Modal analysis of rock forming minerals: Contribution of XRD/Rietveld analysis compared to the classic point counting method

Abstract

The determination of the mineral composition of aggregates which constitute an important component of concrete is essential to understand and estimate the durability of structures. The modal (quantitative mineralogical) analysis of rocks is generally determined by point-counting method on thin sections or slabs. However, there is a difficulty of mineral identification depending not only on the experience and capability of the operator but also on the size of observable grains. In this study, XRD (X-Ray Diffraction)/Rietveld analysis is proposed as an alternative and a comparison study is performed for eight different rocks. The results show equivalent proportions to those of the point counting method for the major phases (minerals), although discrepancies are observed for the minor minerals. Complementary tests as XRF (X-Ray Fluorescence) and density measurements are also performed to pre-characterize and confirm the obtained modal analysis. For instance, the density calculations based on XRD/Rietveld analysis provide close values to the measured densities. Overall, this method can be an excellent alternative to the point counting method especially in the context of construction materials laboratory.