Hydrothermal transformations and weathering are important mineralizing processes in carbonatite-related ores. But the transformation products of mafic minerals, such as the clay minerals, vermiculite and interstratified phases, preclude the use of X-ray diffraction, particularly the Rietveld method, for mineral phase quantification. After treatment with KCl, vermiculite and interstratified phases (all peaks roughly at 1.2, 1.4-1.5 and 2.4 nm) are converted to a phlogopite-like structure with d(0 0 1) = 1.0 nm, which can successfully be refined. The method was tested using three carbonatite-derived ores, with a total phlogopite-like phase content ranging from 1.2 to 32.3 wt.%. Comparison of the chemical analyses with the calculated chemical composition, based on the phase quantification and the mineral composition is in good agreement for major elements, such as Si, Fe and Al. For P results were also very good, but Ti and Nb are generally underestimated, and Ba is overestimated. Minerals with lower contents, as pyrochlore, approach the diffraction detection limit. Problems to accurately determine mineral composition for some of the phases also impacts on the reconciliation. The chemical composition of the pyrochlore group minerals changed, but not to an extent to constrain the usefulness of the technique. Cation exchange successfully extended the amenability of the Rietveld method for phase quantification. (C) 2011 Elsevier Ltd. All rights reserved.