Ilmenite in soil forming from a Brazilian amphibolite

Ilmenite-rich samples were obtained by chemical extraction methods from materials collected from a soil developing on an amphibolite and from its altered parent rock. The soil ilmenite of this mantle was found to be inherited from the rock, but its composition varied significantly from one soil fraction to another and according to its mineralogical association. Unit cell parameters are smaller than those of pure ilmenite: the a-dimension of the hexagonal structure varies from 0.5076 +/- 0.0004 to 0.5083 +/- 0.0007 nm. Mossbauer patterns at room temperature reveal that this mineral, which is most often associated with the iron-rich spinel phase, actually corresponds to a member of the haematite-ilmenite series having the general formula xFeTiO(3).(1 - x)Fe2O3, with 0.83 less than or equal to x less than or equal to 0.95. There is a correlation (r = 0.95) between x, the molar fraction of FeTiO3, and the a-dimension of the unit cell.