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Clay mineralogy and properties of soil and underlying plio-quaternary sediments of SE Mt. Medvednica (North Croatia)

Plio-quaternary sediments of the southeastern part of Mt Medvednica consist of colluvial and alluvial unconsolidated sediments (BASCH, 1983). They are covered with a thin and young soil profile. By investigating clay mineralogy of these profiles at different depths, we tried to gain more insight into weathering processes and pedogenesis. Samples were collected at two locations: Oporovec at depths of 25, 40 and 70 cm and Dotrščina at depths of 25, 120 and 300 cm. Mineralogical composition of the bulk samples was determined using XRD on randomly oriented mounts. Clay minerals were analyzed further after the removal of organic matter, carbonates and Fe/Mn/Al oxides using XRD on oriented slides on <2 µm and <0.2 µm fractions, and FTIR-ATR on <2 µm and <0.2 µm fractions. Additionally, CEC was determined for bulk samples, before and after removal of organic matter, carbonates and Fe/Mn/Al oxides, and the share of Fe/Mn/Al oxides was determined after their removal with Tamm reagent. The XRD results show the presence of quartz, plagioclase and some muscovite in the bulk samples in addition to clay minerals. Clay minerals in the samples include expandable clays, illite and kaolinite with some presence of mixed-layered clays detected. CEC values are much higher for untreated bulk samples, than samples from which organic matter, carbonates and Fe/Mn/Al oxides were removed. Samples contain a very low content of Fe/Mn/Al oxides (under 1%) with Mn being the least abundant. Slight decrease in their concentration is noticed in the underlying sediment compared to the soil horizons, probably due to Holocene weathering of minerals in the upper horizons (HUANG et al., 2011). It is obvious that the mineralogy of the samples does not vary much with depth. Clay minerals present show slight variations between different fractions, e.g. 10 and 7 Å peaks are more prominent in <2 µm fraction which can be explained with larger crystallites of illite and kaolinite. This can be confirmed for kaolinite with FTIR-ATR results. It could be concluded that the observed non-prominent variation in mineral composition within soil profiles developed on investigated plio-quaternary sediments might be related to an early stage of pedogenesis occurred on unconsolidated young sediments.

clay minerals, weathering, soil profile

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