The calcare cavernoso of the Montagnola Senese (Siena, Italy): mineralogical-petrographic and petrogenetic features

As a part of a broader research programme on the stones used in the historical buildings of the Siena area, we performed a mineralogical-petrographic study of the Calcare Cavernoso outcropping in the Montagnola Senese. It is a dolomitic limestone genetically related to the epidiagenetic evolution of the Triassic evaporitic-dolomitic succession known as Burano Anhydrite Formation. Various investigations (using polarizing microscope, X-ray diffractometry, SEM/EDS, cathodoluminescence, WDS electron microprobe) were carried out on the Calcare Cavernoso and its parent-rock, aimed to the characterization of the material. Our observations have allowed us to propose an outline of the petrogenetic evolution of the rock. Calcare Cavernoso appears to be the result of a massive process of dedolomitization occurring at the expenses of the dolomitic facies of the original Triassic evaporitic-dolomitic succession, following its re-exhumation at the end of the Apennine orogenesis. The intense tectonic fracturation of the dolomite favoured the circulation of meteoric waters in continental phreatic environment, with the consequent dissolution of sulphates and dolomite and the neoprecipitation of calcite. A late, pervasive vadose circulation, easily occurring in the already very porous rock, led to the development of karstic phenomena on various scales; increasing the porosity of the rock, they gave it the characteristic vacuolar appearance observed today.