Date of Award
Doctor of Philosophy
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.;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.;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.
Osborne, Margery Diane, "Crystal Chemistry Of Chromium - Aluminium Spinels" (1983). Digitized Theses. 1259.