Cr2O3 adopts the corundum structure, consisting of a hexagonal close packed array of oxide anions with 2/3 of the octahedral holes occupied by chromium. Similar to corundum, Cr2O3 is a hard, brittle material (Mohs hardness 8-8.5).^{[1] It is antiferromagnetic up to 307 K, the Neel temperature.[2][3] It is not readily attacked by acids or bases, although molten alkali gives chromites (salts with the Cr
2O2−
4 anion, not to be confused with the related mineral chromite). It turns brown when heated, but reverts to its dark green color when cooled. It is also hygroscopic.}