Le argille curative in uso negli stabilimenti termali emiliano-romagnoli: verifica della composizione e delle proprietà

The mineralogical and chemical (major and minor elements) composition of the clays used in some thermal centres of the Emilia-Romagna region have been studied, as well as important physical and chemical parameters (such as the grain size distribution, the Atterberg's limits and the cation exchange parameters). The comparison between the matured and virgin clay was carried out in selected cases. The results indicate that two alternative criteria are used to select the raw materials: availability of smectitic clays on the market, and the occurrence of local clayey silt. In the latter case, the compositional characters are slight diferent among the samples, because of the different geologic provenance and because some thermal centre mix different raw materials. The mineralogical composition of the smectitic clay is dominated by expandable clays, whereas the other raw materials represent the mineralogy of the geologic formations they belong to. In facts, quartz, carbonates (calcite and dolomite) and common sheet silicates occur in all these samples. The mineralogy is clearly correlated with the cation exchange capacity and the Atterberg's limits. Also the chemical composition (included some trace elements such as As, Pb, Hg, Cd, Se, Te, Tl e Sb) does not show any anomaly if compared with the composition of similar lithologic types, all over the world. The maturation process does not seem to cause significant compositional changes of the main mineralogical and chemical composition, at least for the inorganic fraction of the muds. Possible toxicological effects are sketched, considering that very toxic elements occur at trace levels, and that quartz occur among the main minerals. In the studied case the very toxic elements are within the natural variation range of detrital fine-grained rocks, suggesting that they are hosted into detrital grains, and then are not readably bio-available during the thermal application. If the maturation waters are enriched, to some extent, in toxic elements, a monitoring of the bulk composition and of the bio-availability could be necessary, because of the sorption behaviour of clays detected in the raw materials. The dangerousness of quartz should be controlled (mainly if the raw material is ground), but the maturation process likely reduce it because the ageing of the surfaces is among the factors that decrease the risk.