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Occurrences, compositions and crystallization of polyhedral serpentine were investigated by SEM, AEM/TEM and μ-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 °C), and 3) the presence of trivalent cations (Al³⁺ in this study, ⩾ 0.1 atoms per serpentine formula unit). μ-XANES data at the iron K-edge indicate that Fe is predominantly ferrous and octahedrally coordinated in our Al-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 …