In the sedimentary basin of Rieti (Central Italy) a Mollic Fragiudalf was studied in order to verify, through the analysis of differential dissolution kinetics of Si and Al, wether the presence of this two elements could be involved in determining the fragic characters of its deep horizons. The reuslts of the main physico-chemical analysis confirmed the fragic features of the deep horizons Bt3x and Ctgx which have been already observed in the field study. Differential dissolution of Si and Al was performed using 0,5 N NaOH at 25°C, and the values of silica and alumina obtained after each of ten consecutive extractions were cumulated to trace the cumulative curves of dissolution. Slope changes of the curves, which indicate changes in dissolution rate, allowed os to recognize three mineralogical phases with different degree of solubility. The total quantity of SiO2 and Al2O3 dissolved in all the extractions and relative to each of these three phases was estimated, by a graphic method, together with SiO2(Al2O3 molar ratio. This one was indicative of the mineralogical composition and behaviour of each horizon. This study showed an accumulation of free silica which could be present in the form of phytoliths or diatoms as well as a fine precipitate of opal-A and silica-gel in the whole soil, mainly in teh Ap and Ctgx horizons. Moreover neogenesis of clay was more evident in Bt3x horizon. The volcanic origin of the parent material, a trachytic ash fall, probably deposited in a lacustrine environment, could have favoured the availability of Si and the authigenic formation of halloysitic clay from the weathering of galss and feldspars. Our study therefore corroborates the findings of previous studies by Chadwick et al. (1987) and Karathanasis (1989) indicating the important role played by silica, probably as bonding agent, in contributing to produce characteristics of a fragipan. (Résumé d'auteur)
