Reliability Of Multi-Method Mineralogical Identification In Expansive Soils

The swelling characteristics and engineering performance of expansive soils are primarily governed by their clay mineralogy; however, mineralogical identification based on a single method may be insufficient for natural clays composed of multiple clay minerals together with non-clay constituents. This study aims to determine the mineralogy of a natural expansive clay from the Çubuk district of Ankara, Türkiye, using an integrated set of direct and indirect methods, and to demonstrate the limitations of relying on a single technique while proposing a practical combination of tests for more reliable mineralogical identification. Direct analyses included X-ray diffraction (XRD) and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDX), while indirect indicators comprised methylene blue adsorption, Atterberg limits, clay fraction content (<2 μm) and activity. The results show that the Çubuk clay contains a significant smectitic (montmorillonitic) component together with non-clay minerals such as quartz and calcite. Methylene blue results indicate high cation exchange capacity and specific surface area, and SEM–EDX spectra support the presence of interlayer cations consistent with a smectite-dominant clay fraction exhibiting illite-smectite characteristics. The results highlight the advantage of a multi-method approach and provide a practical framework for more robust mineralogical characterization.