Ilmenite in soil forming from a Brazilian amphibolite

Ilmenite-rich samples were obtained by chemical extraction methods from mat erials collected from a soil developing on an amphibolite and from its alte red parent rock. The soil ilmenite of this mantle was found to be inherited from the rock, but its composition varied significantly from one soil frac tion to another and according to its mineralogical association. Unit cell p arameters are smaller than those of pure ilmenite: the a-dimension of the h exagonal structure varies from 0.5076 +/- 0.0004 to 0.5083 +/- 0.0007 nm. M ossbauer patterns at room temperature reveal that this mineral, which is mo st often associated with the iron-rich spinel phase, actually corresponds t o a member of the haematite-ilmenite series having the general formula xFeT iO(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 F eTiO3, and the a-dimension of the unit cell.