MINERALOGICAL AND CHEMICAL CHARACTERIZATION OF VITRIFIED ROCKS FROM SERRA DI BUDA (ACRI, ITALY)

In 1971 a crust of melted rocks of 50-90 cm thick was found on the top of a hill called Serra di Buda near Acri (Calabria, Italy, 39° 30’ 37”N; 16° 22’06”E). The area is part of the Sila Plateau (Palaeozoic granitoids) where Kinzigitic Formation (high-grade metamorphic rocks, gneiss) dominates. A preliminary investigation was reported by Bertolani (1972), but the origin of the vitrified rocks of Serra di Buda is still uncertain. The goal of the present research is to shed light on the genesis of the crust of melted rock from Serra di Buda Acri (Italy) using a multi-method approach. X-ray powder diffraction (XRPD) was used to reveal the presence of newly-formed high-temperature and high-pressure minerals. The XRPD pattern showed the presence of quartz, cristobalite, coesite, magnetite, plagioclase, and mullite. Determination of the content of amorphous material was performed using Rietveld method. The presence of cristobalite and coesite would indicate induced temperatures above 1700 ºC and pressure above about 2.5 GPa if thermodynamic equilibrium were obtained. Scanning electron microscopy (SEM) showed the presence of partially melted fragments of plagioclase in a glassy porous matrix and newly formed crystals of magnetite likely formed due to heat-induced oxidation. Electron dispersive X-ray analyses revealed that the sample is mainly composed of SiO₂ and Al₂O₃ and has a porosity of about 5–7 area% in the studied zones. The limited amount of Na, Ca, and K in the glass suggests that alkalis were liberated from decomposing feldspars during heating. The sample studied displays the effects of impact-related processes owing to lightning or meteor impact. The mineralogical and chemical features of the sample from Serra di Buda provides insights into the physical effects occurring in rocks in response of shock-impact events such as lightning strike or impacting bodies.

Reference. Bertolani, M. (1972) An enigmatic outcrop of vitrified rocks near Acri (Cosenza). Boll. Soc. Geol. It., 91, 683-692