The Hematite Group is a more or less informal group of closely related trigonal oxides. Their relationship is linked through their similar structures. The general formula for this group is A2O3. The A cations can be either iron, titanium, aluminum, chromium, vanadium, magnesium, antimony, sodium, zinc and/or manganese.
The structure is composed of alternating layers of cations and oxygens. The cations occupy spaces in layers between the oxygen layers and each are bonded to three oxygens in the above layer and three oxygens in the bottom layer. Not all of the sites for these cations are occupied as only two out of three are filled. If all the sites were filled then the formula would be AO in stead of A 2O3.
In the Ilmenite Subgroup, alternating layers of cations are occupied by only titanium or antimony and the other cation layers are occupied by a dissimilar cation.
This lowers the symmetry of this subgroup (bar 3); from the other members of this group (bar 3 2/m).
The other members are not similarly affected because their A cations are all the same and thus there is no asymmetry to their stacking sequence.
The same symmetry phenomena occurs between the
Calcite Group and the
Dolomite Group of