Advances in Quantitative X-Ray Mineralogy: Mixed Crystals in Bauxite

Abstract

Comparing results of quantitative Rietveld refinement to chemical analysis may be difficult due to the presence of crystals with variable stoichiometry. In principle, variations in composition could be determined by refining site occupancy parameters in the Rietveld software. Unfortunately, in quantitative phase analysis the measured data usually do not permit one to do so. However, from these kinds of data the lattice parameter may be obtained very precisely. Linking the lattice parameters to site occupancy factors in the TOPAS software package improves quantitative phase analysis. The example of Al substitution in goethite, FeOOH, for iron-rich bauxite is discussed. The quantitative mineralogy of 11 CETEM-certified reference bauxites was determined using Rietveld refinement of XRD data. For a highly substituted material, agreement within 1% compared to XRF data may be achieved.