Estudio petrográfico, geoquímico y mineralógico de la fulgurita San José de Lourdes, Zacatecas, México

Abstract

Petrographic, chemical and mineralogical studies were performed on a fulgurite formed by melting of soil by lightning strike located at San José de Lourdes, northeast of Fresnillo, State of Zacatecas, Mexico. Polarizing microscopy, scanning electron microscopy with EDS, microprobe WDS analyses, micro-Raman spectroscopy, X-ray diffraction and X-ray fluorescence were used as analytical methods. Most of the specimens are tubular and have a smooth, lustrous, black, glassy texture while the exterior is encrusted with soil grains. Internally, this fulgurite is partly composed of heterogeneous vesicular glass mostly of andesitic composition containing fragments of skeletal crystals of wollastonite and undetermined Fe- silicate, spheroids of SiO2 similar to ballen quartz within the glass. The presence of very fine and scarce grains of ilmenite, zircon, apatite, silver and gold besides some Si-Fe alloys (silicides) of variable composition and arsenic-calcium compounds (which is found for the first time in fulgurites) is also reported. We estimate that the temperatures of crystallization were above 1,700 °C considering into account the melting of quartz and presence of reduced phases.