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PREFERRED ORIENTATION OF MINERAL GRAINS IN SAMPLE MOUNTS FOR QUANTITATIVE XRD MEASUREMENTS: HOW RANDOM ARE POWDER SAMPLES?

¹ TU Bergakademie Freiberg, Institute of Mineralogy, Brennhausgasse 14, D-09596 Freiberg, Germany
² Department of Earth Sciences, University of Ottawa, Marion Hall, 140 Louis Pasteur, Ottawa, Ontario K1N 6N5, Canada
³ Macaulay Institute, Craigiebuckler, Aberdeen AB15 8QH, UK

^* E-mail address of corresponding author: kleeberg{at}mineral.tu-freiberg.de

The degree of preferred orientation of mineral grains in powder X-ray diffraction (XRD) samples prepared by standard techniques has been evaluated by means of a correction model implemented in the Rietveld program, BGMN. It is demonstrated that neither front- nor side-loading of mineral powders obtained by wet grinding in a McCrone micronizing mill yield powder mounts with randomly oriented particles. Despite fine grinding, the primary sizes and shapes of mineral grains contained in multi-phase samples influence the degree of preferred orientation in XRD powder mounts. Two minerals, both of platy habit, were found to show different degrees of preferred orientation in front- and side-loaded samples. In contrast to these methods of sample preparation, the spray-drying technique yielded perfect randomness of the particles. The experiments on artificial mineral mixtures demonstrate that the model applied can effectively correct for preferred orientation allowing reliable Rietveld quantitative phase analysis of moderately textured samples prepared by standard techniques.

Key Words: Correction of Preferred Orientation • Quantitative Phase Analysis • Random Powder • Rietveld Method • Sample Preparation • X-ray Diffraction