The alpha -quartz trigonal symmetry revealed by fracture tests

Subcritical crack growth experiments on quartz single-crystals show the prominent role played by water in controlling the crack velocity. Crack propagation tests run in two opposite directions show striking differences, confirming previous results; these differences are mainly due to the anisotropy of the quartz surface energy. Microstructural observations of the fracture surfaces for two opposite crack propagation directions give evidence for this anisotropy which is the consequence of the alpha -quartz trigonal symmetry. The results confirm the subtle difference in adjacent m directions, as already shown by electron chanelling patterns.