Weathering of Valle Ricca stiff and jointed clay

The study proposes a weathering model of Pliocene-Pleistocene stiff and jointed blue-grey clay transforming into yellow clay. Physical, mineralogical, chemical and textural changes, as well as the weathering profile were investigated in a quarry of central Italy. Based on geological records and inferences, these changes are likely to have occurred within a time-span of about 50,000 years BP, upon overburden stress unloading and in a stress regime that is locally controlled by suction. Weathering propagated into the clay at a rate of about 0.3 mm/year and was enhanced by the enlargement of the pre-existing tectonic discontinuities and by the formation of new joints. A mass loss of about 22-25 wt.% was calculated. Considering Fe and P as immobile elements, the individual oxides contribute to mass loss in the following order: SiO2 > CaO = CO2 > Al2O3 > MgO > K2O > S > Na2O > TiO2 > MnO. The Fe2O3 / (Fe2O3 + FeO) ratio varies from 9-29% in the blue-grey clay to 75-82% in the yellow one. Oxidation and/or dissolution of 7 Å-Fe2+-bearing clay minerals, mica-like minerals and calcite and parallel increase of smectite and Fe-hydroxides play a critical role in the chemical changes and explain the higher plasticity of the yellow clay with respect to the blue-grey one. The role of water during the weathering process was inferred to occur in cyclical steps: 1) seepage of meteoric water; 2) dissemination of highly oxidizing meteoric water; 3) triggering of oxidation and dissolution of minerals; 4) water evaporation; 5) partial migration of the elements contained in the aqueous solution and consequent deposition of minerals in the joints.