Shocked quartz in Sahara fulgurite

Comparative strain analyses were performed on quartz crystals within a Sahara fulgurite using neutron diffraction experiments. Besides, 3D-mu XRCT (three-dimensional micro X-ray computed tomography) studies of the fulgurite were implemented in order to quantify the fusion behaviour, the expansion of the fulgurite as well as the distribution of the stressed quartz crystals. The study of the neutron diffraction pattern was carried out by applying an integral breadth method, more precisely a Rietveld and a Le Bail strain-size evaluation with the software FullProf. The absorption contrast of the neutron diffraction pattern shows a concentration of the shocked quartz crystals in the outer shell of the Sahara fulgurite, while the inner part of the fulgurite is mostly glassy. On the basis of the diffraction results, the quartz crystals in the fulgurite can be assigned to a stage of deformation for which the causal shock pressure reaches about 25 GPa, as compared to a melting threshold pressure for quartz of about 50 GPa. Comparing the average density of the fulgurite with the average bulk density of quartz-based sand, it can be concluded that the fulgurite expanded by more than 35 % during its formation.