“Trachytes” from Sardinia: geoheritage and current use

“Trachyte” is the term which was used to indicate clear, rough rocks outcropping in Greece in the area of Thrace in ancient times. Another etymological interpretation of the word might derive from the Greek word “turacz” which means “rough”. From the petrographic point of view, trachyte is the volcanic equivalent of syenites, which are rocks formed with K-feldspar, subordinate plagioclase oligoandesinic and mafic, such as green hornblende and biotite. Quartz is generally missing. Differently, “Trachite auctorum” is the latin term which, in the past, indicated a multiplicity of vulcanites, from acidic to neutral (rhyolites, riodaciti, dacites and andesites), found in Central and North-Central Sardinia (Ottana, Sedilo, Allai, Fordongianus, Bosa, Ozieri Oschiri and Osilo), southern (Serrenti) and southwestern (Sulcis). This term is still in use in the commercial field to identify this wide range of stone materials. Volcanic rocks are very different in texture (lavas, ignimbrites, pyroclastic rock welded in different ways, stagnation dome), easy to carve, they have variable porosity, and present very different color features, often characterized by a wide variable intensity: pink, red, yellow, gray, dark gray, green, green-blue.