Lattice Defects and Mechanical Behaviour of Quartz SiO$_2$

Quartz has the chemical composition SiO$_2$. It is an abundant mineral in the Earth crust and its mechanical properties govern the rheological behaviour of a number of crustal rocks. Quartz is also a material widely used in industry (resonators, filters ...) owing to its remarkable piezoelectric properties. Very small amounts of water in the quartz lattice dramatically affect its mechanical properties. The mechanical strength of “wet" quartz is at least 10 times lower than that of “dry" quartz. This effect of water on quartz has been called hydrolytic weakening. The piezoelectric performances of resonators are also dramatically reduced by the presence of amounts of water as small as $\rm [H]/[Si] \approx 300$ at. ppm. In order to get better performances, crystal growers have investigated the grown-in lattice defects (dislocations, twins, occurrence of water and other chemical impurities) and their influence on piezoelectric performances. The community of mineral physicists have also investigated the lattice defects in quartz, especially dislocations, and the physical factors influencing their mobility. They have also investigated the behaviour of quartz under extreme dynamic pressure; this situation corresponds to shock waves generated by the impact of meteorites on the Earth's surface. The aim of this article is to review the mechanical behaviour of quartz under various thermomechanical conditions and the role of the impurity “water” on the relevant physical properties of quartz