Clay mineralogy of soils developed from Miocene marls of Medvednica Mt., NW Croatia: Origin and transformation in temperate climate

Five young soil profiles, formed from Miocene marls and argillaceous limestone, were investigated to determine changes in their clay mineralogy. Soil profiles were described, sampled, and their basic properties analyzed (particle size distribution, pH, cation exchange capacity (CEC), CaCO3 and organic C contents). The soils were classified according to the WRB classification as: 1) Calcaric Mollie Leptosol (Humic); 2) Calcaric Lithic Leptosol (Humic); 3) Calcaric Regosol (Loamic, Relocatic); 4) Calcaric Regosol (Siltic); and 5) Calcaric Cambisol (Densic, Endoloamic, Episiltic). Mineralogy was determined via X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) on bulk samples and clay fractions, and chemical composition was determined by inductively coupled plasma emission spectroscopy (ICP-ES). Calcite, quartz and some plagioclase alongside phyllosilicates were identified in parent material. Dolomite was found in the parent material of younger age. Clay fraction from parent material consists of expandable clays (smectite-rich illite-smectite and vermiculite), illite and kaolinite, with chlorite present in younger parent materials. In most soil horizons dissolution and leaching of less stable phases (calcite, dolomite, chlorite) occurred. Initial stage soil formation is consistent with the dominant influence of parent rock mineralogy on soil clay mineral assemblage. Analyses showed substantial homogeneity between and within profiles; nevertheless, slight changes in the clay mineralogy were noted: decrease of illite layers in illite-smectite in topsoil samples and occurrence of mixed-layered chlorite-vermiculite and/or hydroxyl or organic matter interlayering in soil horizons. Chemical composition of bulk samples indicated mobility only of alkali elements, confirming weak chemical weathering. The results show slight changes in clay mineralogy, which can be detected in young soils developed from calcareous parent material under high pH conditions. Those weak alterations of parent clay minerals assemblage, often through unstable phases, can modify soil properties, but are also indicators of subtle clay transformation in calcareous soils in temperate climate.