Dehydration reactions and micro/nanostructures in experimentally-deformed serpentinites

High-T torsion experiments on lizardite + chrysotile serpentinites produced mineralogical and micro/nanostructural changes, with important implications in rheological properties. High-resolution TEM showed that specimens underwent ductile [by microkinking and (001) interlayer glide] and brittle deformation (by microfracturing), together with dehydration and break-down reactions. Lizardite is affected by polytypic disorder and microkinking [kink axial planes at high angle with respect to (001) planes], that were not present in the initial ordered 1T-lizardite. Chrysotile fibres are deformed, resulting in elliptical cross-sections, with strong loss of interlayer cohesion. Both lizardite and chrysotile break down to a fine intergrowth of olivine (up to 200 nm), talc (up to 30 nm) and poorly-crystalline material. Lizardite-out reaction preferentially occurs at kink axial planes, representing sites of preferential strain and enhanced reactivity; conversely, chrysotile break-down is a bulk process, resulting in large healed olivine aggregates, up to micrometric in size. Overall observations suggest that dehydration and break-down reactions are more advanced in chrysotile than in lizardite.