The effect of weathering on clay mineralogy of Plio-Quaternary sediments and the overlying material on SE slopes of Medvednica Mt., Croatia

Abstract

Abstract Clay mineralogy of Plio-Quaternary sediments of SE slopes of Medvednica Mt. was compared to that of the overlying weathering horizons. Three locations of Plio-Quaternary sediments, representing past weathering products, were sampled along with the overlying material, representing more recent weathering cycles. Particle size distribution, pH and cation exchange capacity (CEC) of the samples were analyzed. Bulk mineralogy was determined using X-ray diffraction (XRD). Clay mineralogy was determined, after carbonate cement, organic matter and free Fe oxides removal, on fractions < 2 and < 0.2 µm using XRD and Fourier-transform infrared spectroscopy (FTIR). Average layer charge of the expandable layers was determined using the O-D method. Major elements content was determined using inductively coupled plasma optical emission spectroscopy (ICP-OES). Bulk mineralogy comprises quartz, feldspars, mica, clay minerals and occasional Al and Fe (oxy)hydroxides. Clay mineralogy differs subtly between samples; dioctahedral expandable clay minerals dominate with illite and kaolinite present. Hydroxyl interlayering of expandables is noted in the surface samples, corresponding to lower CEC values. Average absolute layer charge of the expandable layers is higher in the surface samples and decreases with depth on two of the investigated locations. Overall, clay mineralogy of the Plio-Quaternary sediments reflects a more intensive weathering regime, confirmed by the presence of Al (oxy)hydroxides and lower absolute charge of the expandable layers. The weathered material and soil overlying these sediments shows heterogeneity in genesis, but corresponds to less intense weathering regime, confirmed by the formation of hydroxy-interlayered minerals and higher absolute charge of the expandable layers.