Trends in the Unit Cell Parameter for Kimberlitic Versus Non-Kimberlitic and Diamond-Indicating Chromite

Abstract

Abstract Spinel-group minerals are among the best-known and widely used minerals in diamond exploration due to their ubiquity, resistance to weathering, and utility as petrogenetic indicators. The kimberlite indicator mineral chromite is investigated in this study using micro-X-ray diffraction (µXRD) to measure chromite unit cell parameter ao. We used epoxy-mounted chromium-rich spinel (henceforward called ‘chromite’) mineral separates with known chemical composition from kimberlitic and non-kimberlitic sources to evaluate structural-chemical correlations for potential use in diamond exploration. Chromite grains of <300 µm size from the Koala, Misery, and Sheiba kimberlites in the Ekati property (Northwest Territories, Canada), as well as from exploration programs in Botswana and Gabon, Africa, were examined in situ, as mounted for standard electron probe microanalysis (EPMA). Unit cell parameter ao was measured by µXRD for several natural kimberlitic and non-kimberlitic chromite grains, and these data have been correlated with chemical composition as determined by EPMA on a grain-by-grain basis. Conventional chemical discrimination plots with unit cell size denoted by color demonstrate clearly discernable unit cell trends that are useful for classification. Two kimberlitic chromite compositional trends can be discriminated by chromite unit cell size. The kimberlitic phenocryst trend is delineated by a distinct increase in unit cell size (ao > 8.336 Å), whereas the kimberlitic xenocryst trend is delineated by a distinct decrease in the unit cell (ao < 8.322 Å). The latter trend is also followed by the Gabon non-kimberlitic samples. Notably, the unit cell parameters for chromite in the diamond-indicating field have a tightly determined value of ao = 8.329 (± 0.007) (or 8.322–8.336 Å). This field partially overlaps with the unit cell values for some non-kimberlitic chromites (e.g., Botswana). Unit cell values of chromite grains recovered from heavy mineral concentrates could serve as a preliminary screening technique for identifying diamond-indicating chromites prior to chemical analysis if their kimberlitic provenance is known. More broadly, the μ-XRD unit cell technique is a useful, non-destructive tool that shows promise for application to other kimberlite indicator minerals.