PETROGRAPHIC AND MINERALOGICAL CHARACTERIZATION OF VOLCANIC ROCKS AND SURFACE-DEPOSITIONS ON ROMANESQUE MONUMENTS

The stone used on the monuments, especially if they come from ancient times (e.g. medieval period), when exposed to the atmosphere, they are frequently affected by weathering and bio-deterioration processes. Thus, these latter produce various chemical and mineralogical transformations of substrate at the interface with the atmospheric agents as function of intrinsic compositional and physical characteristics of the material. Moreover, if there also are ancient treatments on the surface of geomaterials, the investigation field is further complicated. This research aims to analyse the surfaces of different volcanic lithology (basalts, andesite, pyroclastic rocks) used in the medieval Romanesque churches of Sardinia (XI-XIV cent.) having high historical-architectural and cultural relevance. By mineralogical and petrographic analysis (OM) and other different analytical methods (XRPD, colorimetry) the investigations addressed to define: i) petrographic features of volcanic rocks substrate; ii) chemical and mineralogical composition of coatings/crusts and any salt fanoand cryptoefflorescence; iii) compositional and microstratigraphic characterization of ancient treatments (i.e., Ca-oxalate films) on the surface of facades; iv) chromatic modifications of surface with respect to the substrate due to the alteration processes. The results show the incidence of several factors in the alteration process, as function of the occurred time, petrophysical features and composition of geomaterials, their position in the monument structure, exposition to the weathering processes, microclimatic characteristics, environmental conditions.