Multispectroscopic methodology to study Libyan desert glass and its formation conditions

Abstract

Libyan desert glass (LDG) is a melt product whose origin is still a matter of controversy. With the purpose of adding new information about this enigma, the present paper analyzes the inner part of LDG specimens and compares them with the results of LDG surfaces. An integrated analytical methodology was used combining different techniques such as Raman spectroscopy, in point-by-point and imaging modes, scanning electron microscopy with X-ray microanalysis (SEM-EDS), energy-dispersive micro X-ray fluorescence spectrometry (μ-EDXRF), electron probe micro analyzer (EPMA), and optical cathodoluminescence (Optical-CL). According to our results, flow structures of the melt and the amorphous nature of the matrix could be discerned. Moreover, the observed displacement of Raman bands, such as in the cases of quartz and zircon, and the identification of certain compounds such as coesite (the most clarifying phase of high pressures), α-cristobalite, gypsum, anhydrite, corundum, rutile, amorphous calcite, aragonite, and calcite allowed us to know that LDGs could be subjected to shock pressures between 6 and more than 30 GPa, and temperatures between 300 and 1470 °C. The differences of temperature and pressure would be provoked by different cooling processes during the impact. Besides, in most cases the minerals corresponding to high pressure and temperatures were located in the inner part of the LDGs, with some exceptions that could be explained because they were trapped subsequently to the impact; there was more than one impact or heterogeneous cooling. Furthermore, nitrogen and oxygen gases were identified inside bubbles, which could have been introduced from the terrestrial atmosphere during the meteorite impact. These data helped us to clarify some clues about the origin of these enigmatic samples.