Tomás Martín Crespo

The fulgurite of Torre de Moncorvo (Portugal): description and analysis of the glass

The fulgurite of Torre de Moncorvo (Portugal) was formed by lightning striking a small electricity pylon; it consists morphologically of a central cylinder with radial ramifications several metres long. It is largely composed of glass, with the remains of quartz crystals and neoformed cristobalite. This paper reports a textural and geochemical study of the lithologically different axial and radial facies identified in the fulgurite. The axial facies is formed of a dark, vitreous rock, shows a concoidal fracture, and has a central tube some 20 cm in width. The fulgurite shows a large quantity of small vesicles and partially melted quartz remains, along with fractured crystals of cristobalite and quartz. Analysis of this facies showed it to be composed of an aluminosilicate material with Na, K and Fe, formed from a granitic protolith and some allochthonous materials. Two types of glass were characterised, different from a textural and geochemical point of view, formed in two phases of incongruent melting. The radial facies are similar to their axial counterpart at the point of contact with the central cylinder, but these very vesicular, dark cylinders gradually take on different properties with distance from the axial facies. The chemistry of these branches indicates that they formed exclusively from native granitic material. The radial facies are made of the same types of glass as the axial facies, but cristobalite crystals have not been identified in this facies.