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MICROSTRUCTURES OF COMMON POLYGONAL SERPENTINES FROM AXIAL HRTEM IMAGING, ELECTRON DIFFRACTION, AND LATTICE-SIMULATION DATA^¶

Alain Baronnet^1, and Bertrand Devouard^2,

¹ CRMCN–CNRS^,*, Campus Luminy, Case 913, F–13288 Marseille Cedex 09, Franc
² Département des Sciences de la Terre, UMR 6524 OPGC – CNRS – niversité Blaise Pascal, 5, rue Kessler, F–63038 Clermont-Ferrand Cedex, France

E-mail addresses: baronnet{at}crmcn.univ-mrs.fr, devouard{at}opgc.univ-bpclermont.fr

Imaging with high-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), and numerical simulations of the diffraction patterns are combined to further document the microstructures of the most widespread types of 15-sector and 30-sector polygonal serpentines. Imaging, diffraction and simulation data recorded along the fiber axis are reconciled with a model involving lateral continuity of the 1:1 layer structure and a curvature without inversion of the sheets of tetrahedra across the sector boundaries. Axial fivefold symmetry is confirmed for polygonal serpentines. This model differs from sharp-boundary models proposed by Chisholm (1991, 1992), without inversion, and by Dodony (1993, 1997a), with inversion. Streaked diffraction chords connecting Bragg reflections are typical of all polygonal fibrils. They are due to curved layers with constant curvature at sector boundaries, involving extra spacing and locally distorted H-bonds between layers. This is a peculiarity of polygonal serpentines, not present in other varieties of rolled serpentine.

Keywords: serpentine-group minerals, polygonal serpentine, microstructure, HRTEM, electron diffraction, dislocation, fast Fourier transform.

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