Importance of grain size distribution in quantitative analysis using Rietveld Method: An experimental study

The Rietveld method refines a theoretical line profile until it fits the measured profile using a least squares methodology. In Rietveld method, complete qualitative mineral determination, mineral chemistry, appropriate XRD analytical conditions and sample preparation steps should be applied correctly. This study aims to determine the grain size-dependent variations in the quantitative compositions of minerals with various chemical compositions and crystallographic characteristics. Eight pure minerals (quartz, calcite, halite, colemanite, barite, polyhalite, gypsum, thenardite) are been selected, powdered, pulverized and separated into 3 different grain sizes as 228μm (Dv90), 29μm (Dv90) and 8μm (Dv90). These minerals are mixed in certain proportions to prepare three mixture samples. Mixture samples are evaluated using the Rietveld Method on XRD whole rock patterns and then compared to the actual reference mixture sample whose mixing ratio is known for each grain size. Sample with a grain size of 8μm (Dv90) with random orientation gave the closest result compared to the reference content. If the structural and physical refinement stages are processed properly, accuracy of percentage mineralogical composition increases when the grain size decreases.