HIGH-RESOLUTION TRANSMISSION ELECTRON-MICROSCOPY OF POLYSOMATISM AND STACKING DEFECTS IN ANTIGORITE

High-resolution TEM imaging of antigorite requires extreme care to minimize beam damage by using low-dose procedures for the setup and recording of images. Interpretation of fine image detail is possible only if the crystal is well aligned along its [010] zone axis, has not suffered much beam damage, and is thinner than about 15 nm. The high resolution achieved demonstrates that the conspicuous (100) fringe, observed in experimental but not in simulated images, corresponds to the antigorite half-wave reversal containing the six-membered rings of tetrahedra and not the one containing the eight-membered rings, as proposed previously. Four types of microstructures occur in the antigorite studied: polysomatic variations in wavelength, produced by the addition or removal of lizardite modules; dislocations, where two antigorite waves merge into a single one, possibly with lizardite inserted where the wavelength becomes very long; a-glide twins, whose formation mechanism is probably due to attachment of a Mg cation to the wrong basal anion triad of an octahedron during nucleation of a new Mg octahedral sheet; and (100) offsets, where the wave reversals are shifted along +a or -a in steps of usually one, but occasionally more, lizardite modules, implying the occurrence of a single talc-like unit (+a shift) or a brucite-like unit (-a shift).