Vitreous silica formation within shear bands in a dynamically compacted granular crystalline α-quartz

Abstract

A shear band is a heterogeneous, narrow seam within a solid material whose formation is caused by intense localized shearing when a sufficiently large amount of deformation occurs. If that deformation occurs at a sufficiently rapid rate, with operative friction, then co-located ephemeral heating will occur in the shear band. In this study, shear bands were produced from dynamic shear-induced compaction of a granular form of crystalline α-quartz (SiO2). The produced shear bands were approximately 25-µm thick and were examined with scanning electron microscopy/electron backscatter diffraction, transmission electron microscopy, Raman spectroscopy, and nanoindentation. They were found to contain a mixture of vitreous silica and small-sized crystallites. This finding is significant because the presence of the vitreous silica within the shear band is a postmortem indicator that the localized temperature had reached or exceeded the melting temperature (∼1723°C) of crystalline SiO2 during the rapid shear and compaction and then sufficiently rapid cooling quenched in that vitreous state.