Occurrence, composition and growth of polyhedral serpentine

Occurrences, compositions and crystallization of polyhedral serpentine were investigated by SEM, AEM/TEM and mu-XANES analysis of samples from different ultramafic units. Polyhedral serpentines are identified in all of these contexts and form as an alteration product of orthopyroxene (enstatite) and as late veining events. They are always the last serpentine type to crystallize. Their formation requires a combination of three factors: 1) open space, 2) a relatively low temperature (T < 200-300 degrees C), and 3) the presence of trivalent cations (Al3+ in this study, >= 0.1 atoms per serpentine formula unit). mu-XANES data at the iron K-edge indicate that Fe is predominantly ferrous and octahedrally coordinated in our AI-rich samples. This microstructure therefore cannot be systematically used as a marker of oxidizing conditions. Textural and microstructural criteria suggest that polyhedral serpentine crystallizes via a gel precursor first reorganized into a poorly-crystallized proto-serpentine, in which onion nucleation takes place as nested, discontinuous sheets. Grains expand radially, inwards and outwards, by a layer-by-layer mechanism. Thick layers, made of tens of serpentine sheets, propagate laterally in the (001) plane and result in a pseudo-spherical onion-like morphology. By analogy with available clay synthesis experiments, the relatively low temperature conditions under which polyhedral serpentine form may favor a segregation of trivalent cations in the structure. This could create locally dioctahedral components in the structure that may explain the peculiar bending along < 010 > responsible for the faceted morphology of polyhedral serpentine.