IMPROVED MODAL ANALYSIS FROM X-RAY POWDER DIFFRACTION DATA. W.G. Mumme, G. Tsambourakis, R.J. Hill and I.C. Madsen, CSIRO Division of Minerals, P.O. Box 124 Port Melbourne 3207 Australia

Mineral phase abundances have been determined on a range of naturally occurring rock types by Rietveld analysis of X-ray and neutron powder diffraction data.

The phase systems analysed in this work include (i) igneous, metamorphic and volcanic rocks, (ii) sedimentary rocks, and (iii) massive sulphide ores. The Rietveld results are compared with values obtained by (i) optical modes for the medium to coarse grained samples obtained by point counting, and (ii) normative calculation from bulk chemistry. For the sulphide ores, the X-ray results have been improved by the inclusion of a correction for microabsorption effects.

The presence of severe preferred orientation in some phases required the collection of X-ray data using Debye-Scherrer geometry with samples packed in 0.5mm capillaries. While preferred orientation was virtually eliminated by this method, care had to be taken to ensure that the sample examined (<2mg) adequately represented the bulk material.

One of the major benefits of a Rietveld based approach over traditional quantitative XRD methods is that the crystal structure can be refined during the analysis. Since this involves the unit cell dimensions and contents, the `calibration constant' (ZMV) can be automatically updated during the refinement. Not only are more accurate phase abundances produced, but supplementary information is obtained about individual phase chemistry.