Fractal Characterization of Particle Size Distributions in Chromitites from the Great Dyke, Zimbabwe

Abstract

—Chromitite from seams in the early Proterozoic Great Dyke, Zimbabwe, has three types of microstructure. Grains in intact samples have an average of just over five slightly curved grain boundaries around each polygonal grain, and triangular-shaped triple grain junctions, some with grain boundaries intersecting at 120°. These features show adjustment to a minimum surface energy configuration. Samples with extension microcracks have smaller particles on average, which are more inequant and have a stronger preferred orientation than particles in the intact samples, due to fragmentation by impingement microcracking. Microfaults have still smaller average particle sizes, but more equant and less well orientated angular fragments, formed by sliding and rotation of particles after linkage between extension microcracks. Intact samples have a curved relationship on a log-log plot between cumulative numbers of particles and grain size. This particle size distribution evolves with strain to a linear, fractal relationship in the microfaults, with a fractal dimension of 2.8. The changes in particle size distribution are consistent with constrained comminution, and an additional process of selective fracture of larger particles. The degree of cataclasis is an important factor in determining chromite ore quality.