engleski

Illitization of smecites in paleosols from Istria, Croatia

Greenish clays often occur as thin layers in Early Cretaceous shallow- water carbonate sediments of Istria where they mark the top of shallowing upwards sequences. When they show evidence of subaerial exposure and pedogenesis they are considered paleosols. Paleosols bear important information on material supply, climate, relief, and duration of subaerial exposure phases Clay minerals in <2m and <0.2 m fractions were analyzed using XRD and FTIR. Chemical composition (main and trace elements) was employed using XRF and ICP-MS. The clay mineralogical composition is as follows: (1) illitic material, (2) randomly oriented I/S mixed layer, (3) regularly oriented I/S mixed layer, (4) kaolinite and (5) chlorite. In the Selina profile the clay mineral distribution shows a clear trend. The main clay mineral in the upper part is illitic material up to 78 wt.%. In the lower part of the profile illite values are below 50 wt. %. The second main clay mineral group are ordered and irregularly oriented illite/smectite mixed layer minerals respectively. Mineralogical as well as chemical data point out, that a transformation of smectite (from the mixed-layer minerals) to illite must have occurred. Wetting and drying experiments and layer charge measurements support that theory. K-fixation necessary for illitization of smectites could have been achieved at surface temperatures by repeated wetting and drying. The K+ might have been provided by plants, marine waters, volcanic rocks or other various sources.

illitization of smectites; paleosols; Istria; Croatia

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