Quantitative analysis of alkali feldspar minerals using Rietveld refinement of X-ray diffraction data

Microcline + albite and albite + quartz standard mixtures were quantitatively analyzed using Rietveld refinement to investigate the reliability of the method. The method was also applied to the determination of weight percentages of component minerals in a perthite sample. The analysis resulted in a standard deviation of approximately I wt% for the microcline + albite mixtures and 4 wt% for the albite + quartz mixtures, indicating that the accuracy of quantitative Rietveld analysis is far better than that of conventional XRD methods in which only a few select peaks are used. The inclusion of the Dollase-March preferred-orientation correction function slightly improved the accuracy of the results. That function also improved the rate of convergence and the accuracy of unit-cell parameters. However, the Rietveld-Toraya correction function did not work well with the quantitative analyses. The result of the quantitative analysis of the albite + quartz mixtures was poorer than that of the microcline + albite mixtures, and the weight fraction of albite tended to be overestimated. The analytical result of the microcline + albite mixtures implies that the Rietveld method is more accurate when component minerals have similar preferred-orientation characteristics. However, if component minerals have different crystal habits, as represented by the albite + quartz mixtures, the deviation is much greater and systematic errors may occur. Quantitative phase analysis of a perthite specimen using the Rietveld method resulted in very accurate values for the mineral proportion and unit-cell parameters of the constituent alkali feldspars, demonstrating the power of the Rietveld method to analyze multiphase samples.