Time resolved X-ray diffraction experiments have been performed in order to investigate the kinetics of the α-quartz-coesite transformation of SiO₂ at around 6 GPa and 700–900°C, using the “MAX80” cubic anvil type high pressure apparatus with synchrotron radiation. The correlation between a volume fraction of transformed coesite and relative intensities of reflections from α-quartz and coesite was experimentally determined. On the basis of the correlation, the volume fraction transformed coesite was estimated as a function of time. The Avrami rate equation was used for the determination of the kinetics. The value of the exponent n is determined as almost 1 and this result indicates that the grain growth of coesite is dominant mechanism of the transformation. The empirical activation energy, which is derived from the temperature dependency of the determined rate constant k, was estimated as 200 (±40) kJ/mol at the pressure overstep of ca. 3.5 GPa from the α-quartz-coesite equilibrium conditions. This value is similar to a reported activation energy of oxygen diffusion in the quartz structure. This consistency suggests that the oxygen diffusion is able to control the growth rate of the coesite phase at the present experimental conditions.

抄録全体を表示

Cleaved surfaces in biotite mica have been investigated using high-resolution TEM with the beam direction parallel to the cleaved surfaces to reveal the location of some potassium ions. Numerous (001) cleavages were formed along thin edges of the crystals, probably due to tensile strain induced by ion milling sample preparation. The cleavages are formed at interlayer positions in the biotite structure and opposing cleavage surfaces have distinct contrasts in atomic-resolution electron micrograph. In part, the contrast difference is explained by multislice image simulation calculated for the model where a certain amount (about half a monolayer in the maximum) of potassium ions are located on one side of the cleavage and few potassium ions are located on the other side. Some cleavages are healed by beam radiation, leaving no trace in the structure image of the recovered interlayer, indicating diffusion of potassium into the interlayer from the specimen surfaces.

抄録全体を表示

Crystals of ferrian aluminian diopside were synthesized and the structures were investigated by means of a single-crystal X-ray diffraction method in detail. EPMA gave the chemical formula of Ca_1.00(Mg_0.55Fe³⁺_0.37Al_0.08)(Si_1.55Al_0.45)O₆ for the crystals as an average. Structure refinement indicated that about 9 % of Fe³⁺ ions are at T sites, giving the formula of Ca_1.00(Mg_0.55Fe³⁺_0.34Al_0.11)(Si_1.55Al_0.42Fe³⁺_0.03)O₆. Existence of Fe³⁺ at T sites was, however, not confirmed on a difference Fourier map synthesized by making use of anomalous dispersion effect at the Fe K absorption edge. The bond distances are reasonable for a ferrian aluminian diopside with a chemical composition found for the present crystals.

抄録全体を表示

1M is the simplest and one of the most common mica polytypes. Because of the special cell dimensions, twinning by reticular pseudo-merohedry is frequently observed. The possible orientations of the individuals in the twin are approximately the same as the possible orientations of layers in polytypes. Because of the presence of a pseudo-hexagonal twin lattice, five pairs of twin laws are possible and they give rise to five pairs of corresponding twins. Thirty-one composite twins can be enumerated, which however give rise to ten different diffraction patterns. In the twin lattice there are nine geometrically independent rows parallel to c^*, defining a minimal rhombus in reciprocal space, which allows us to identify the ten independent patterns. A larger unit of translation, called tessellation rhombus, can be built on it; by tessellation of this unit all the reciprocal space can be covered.

抄録全体を表示