Secondary phosphate phases in altered trachyte from S. Miguel Island (Azores, Portugal)—A possible contribution to the stone degradation

Publisher Summary This chapter discusses the geochemical and mineralogical research that were undertaken to determine the origin of degradation observed in stone monuments of the Misericordia church. This facade was constructed using volcanic rocks of trachyte composition. The degradation process in these stones is correlated to the precipitation of secondary phases as saline minerals and rare earth elements (REE) phases. During alteration processes, the behavior of REE depends on several factors, for example Eh, pH, the presence of organic and inorganic ligands, and the mineralogical distribution of REE in the parent material. Among the REE, cerium (Ce) is important because it occur in nature as Ce 3+ like the majority of lanthanides, or as Ce 4+ in oxidizing conditions. The mineralogical transformations (dissolution/precipitation), including the nature of secondary minerals incorporating the REE, is a fingerprint to mark out the degradation processes. Loss of material results from the dissolution reactions of primary minerals inducing an increase in the porosity, whereas the precipitations of secondary phases cause physical constraint increasing the fragility of the stones. This chapter evaluates the REE-bearing minerals in altered trachyte from the church by combining the scanning electron microscopy (SEM) observations made in both monument stones and quarry samples. From the analyses, it is concluded that the occurrence of these secondary phases in the first stages of weathering of trachytes in Azores Islands indicates a high porosity of these rocks allowing an effective water-rock interaction.