Quantitative mineralogy of sandstones by X-ray diffractometry and normative analysis

Abstract

ABSTRACT Quantitative X-ray diffraction, normative calculation, and whole-rock chemical analysis have been compared for Permo-Triassic quartz-lithic to litho-feldspathic sandstones from eastern Australia. The quantitative XRD technique provides mineralogical percentages that correlate well with the chemical composition of the sandstones, and also with most of the mineral percentages derived from normative calculation. The need for assumptions on potassium distribution between minerals provides a limitation on estimates of K-feldspar and illite using the normative technique, although these can be improved on a sample-specific basis if necessary using complementary thin-section observations or qualitative X-ray diffractograms. Quantitative XRD analysis possibly indicates a small proportion of amorphous material in some of the sandstones, especially materials with abundant lithic fragments or clay minerals, but the proportion of any amorphous material overall is probably negligible. Both quantitative XRD and normative calculation avoid problems associated with sample heterogeneity or operator inconsistency that may affect point counting of thin sections. They can also be used to provide quantitative information on sediments with abundant rock fragments or matrix components, the actual mineralogy of which is not resolvable in conventional microscope investigations.