| Mossbauer spectra for a variety of spinel phases, both natural and synthetic and representing a very wide range in composition, indicate an essentially ordered arrangement of iron atoms in the crystal structures of natural chromite and Cr-bearing spinels. All the Mossbauer spectra of natural samples have been rationalized by fitting just two doublets, assigned to Fe('2+) in the tetrahedral (T) cation site and Fe('3+) in the octahedral (M) site, using initial Mossbauer parameters for synthetic end-member compositions. Calculated Fe('3+) to Fe('2+) ratios agree well with wet chemical data.;Synthetic iron-bearing spinels, with a normal cation arrangement, provide a perfect medium in which to study the effects of next-nearest neighbor cations on the Mossbauer spectra of minerals. The tetrahedral cation site maintains perfect T(,d) symmetry while the surrounding next-nearest neighbor oxygen sphere is comprised of 12 octahedrally coordinated cations. Mossbauer work on naural Cr-bearing spinels shows anomalously broad Fe('2+) peaks. This is attributed to the next-nearest neighbor effects of the random distribution of trivalent cations in the M-sites surrounding the ferrous iron.;In order to further quantify these next-nearest neighbor effects, selected compositions within the spinel solid solutions, FeCr(,2)O(,4)-FeAl(,2)O(,4), FeCr(,2)O(,4)-MgCr(,2)O(,4), FeCr(,2)O(,4)-ZnCR(,2)O(,4), FeCr(,2)O(,4)-FeGa(,2)O(,4) and FeCr(,2)O(,4)-Fe(,2)TiO(,4) have been synthesized and examined by Mossbauer spectroscopy. Mossbauer parameters obtained on the Cr-Al system show a direct correlation between halfwidth, quadrupole splitting and both number and type of next-nearest neighbor configurations. Partial quadrupole splittings have been determined for various next-nearest neighbor arrangements and have been used to calculate quadrupole splittings for natural and synthetic compositions. Agreement is found to be very good between calculated and empirically derived Mossbauer parameters.;Mossbauer site occupancies were used to constrain the crystal structure refinements of selected natural spinels using single crystal X-ray diffraction data. Refinement of a Bushveld chromite (a = 8.3042(5), weighted R = 0.026) confirms the expected site occupancies for a normal spinel with divalent cations ordered into the T-site and trivalent ordered into the M-site. Refinement of a gahnite from Hood River, N.W.T. (a = 8.1093(7), weighted R = 0.036) also yielded "normal" site occupancies. Refinement of a spinel from Madagascar (a = 8.1088(5) weighted R = 0.015) gave a somewhat ambiguous distribution with possible indications of a slight inverse component in the cation distributions. |
