TEM observation of dissociated dislocations with b = [010] in naturally deformed olivine

Naturally deformed olivine specimens from the mylonitized and recrystallized Uenzaru peridotite in the Hidaka metamorphic belt, northern Japan, have been examined by high-resolution transmission electron microscopy (HRTEM). The olivine grains have a high density (approximately 10¹² m^-2) of [100] dislocations, a moderate density (approximately 10¹¹ m^-2) of [001] dislocations and a low density (≤ 10¹⁰ m^-2) of [010] dislocations. High-resolution lattice-fringe images revealed that the [010] edge dislocations parallel to [100] are split into partial dislocations according to one of the following two dissociation reactions: [010] = 1 4[011] + 1 4[011] + 1 4[011] + 1 4[011], with stacking faults parallel to the (021), (010) and (021) planes; [010] = 1 4[011] + 1 4[011] + 1 2[011], with stacking faults parallel to (021) and (021). The separation between partials ranges from 5 to 10 nm. Displacement vectors of the type 1 4〈011〉 preserve the hexagonal close packing of oxygens in the olivine structure, and stacking fault planes of the type {021} and (010) do not cut the strong SiO bonds. The geometrical configurations of the partial dislocations indicate that they formed by climb dissociation. Based on the geometrical structures observed for the dissociation of [010] dislocations, a possible structure is proposed for dissociated [001] dislocations.