Petrographic, geochemical and physical characterization of volcanic rocks from the fortification of Bosa Castle (western Sardinia, Italy)

Abstract

The fortification of Bosa Castle overlooking the valley of the river Temo and the medieval old town of Bosa (XII-XIV centuries). His tactical position militarily, which allowed control of the land from the sea to the river, setting up urban in the valley (Bosa Vetus), allowed the exploitation for centuries, even as a place of sighting to face sudden attacks of brigands or Saracen enemies. The site initially belonged to the Giudicato di Torres (around the X cent.), And finally twelve hundred to Malaspina (from Lunigiana). Later the castle suffered many renovations of Turrritani, Malaspina, Arborea, the Aragonese, although overall preserves the defensive structure built by the Marquis of Tuscany and Liguria. Its decline began in the second half of the sixteenth century in favor of nearby Alghero populated by Catalans. This research proposal intends to define the petrographical and physical features and the weathering alteration of geomaterial used for the castle. The study of petro-volcanological features (i.e. structure, sialic and mafic minerals, welding degree) by macroscopic observations is the starting point for preliminary recognition of the different stones. Then, the analysis of texture and paragenesis on thin section by optical observations are made. Laboratory tests have been conducted to evaluate the physical-mechanical properties (density, water-absorption, porosity, PLT punching index, compressive and traction strength). The mapping of alteration forms and the causes of decay, in relation to varying volcanites and microclimatic-environmental conditions, have been studied.