TEM and XRD study of antigorite superstructures

Samples of antigorite exhibiting variety in superstructures, taken from low-grade to high-grade serpentinites from southwestern Japan, were studied by X-ray diffraction (XRD), selected-area electron diffraction (SAED), and high-resolution transmission electron microscopy (HRTEM). Indexed XRD patterns indicate an A cell parameter of the supercell in the range 35.4 - 53.6 Å, and the following dimensions of the subcell: 5.407 < a < 5.466, 9.238 < b < 9.262, 7.241 < c < 7.279 Å, 91.07 < β < 91.65○. SAED patterns show a wider range of A parameters, from 28 to 61 Å (5.0 < M < 11.4, M = A/a). Antigorite with a 40.0-42.5 Å supercell (M = 7.5) is the common structure in southwestern Japan. A new method for supercell measurement by powder XRD is proposed. M values show a linear relation with the XRD-derived parameter, Δ2θ: M = 14.26 - 2.41 Δ2θ. A sample of antigorite with A = 43 A (M = 8) belongs to space group Pm. Diffraction patterns (single-crystal XRD and SAED) and c-axis HRTEM images of antigorite having M = (2n + 1)/2 indicate that the true periodicity of the superstructure along the X direction is 2A, corresponding to two waves, and the space lattice is C-centered.