  Plastic instabilities in amorphous materials are often studied using
idealized models of binary mixtures that do not capture accurately molecular
interactions and bonding present in real glasses. Here we study atomic scale
plastic instabilities in a three dimensional molecular dynamics model of silica
glass under quasi-static shear. We identify two distinct types of elementary
plastic events, one is a standard quasi-localized atomic rearrangement while
the second is a bond breaking event that is absent in simplified models of
fragile glass formers. Our results show that both plastic events can be
predicted by a drop of the lowest non-zero eigenvalue of the Hessian matrix
that vanishes at a critical strain. Remarkably, we find very high correlation
between the associated eigenvectors and the non-affine displacement fields
accompanying the bond breaking event, predicting the locus of structural
failure. Both eigenvectors and non-affine displacement fields display an
Eshelby-like quadrupolar structure for both failure modes, rearrangement or
bond-breaking. Our results thus clarify the nature of atomic scale plastic
instabilities in silica glasses providing useful information for the
development of mesoscale models of amorphous plasticity.
