State-of-the-art and trends in quantitative phase analysis of geological and raw materials

The accuracy of X-ray powder diffraction based techniques in quantitative phase analysis of geological materials is discussed. Based on the outcomes of recent Round Robin projects, the general quality of routinely applied methods must be estimated to be much lower than claimed in some individual publications. Estimated standard deviations for simple mixtures and intermediate mass fractions are typically below 3% (absolute) and can reach 1% if outliers are eliminated. In case of more complex compositions like clay-bearing rocks, the top laboratories are able to match the true phase abundances of most minerals with a similar accuracy, but the bias for clays or high absorbing minerals is mostly found to be higher. The majority of the laboratories have significant problems with the correct qualitative identification of minerals in complex mixtures. The Rietveld method tends to become the most popular technique for quantification, even for disordered structures like clay minerals. The general level of QPA could be enhanced by teaching the basics of X-ray diffraction physics and crystallography, and by providing user-friendly software with stable mathematic algorithms and structure data optimized for quantification.