Reaction progress related to indentation structures at glaucophane/glaucophane contacts in an impure marble from Syros, Greece

Microstructures of brittle mineral grains embedded in a ductile matrix provide information on the influence of stress concentration on reaction progress during metamorphism. Under non‐hydrostatic conditions, contrasts of mechanical properties between minerals possibly cause stress concentrations at specific grain boundaries, which may result in a localization of subsequent reactions. In an impure marble from Syros, Greece, glaucophane grains occurring in a calcite matrix commonly impinge on other glaucophane grains to form concavo‐convex boundary contacts. The geometrical relationship between grain boundary and growth zoning within glaucophane grains indicates that indentation and dissolution occurred preferentially at the glaucophane/glaucophane contacts in response to compression normal to the foliation. In contrast, idioblastic surfaces of glaucophane were preserved at glaucophane/calcite boundaries, indicating that stress concentrated at glaucophane/glaucophane contacts. The retrograde minerals (winchite, albite and biotite) that were formed by consuming glaucophane are observed not only at the extensive site, but also at the ‘compressive’ site where indentation occurred. The precipitation of retrograde minerals at the extensive sites was accompanied by glaucophane dissolution at the indentation stage. In contrast, the formation of retrograde minerals at the ‘compressive’ site indicates that retrograde replacement preferentially proceeded at the indented sites even after stress relaxation. Such a localization of the replacement suggests that the formation of damaged zones around glaucophane/glaucophane contacts induced by stress concentration would have provided the preferential sites for glaucophane breakdown at the later stage.