Chromatism and the chromatic alteration of the florentine sandstone in Pietra Serena

Pietra Serena is one of the materials more used in Florentine architecture. It is a sandstone that crops out in the hills north of the city in the municipality of Fiesole and it has been employed mainly for ornamental purposes. This litotype belongs to the Macigno/Monte Modino Formation (Oligocene Upper- Miocene Lower) which consists of beds of turbiditic sandstones separated by pelitic levels which are the finest components of each single turbidity layer. Petrographically, Pietra Serena can be defined as a medium-coarse-grained lithic arkose made of quartz, feldspars, micas, fragments of metamorphic and magmatic rocks. The clayey matrix is quite abundant while the carbonatic cement is present in a lower amount . Generally the decay process of sandstones is related to the type of matrix, the amount of cement, the kind of clay minerals and to the pore size distribution: the water infiltration causes swelling of the clay minerals, loss of the clayey matrix, with resulting exfoliation, peeling of the stone artefacts and change of colour. Pietra Serena usually has a bluish-grey colour in fresh cut but with time it can loose its original colour acquiring a brown and/or reddish colour on the surface of buildings. In this research the correlation between the composition and physical characteristics of Pietra Serena employed in architectural decorations in the centre of Florence and its chromatism and chromatic alteration has been investigated. The results point out that, given the almost constant iron content, the kind of exposition to atmospheric agents and the petrographical and physical characteristics (eg. amount of calcitic cement, porosity) play a main role on the chromatic alteration of the stone. In the case of the formation of a red chromaticity, it is possible to assume that the source of iron is mainly related to biotite K(Mg,Fe2+)3[AlSi3O10(OH,F)2 and chlorite (Mg,Fe2+)5Al(AlSi3O10)(OH)8: the chemical alteration of these minerals due to hydrolysis gives rise to release of Fe2+, easily oxidized to Fe3+ in an oxidative environment. In case of an high amount of calcite cement which occludes the porosity, the chemical alteration of biotite and chlorite is inhibited due to a lower action of the percolating water thus hindering the diffusion of iron, preserving the original grey colour of the rock. The case of the variety of Pietra Serena called Pietra Bigia is different; this sandstone has a typical yellow-brown colour, not only superficially but in the whole rock outcrop; the colour was formed in geological times in areas close to fractures from which a front of oxidation propagated, developing this particular chromatic variation different from the typical grey colour.