Nobuyuki Aikawa

Formation process of calcium carbonate from highly supersaturated solution

Abstract The precipitation process of calcium carbonate from highly supersaturated solutions was observed in situ by mixing CaCl2 and Na2CO3 aqueous solutions at about 20°C under an optical microscope and an infrared microspectroscope. After an amorphous phase forms, spherulitic vaterite and calcite in a rhombohedral shape nucleate simultaneously but separately, and grow by forming the precipitate-free zone around them. This transformation is solvent-mediated, and the measurement of the growth rate suggests that the rate-controlling process is the diffusion of elements in the earlier stage of this process, which then changes to surface kinetics.

Temperature dependence of intersite distribution of Mg and Fe in olivine and the associated change of lattice parameters

The cation distribution of natural and heated ferromagnesian olivine with chemical composition, Fo67Fa33, from metagabbro was examined by X-ray diffraction. Heating and quenching experiments were made by a newly devised apparatus which enables us to obtain very fast quenching speed in comparison with the usual technique. The distribution constants, KD=(Fe+2/Mg)M1/(Fe+2/Mg)M2, of the natural samples were less than 1.07, and those of heat-treated samples were more than 1.15, indicating that cation ordering takes place with temperature. The distribution of Fe+2 and Mg is nearly random at low temperatures, whereas Fe+2 shows a slight but significant preference for a smaller M1 site at high temperatures. The change of the distribution constant was observed on specimens which were heated for a short period of time (6–1,060 s) and quenched within 10 ms. Thus the rate of the cation reordering reaction is a very fast process. The lattice parameters b and c decrease whereas a increases with the increase of distribution constant. The overall effect on unit cell volume is a decrease with the increasing distribution constant, suggesting the presence of significant pressure dependence of the cation distribution towards the ordering of Fe at M1 site in ferromagnesian olivine.

Pressure effect on the divalent cation distribution in nonideal solid solution of forsterite and fayalite

The divalent cation distribution in olivine (Mg, Fe)2SiO4 under high pressure and temperature was studied to clarify the detailed state of olivine in the mantle. Single crystal samples were heated for a sufficient period of time for the cations to migrate and quenched fast enough to preserve the equilibrated state under high pressures, and the crystal structure was determined with X-ray method. The pressure effect on the distribution coefficient KD[= (Fe/Mg)M1/(Fe/Mg)M2] was determined for the first time; dKD/dP≅0.02 GPa−1. A set of five thermodynamic parameters required to describe the regular solution model was determined from data concerning the pressure dependence and the known temperature and compositional effects. As a result we have shown how KD depends on pressure, temperature, and composition. The notable feature clarified is the very large contribution of nonideality in the olivine solid solution. The KD of olivine crystals in the mantle is predicted; KD increases to ∼ 2.2 at the depth of 400 km, in contrast to 0.9 ∼ 1.2 of natural samples available at the surface of the Earth.

Polarized infrared spectroscopic study of diffusion of water molecules along structure channels in beryl

Abstract Incorporation of water in anhydrous synthetic beryl was studied at 500-700 °C and 50-150 MPa of confining water pressure to measure the diffusion of water molecules along the channels in a cyclosilicate. A series of polarized IR spectra series were taken with E parallel to the channel direction, which is parallel to the c-axis, along a traverse parallel to this axis. Water concentration profiles were determined from absorbance of H2O peaks. The IR spectra showed that the dominant diffusing species is type I water molecule, whose H-H vector is parallel to the c-axis (sharp peak at 3700 cm−1). No pressure dependence on water diffusivity can be recognized under these experimental conditions. The Arrhenius relation gives the activation energy of 133 ± 12 kJ/mol, with a pre-exponential factor of 10−2.6 (cm2/s). Diffusion of water is much faster in the beryl channels than volume diffusion in other silicates, but the activation energy and diffusion coefficient values for beryl are similar to the corresponding values previously reported for grain boundary diffusion in quartz aggregates.

The orientation of the OH− dipole in an optically anisotropie crystal: An application to the OH− dipole in topaz

For the purpose of determining the orientation of the OH− dipole in an optically anisotropic crystal, distribution of polarized IR absorbance is formulated under Fourier transform microspectroscopy. The formulatd absorbance distribution suggests that the degree of pleochroism of absorbance depends on the angle between the orientation of the OH− dipole and the principal orientation of the optically anisotropic crystal. As its application, the general orientation of the OH− dipole in topaz is determined to be inclined 27.3° from the c-axis in (010).