Line broadening in chromites from Zimbabwe using high-resolution X-ray diffraction

Abstract

Chromite grains in ores from the Great Dyke, Zimbabwe, exhibit varying degrees of shearing when viewed by optical microscopy. High resolution diffraction data revealed that line broadening from powder samples of sheared chromites is largely due to two or more spinel phases with slightly different cell parameters, the number of phases increasing with the degree of shearing. The dominant or “parent” phase, with parameters ranging from 8.3123(2) to 8.2676(2) Å, constitutes 57% to 76% of most samples. The cell parameters of secondary phases are generally less than that of the parent phase by δa/a0 in the range −1.3 to −4.0×10−3, the difference again tending to increase with shearing. Most reflections for the parent phase are relatively sharp whereas those for the secondary phases exhibit line broadening that could be analyzed in terms of crystallike (domain) size and rms strain. The crystallite diameters, assuming spherical particles, are relatively large for the parent phases [260(2)–100(5) nm], while those for the secondary phases range from 165(70) to 70(25) nm. The rms strain is not large for any sample and is negligible or small for the unsheared and weakly sheared material. The microstrain in secondary phases is greater than that in the parent phase and tends to increase with shearing, the maximum rms strain being 1.6(3)×10−3. Two new forms of chromite in highly sheared material are reported, one with a slight (0.33%) tetragonal distortion and the other with a cell parameter of 17.561(2) Å, greater than that of normal spinel chromites by a factor of about 3/√2, and a lowering of symmetry from Fd3¯m to Fm3¯m. Changes in chemical composition, indicated by a range in cell parameter, are attributed to tectonics that affected much of the Great Dyke. The strain is partitioned between failure by brittle deformation of the chromite grains and stress-induced cation migration leading to cells with slightly different composition.