The Distribution of Contaminants and the Nature of the Alteration Products in Physically Separated Grains of Altered Ilmenites

Abstract

So-called ' ilmenite ', 'leucoxene' and 'rutile' grades of commercial heavy mineral concentrates were fractionated magnetically, in the laboratory. In the less magnetic ractions, secondary TiO₂ (both anatase and rutile ), formed by the alteration of ilmenite grains, was separated from primary rutile, on the basis of density. The separated fractions were examined by optical and scanning electron microscopy (SEM) and X-ray diffraction (XRD) methods, thereby insights being gained into the nature of secondary contamination in altered ilmenite. Grains of secondary TiO₂ are very porous and report with an effective density <4.0 g/cm₃ in a Magstream separator. Nearly all of these grains, which were distinctively fawn or dull grey in colour, were shown to contain epitaxially oriented, polycrystalline rutile , by single-grain XRD. The porosity of the grey grains was fine and uniform,   whereas the fawn grains contained coarse pores within a trellis of the oriented rutile. Randomly oriented anatase powder occurred with the oriented rutile in some fawn grains. Such trellis textures could facilitate the subsequent ingress of contaminants.

Electron probe microanalysis (EPMA) indicated considerable variation in the level of contaminants, between grains, but the averaged analyses for a given fraction were in broad agreement with bulk analyses by X-ray fluorescence (XRF). XRD of single grains showed that anatase occurred significantly more often in fawn grains that were extracted from EPMA mounts because of their high levels of contaminants. The anatase may provide a high surface area for adsorption of contaminants, and/or be an indicator of a past environment where 'insoluble' oxides have been more mobile.