THE SOLUBILITY OF [4H](SI) DEFECTS IN ALPHA-QUARTZ AND THEIR ROLE IN THE FORMATION OF MOLECULAR WATER AND RELATED WEAKENING ON HEATING

The nature of the solubility of water as [4H](si) defects in quartz, a nd their role in providing a source of molecular water on heating, is investigated. Existing nb inito energy calculations on the incorporati on of water in quartz are used to show that energetically 4H for Si su bstitution is likely to constitute the most prevalent mode of water up take on the atomic scale in quartz under equilibrium conditions, and t hat the planar defects previously observed by a number of different au thors by electron microscopy in wet quartz are likely to be planar raf ts of aggregated [4H](si) defects which are formed on supersaturation. These new conclusions call into question the previous identification of the planar defects as high pressure water clusters and require that their role in the production of molecular water in the context of rec ent theories of hydrolytic weakening be re-assessed. Accordingly the e xisting ab initio results have been used to establish the characterist ics of the phase diagram for the system quartz-water in the temperatur e and pressure range of interest in hydrolytic weakening. Additional e lectron-optical experiments on wet quartz show that, on annealing at t emperature in the electron microscope, similar planar defects develop in wet quartz by a diffusion process. In the context of existing theor ies of hydrolytic weakening it is now proposed that the conversion of [4H](si) defects to molecular water, where this is dictated by the equ ilibrium phase diagram, leads to a relatively large increase in volume and to the appearance of the bubbles of free water and the nucleation of associated prismatic dislocation loops of Burgers vector b = 1/3 a <11 (2) over bar 0> as previously observed. Ultimately the development of these loops leads to dislocation-induced plasticity.