THE QUANTIFICATION OF LATERITIC BAUXITE MINERALS USING X-RAY-POWDER DIFFRACTION BY THE RIETVELD METHOD

The application of the Rietveld method to quantify mineral components of bauxite and lateritic samples was carried out in order to determine the ability of the method to obtain accurate mineralogical abundances for these materials. The method was initially applied to synthetic mi xtures using both Cu and Co Ka radiations, and it was shown that Rietv eld-derived data compared favourably with the weighed compositions. Ap plication to two types of natural bauxite resulted in a high correlati on between Rietveld predicted values and those calculated by proportio ning peak intensities with chemical assays. The use of the whole patte rn rather than selected peak intensities gives greater accuracy, confi rmed by a strong correlation between derived oxide concentrations from XRF assays. Accuracy and precision were improved by the determination of isomorphous substitution of aluminum in goethite and hematite by r efinement of unit cell dimensions. Importantly, the ability of the Rie tveld program to successfully model several goethites with different l evels of isomorphous substitution improved the correlation between pre dicted and calculated values. In addition, crystallinity and crystalli te size that influence the reactivity of the mineral components can be derived from refined peak profiles. (C) 1998 International Centre for Diffraction Data. [S0885-7156(97)01904-0]