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Clay minerals as an indicator of the polygenetic nature of terra rossa in Istria, Croatia

Terra rossa in Istria is situated on the Jurassic-Cretaceous-Paleogene carbonate plain and is considered a polygenetic reddish soil which bears typical terra rossa Fe-oxide characteristics (e.g. Fed and Fed/Fet). In the north-western part of Istria, the Upper Pleistocene loess clearly covers terra rossa. When thinner, this loess may be mixed and digested by terra rossa, and is not easily recognisable. Terra rossa is clay and silty clay composed of quartz, plagioclase, K-feldspar, micaceous clay minerals (illitic material and mica), kaolinites (KlD and Kl), chlorite, vermiculite, low-charge-vermiculite or high-charge smectite, mixed-layer clay minerals (other than illitic material), hematite, goethite and XRD-amorphous inorganic compound. Calcite, dolomite and boehmite are sporadically present and are of local importance. Kaolinites and illitic material are dominant clay mineral phases in the clay fraction of all terra rossa from Istria. Kaolinite which does not form intercalation compounds with DMSO is the dominant mineral phase in fine clay and is considered predominantly authigenic rather then inherited from parent materials. Neither the content and particle size distribution nor the bulk and clay mineralogy of the insoluble residue of limestone and dolomite support development of terra rossa entirely by dissolution of carbonate rock. If terra rossa has developed only from the insoluble residue of limestone and dolomite, its clay content, due to weathering should be higher than that in the insoluble residues which is not the case. Plagioclase was found only in one insoluble residue while all terra rossa samples contain this mineral. Moreover, insoluble residues do not contain vermiculite which was observed in small amounts in clay fraction of all terra rossa samples. (Na2O/K2O)x100, Zr/Nb and (Zr/Ti)x1000 ratios in the majority of terra rossa samples are much higher than in the insoluble residue of limestones and dolomites which also supports external material influence in the genesis of terra rossa. Materials other than the insoluble residue of limestones and dolomites which might have contributed to terra rossa are loess sediments, whose deposition was very important recurrent process in Istria probably since the early Middle Pleistocene, flysch sediments which extended much more southwards from its present position and bauxites of the Jurassic and Paleogene ages. Analyses performed indicate that loess older than that of the Upper Pleistocene age, flysch and bauxites might have contributed in the genesis of terra rossa.

clay minerals; terra rossa soils; Istria; Croatia

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