Hitoshi Momoi

Preferred orientation of antiphase boundaries in pigeonite as a cooling ratemeter

Antiphase domains (APDs) of pigeonite lamellae in natural and heated augite crystals from the Hakonetoge andesite have been examined by a transmission electron microscope (TEM). Antiphase boundaries (APBs) of the pigeonite lamellae in natural specimens have a sigmoidal shape cutting the c axis in (010) sections. APBs in specimens heated at temperatures above the high-low inversion and then quenched are nearly parallel to the c axis with almost straight boundaries. These observations indicate that the preferred orientation of APBs in (010) sections depends on cooling rate; at fast colling rates the APBs are nearly parallel to the c axis, whereas at slower cooling rates they are inclined to the c axis.The cooling rate of the natural augite specimen from Hakonetoge is estimated to be about 0.01 °C/h from the experimentally determined time-temperature-transformation (TTT) diagram for the APB orientations.APD sizes are large in specimens quenched from above the inversion temperature; they are at a minimum after cooling rates of around 1–0.1 °C/h, and then become larger with slower cooling rates.

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

Abstract 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 [01 1 ] + 1 4 [01 1 ] + 1 4 [011] + 1 4 [011] , with stacking faults parallel to the (02 1 ), (010) and (021) planes; [010] = 1 4 [01 1 ] + 1 4 [01 1 ] + 1 2 [011] , with stacking faults parallel to (02 1 ) 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 SiO 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.