Rocks and advanced ceramics

Abstract

We show that the structure and chemistry of grain boundaries in natural rocks are comparable to those in advanced ceramic materials. As a result of grain boundary migration, the Zener relationship (log d_ol/d_px against log f_px; where d_ol, d_px and f_px are the grain sizes of olivine and pyroxene, and the fraction of pyroxene, respectively) measured in natural mylonite is remarkably similar to that observed in synthetic fine-grained aggregates of olivine and pyroxene. Superplasticity of the fine-grained synthetic mineral aggregate was demonstrated with observations of the same phase aggregation, dynamic grain growth and development of crystallographic preferred orientation, all of which were consequences of grain boundary sliding. We identify such processes in the natural rocks. We infer that similarities between natural and synthetic rocks are a result of the common behavior of grain boundaries in both materials. We conclude that synthetic fine-grained mineral aggregates are good analogues for polycrystalline rocks, and should be used to investigate grain boundary processes in the interior of the Earth.