Influence of grain boundary structure on dissolution controlled pressure solution and retarding effects of grain boundary healing

摘要

It is widely accepted that the structure of a grain boundary undergoing pressure solution can have a strong influence on the rates at which diffusive transport in the grain boundary occurs. However, the influence of grain boundary structure on internal grain boundary dissolution rates has received little attention, despite evidence that dissolution controlled pressure solution in quartz is slower than expected assuming dissolution kinetics appropriate for free surfaces. In this paper, three hypothetical steady state grain boundary structures are defined and the influence of these structures on dissolution controlled pressure solution rates in an elastic solid are considered by deriving simple models based on internal grain boundary mass and energy balances. It is found that average dissolution rates in a rough grain boundary (island-channel network) are slowed down by up to 13% compared to dissolution in a flat grain boundary containing a thin fluid film. This can only partly account for the discrepancy between models and experiments reported in the literature. In addition a model is derived providing a criterion or “yield stress” for pressure solution, below which the process is prevented by surface energy driven grain boundary healing (progressive reduction of the contact area filled by connected fluid). This “yield stress criterion” for pressure solution offers a further explanation for reduced rates or cessation of pressure solution at low effective stresses in nature and experiment. Using this criterion, limiting porosity depth curves are predicted for sandstones compacting by pressure solution, which show favorable agreement with porosity-depth data for quartz sandstones.