Trachyte stones in monuments of the São Miguel and Terceira Islands, Azores (Portugal)

Publisher Summary Among the volcanic rocks from Azores Islands, trachyte stones have been preferentially used through centuries in many monument facades because of their color and softness. After a few centuries, these stones show remarkable decaying. This chapter describes the study conducted to compare the trachyte stones from quarries and monument to point out the similarities and differences in their petrophysical and chemical characteristics and to determine the decay phenomenon. Angra do Heroismo Cathedral in Terceira Island and Misericordia Church in Sao Miguel Island were the monuments selected for the study on degradation causes and mechanisms. The same methodology was applied to quarry and monument samples from both Islands by using petrographic, mineralogical, chemical, and petrophysical techniques. The Well-marked differences were noticed among the trachytes from Sao Miguel and Terceira Islands; but within each island, outcrops or the monument building stones showed several similarities. Some textural aspects observed in the field showed some variability; the stones had a pore-type porosity of genetic nature. The values of porosity accessible to water were very high for monument samples and higher than the values for quarry samples. The samples of Misericordia Church showed a high porosity, dominated by large pores. It was concluded that these highly porous trachyte materials are susceptible to degradation, namely by salt crystallization mechanisms.