Mineralogy and geochemistry of detrital rutile from the Sibaya Foundation, KwaZulu-Natal.
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Rutile, although not a major component of detrital heavy mineral deposits, is a valuable source of titanium oxide. Theoretically rutile is pure titanium dioxide (TiO2) and should form white or colourless tetragonal crystals with a density of 4.25gm/ml. However, natural rutile although tetragonal, displays a variety of colours ranging from red through brown to black, yellow or blue, variable density between 4.23 to 5.50g/ml as well as a range in the magnetic susceptibility and electrical conductivity. In addition to these variations exhibited by natural rutile, samples from detrital heavy mineral deposits normally contain, in addition to homogenous grains, composite grains, in which rutile is intergrown with one or more mineral species, commonly quartz, feldspar and ilmenite. The Sibaya Formation, like most detrital heavy mineral deposits, has a polymictic source, and as such contains rutile grains formed in many different chemical environments. Homogenous rutile grains display a chemical variation with a preference for the select few elements, which are compatible with the rutile cyrstallographic structure. The ions that substitute for titanium (Ti4+) in the crystal lattice are a reflection of chemical environment in which the crystal formed. The size and charge of the Ti4 + ion greatly restricts the species that may enter the rutile crystal lattice, with Sb3 +, V3 +, Fe3 +, Cr3 +, Sn4 +, M04+, W4+, Mn4+, 8i5+, Nb5+, Ta5 +, Sb5 +, V5 +, being theoretically compatible with the size and charge of the Ti4+ ion. Electron microprobe analysis of detrital rutile grains from the Sibaya Formation, KwaZulu-Natal show that elements, Nb5 +, Ta5+, A13+, Zr4+, Si4+, Fe3+, Cr3 +, and V5 +, commonly substitute for the Ti4 + ion. However, Sb3+, Sn4+, M04+, W4 + and 8i5 + were not present at detectable levels implying that the provenance area is not enriched in these elements. Although the high Fe3+ values were expected in the rutile grains, as Fe3 + is common in many rocks, the high Si4+ values encountered were not expected, as Si4 + is not normally compatible with Ti4 + ion, as noted by their distinct separation in rutilated quartz. The anomalous Si4 + content of certain grains suggests that within the provenance area rutile bearing rocks formed under unusual conditions, such as high pressure, temperature and silicon activity where the high charge density of the Si4 + ion would favour the inclusion of Si4 + into the rutile lattice. The chemical variation of the rutile grains causes significant variation in the magnetic susceptibility and electrical conductivity, and thus has marked effects on mineral processing, which relies heavily on magnetic and electrostatic separation techniques. The data presented indicates that individual homogenous rutile grains displays significant range of chemical composition, commonly containing other oxides from a fraction of a weight percent to well over 10wt%. Data plots of TiO2, FeO and 'other' oxides (Nb2O5, Ta2O5, A12O3, ZrO2, SiO2, Cr2O3 and V2O3), showed that many of the more magnetic rutile grains appeared to be FeO enriched and contained a higher proportion of 'other' oxides. However, some grains that just had higher proportions of 'other' oxides and a lower FeO content were also magnetic. Thus magnetic susceptibility although strongly influenced by the presence of FeO, can also be enhanced by the substitutions of other oxides. The vast majority of rutile grains from the electrostatic fractions were relatively TiO2 pure, and contained low concentrations of 'other' oxides. However, some grains did have slightly enhanced SiO2 and V2O3 concentrations, which appear to enhance the conductivity of the grains. Four main colour groups were differentiated from the population of rutile grains from the Sibaya Formation, these being, reddish brown, black, blue and yellow. No single oxide seemed solely responsible for the colour of rutile grains. However, the red rutile grains had a slightly but significantly higher Cr2O3 and Nb2O5 content, whereas black rutile grains appeared to be V2O3 and Nb2O5 enriched. The blue colour of rutile grains appears to be influenced by a combination of SiO2, Al2O3 and Nb2O5 substitutions. The yellow rutile grains had slightly enhanced FeO and Nb2O5 concentrations. Although these differences are very small, trace quantities of certain elements and different combinations of elements can have a strong effect on colour. Apart from Fe3+, no single element; appears to be solely responsible for variations noted in the physical characteristics (magnetic susceptibility, electrostatic conductivity and colour) of homogenous rutile grains from the Sibaya Formation. However a combination of substituting elements appears to influence magnetic susceptibility and electrical conductivity. An enhanced Fe3+ content normally increases the magnetic susceptibility although combinations of other elements may have the same effect on Fe3+ poor grains. In general terms, the purer the rutile grain, the more likely it is, to be non-magnetic and conductive. Substitutions of 'other' oxides appear to decrease the conductivity of rutile grains. The relationship between grain colour and chemistry is also not very clear, verifying the widely held view that grain colour is often the result of more than just mineral chemistry.