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This study describes the microstructural and chemical development of symplectites, obtained in fluid-mediated mineral replacement experiments. During the experiments polymineralic feldspar-rich samples were exposed to aqueous Na-SiO2 solution at 600 °C and 2 kbar confining pressures for durations of 12 h to 20 days. The resulting reaction rims display high mineralogical and structural complexity and contain two varieties of symplectites, represented by nanometre-scale intergrowths of gehlenite-zeolite and grossular-zeolite grains. The experimental fluid was enriched in 18O isotope in order to trace oxygen redistribution during the reaction. The elevated 18O concentration in the reaction products and the heterogeneity in its distribution suggest that symplectite formation was controlled by dissolution-precipitation mechanisms rather than volume-diffusion processes. Microstructural and chemical observations suggest that symplectite formation occurred in multiple stages in response to spatially heterogeneous and temporarily evolving fluid composition at the reaction interfaces. Hence, our results shed light on the fundamental processes involved in symplectite formation improving our ability to interpret symplectite microstructures. [ABSTRACT FROM AUTHOR]

Deformation microstructures are widely used for reconstructing tectono-metamorphic events recorded in rocks. In crustal settings deformation is often accompanied and/or succeeded by fluid infiltration and dissolution-precipitation reactions. However, the microstructural consequences of dissolution-precipitation in minerals have not been investigated experimentally. Here we conducted experiments where KBr crystals were reacted with a saturated KCl-H 2 O fluid. The results show that reaction products, formed in the absence of deformation, inherit the general crystallographic orientation from their parents, but also display a development of new microstructures that are typical in deformed minerals, such as apparent bending of crystal lattices and new subgrain domains, separated by low-angle and, in some cases, high-angle boundaries. Our work suggests that fluid-mediated dissolution-precipitation reactions can lead to a development of potentially misleading microstructures. We propose a set of criteria that may help in distinguishing such microstructures from the ones that are created by crystal-plastic deformation., Competing Interests: The authors declare no competing financial interests.