Contribution of Portuguese two-mica granites to stone built heritage: the historical value of Oporto granite

Abstract

Abstract The historical centre of Oporto (Portugal), designated a UNESCO World Heritage of the Humanity, has been selected to illustrate the use of the local granite during the development of the city, invariably influencing the history, architecture, religion, culture and the personality of the local people. The Oporto monumental heritage goes back to Roman occupation: the city has been developed by the construction of granite buildings strongly implanted on a hard granite substrate. The Oporto profile gives an impressive grey architecture according to different styles of granite work that characterize the city in all its aspects. The description of selected sites within the historical centre, where it is possible to recognize the importance of the granite in the character of the city, aims to call attention to the inextricable role of geology in the built heritage and culture, as well as to diagnose the deterioration processes, enhanced by the environment and the anthropogenic activity, that affect houses and monuments as a consequence of the intrinsic properties of the granite. The Oporto granite genesis, responsible for the nature of the mineralogy assemblage, the physical properties and the degree of weathering, provides specific features that make this granite particularly suitable for the ornamental work that defines the town monumental constructions. The old city wall, the medieval and baroque churches, the neoclassic houses and also the small humble habitations are a few examples of the profuse stone heritage of which Oporto is proud.