below shows four layers of the structure with higher layers to the right and darker. Oxygen atoms are blue, cations yellow.
Wait a minute. Aluminum, though very reactive, is durable because a monolayer of corundum forms on the surface as soon as it's exposed to air. So why doesn't rust (hematite) protect iron?
The answer is that aluminum has only one natural oxidation state whereas iron has two. Put Fe+++ in contact with Fe0 (metal), and the Fe+++ yanks electrons off the metal to become Fe++, while oxidizing the metal. 2Fe+++ + Fe0 = 3Fe++. Then the Fe++ reacts with oxygen to become Fe+++ (4Fe++ + 3O2 = 2Fe2O3) and the cycle continues. Somebody stop me before I start singing "The Circle of Life."
Created 18 April, 2011, Last Update 25 May, 2011
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