Fractal characterization of particle size distributions in chromitites from the Great Dyke, Zimbabwe

Chromitite from seams in the early Proterozoic Great Dyke, Zimbabwe, has th ree types of microstructure. Grains in intact samples have an average of ju st over five slightly curved grain boundaries around each polygonal grain, and triangular-shaped triple grain junctions, some with grain boundaries in tersecting at 120 degrees. These features show adjustment to a minimum surf ace energy configuration. Samples with extension microcracks have smaller p articles on average, which are more inequant and have a stronger preferred orientation than particles in the intact samples, due to fragmentation by i mpingement microcracking. Microfaults have still smaller average particle s izes, but more equant and less well orientated angular fragments, formed by sliding and rotation of particles after linkage between extension microcra cks. Intact samples have a curved relationship on a log-log plot between cu mulative numbers of particles and grain size. This particle size distributi on evolves with strain to a linear, fractal relationship in the microfaults , with a fractal dimension of 2.8. The changes in particle size distributio n are consistent with constrained comminution, and an additional process of selective fracture of larger particles. The degree of cataclasis is an imp ortant factor in determining chromite ore quality.