Yellowing of a white granite pavement in urban environment: the Fe-rich patina of Piazza Cavalli, Piacenza (Italy)

An integrated petrographic, colorimetric and capillary water absorption study of decayed flagstones of Granito Bianco (white granite) Montorfano from Piazza Cavalli (Piacenza, Italy) can provide important clues on the formation of yellowish patina on leucocratic granites used for buildings and decorative purposes. The granitic flagstones constituting the pavement of Piazza Cavalli rest on a basement consisting of iron-rich (Fe2O3 = 6.53-7.37 wt%) clay bricks. After more than two centuries of exposition to the urban environment, these surfaces acquired a yellowish patina characterised by a significantly higher yellow component that do not occur on the fresh surfaces in the native quarry of Mergozzo (NW Italy). In particular, the intensity of the yellowish patina is not controlled by the modal amount of biotite, the main Fe-rich mineral of Granito Bianco Montorfano. The results of the capillary water absorption test indicate that the aged Granito Bianco Montorfano flagstones have higher water transport capability than samples recently extracted from the Mergozzo quarry. The capillary water rise is controlled by inter/intragranular porosity of the granite flagstones. In particular, iron-rich compounds preferentially occur along the pores, indicating that iron can be efficiently transported by advection from the clay brick basement towards the surface. The results of this study show that (i) external iron sources coupled with iron advection are essential to the development of a Fe-rich yellowish patina on white granites, (ii) the release of iron from femic minerals contribute to the development of rust stains, but cannot account for the yellowish patina covering large surfaces of white granite flagstones.