Keiji Shinoda

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.

Growth of large (1 mm) MgSiO3 perovskite single crystals: A thermal gradient method at ultrahigh pressure

Abstract Large single crystals of MgSiO3 perovskite were successfully synthesized by a thermal gradient method at 24 GPa and 1500 °C. This was achieved by an improvement of high-pressure synthesis technique that allowed us to grow single crystals under such ultrahigh-pressure conditions in relatively large volumes (e.g., 10 mm3). Since crystal growth is hindered by neighboring crystals, the nucleation density was suppressed by reducing the thermal gradient to 20 °C/mm, permitting an increase in free space for large crystal growth. KHCO3-Mg(OH)2 solvent can be used to grow perovskite crystals. However, the carbonate solvent produces melt inclusions. Silicate sources with MgSiO3 composition produce stishovite inclusions, which in turn cause splitting of perovskite crystals. The formation of these inclusions is avoided by using H2O as a solvent and 85MgSiO3-15Mg2SiO4 as a silicate source. The H2O also allows homogeneous crystal growth, probably because of its low viscosity and high silicate solubility. High-quality single crystals larger than 1 mm were successfully synthesized through these technical developments.

Polarized infrared absorbance spectra of an optically anisotropic crystal: Application to the orientation of the OH- dipole in quartz

The polarized infrared (IR) absorbance spectrum was formulated for an optically anisotropic crystal under the conditions of Fourier transform microspectroscopy. The adequacy of this formulation was confirmed by comparison of calculated interference fringes with those observed in polarized absorbance spectra of optically anisotropic crystal. In accordance with the formulated absorbance, an experimental constraint for the correct measurement of polarized IR absorbance spectrum of optically anisotropic crystal is proposed. Under this experimental constraint, polarized IR absorbance spectra of quartz from Arkansas were measured. They show that the orientation of OH- dipole which compensates the charge balance coupled with Al3+ in quartz is highly polarized perpendicular to the c-axis.

OH group behavior and pressure-induced amorphization of antigorite examined under high pressure and temperature using synchrotron infrared spectroscopy

Abstract Infrared (IR) absorption spectra of antigorite were measured up to 27 GPa and 320 °C using synchrotron IR radiation to elucidate OH group behavior under high-pressure (HP) and high-temperature (HT) conditions. The absorption bands attributable to the OH stretching modes of outer OH groups (OHouter) and inner OH groups (OHinner) show positive pressure dependencies. The shift rate of the OHinner band is almost constant at all pressure ranges. In contrast, that of the OHouter band increases slightly at about 6 GPa. This discontinuous change of the shift rate is consistent with the anomalous behavior of the OHouter upon compression, which was predicted in the previous first-principle calculation study. Specifically, the pressure dependence of the OHouter band shows that the hydrogen ion of an OHouter interacts not only with the nearest basal oxygen ion of the SiO4 tetrahedron but also with the second nearest two basal oxygen ions upon compression. The latter interaction becomes dominant over the former interaction at about 6 GPa. Pressure-induced amorphization was indicated from IR spectra measured at 300 °C and 25.6 GPa. This P-T condition is out of the thermodynamic stability field of antigorite. A broad absorption band, which is close to the broad band attributable to natural hydrous silicate glass, appeared after amorphization, which suggests that the pressure-induced amorphization of antigorite does not induce dehydration. Hydrogen atoms are retained in amorphized antigorite as OH groups.

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).

An induction heating diamond anvil cell for high pressure and temperature micro-Raman spectroscopic measurements

A new external heating configuration is presented for high-temperature diamond anvil cell instruments. The supporting rockers are thermally excited by induction from an externally mounted copper coil passing a 30 kHz alternating current. The inductive heating configuration therefore avoids the use of breakable wires, yet is capable of cell temperatures of 1100 K or higher. The diamond anvil cell has no resistive heaters, but uses a single-turn induction coil for elevating the temperature. The induction coil is placed near the diamonds and directly heats the tungsten carbide rockers that support the diamond. The temperature in the cell is determined from a temperature-power curve calibrated by the ratio between the intensities of the Stokes and anti-Stokes Raman lines of silicon. The high-pressure transformation of quartz to coesite is successfully observed by micro-Raman spectroscopy using this apparatus. The induction heating diamond anvil cell is thus a useful alternative to resistively heated diamond anvil cells.

Near-infrared spectra of H2O under high pressure and high temperature: Implications for a transition from proton tunneling to hopping states

The nature of protons in ice VII up to 368°C and 16GPa was investigated with synchrotron near-infrared spectroscopy. The absorption band of the first OH stretching overtone mode divided into doublet peaks above 5GPa at room temperature, suggesting that proton tunneling occurs at the overtone level. As the temperature increased, the doublet peaks gradually reduced to a singlet. This result implies that thermally activated protons hop between the two potential minima along the oxygen-oxygen axis. A pressure-temperature diagram for the proton state was constructed from the changing band shape of the overtone mode.

Minohlite, a new copper-zinc sulfate mineral from Minoh, Osaka, Japan

Abstract Minohlite, a new copper-zinc sulfate mineral related to schulenbergite, was found in the oxidized zone of the Hirao mine at Minoh (Minoo) City, Osaka Prefecture, Japan. The mineral occurs in cracks in altered shale as rosette aggregates up to 100 m m in diameter, composed of hexagonal platy crystals up to 50 μm m in diameter and 10μm m in thickness. The associated minerals are chamosite, muscovite, smithsonite, serpierite, ramsbeckite, ‘limonite’ and chalcopyrite. Minohlite has hexagonal (or trigonal) symmetry with unit-cell parameters of a = 8.2535(11), c = 8.1352(17) Å, V = 479.93(16) Å3 and Z = 1, and possible space groups P6, P6̅, P6/m, P622, P6mm, P6̅2m and P6/mmm (or P3, P3̅, P321, P3m1, P3̅m1, P312, P31m and P3̅1m). The six strongest reflections in the powder X-ray diffraction pattern [d in Å (I) hkl] are 8.138 (20) 001; 4.128 (24) 110; 2.702 (100) 120; 2.564 (76) 121; 1.560 (43) 140; and 1.532 (24) 141. Electron microprobe analyses gave the following values (wt.%): CuO 37.18, ZnO 21.08, FeO 0.49, SO3 16.78, SiO2 0.44, and H2O 24.03 (by difference). The empirical formula, calculated on the basis of Cu + Zn + Fe + S + Si = 9 atoms per formula unit, is (Cu4.43Zn2.45Fe0.06)Σ6.94[(SO4)1.99(SiO4)0.07]Σ2.06(OH)9.64·7.81H2O, which leads a simplified formula of (Cu,Zn)7(SO4)2(OH)10·8H2O where Cu > Zn. The mineral is bluish-green and transparent with a pearly to vitreous lustre. The streak is pale green. Cleavage is perfect on {001}. The Mohs hardness number is less than 2. The calculated density is 3.28 g cm−3. The mineral is named after Minoh City, where it was discovered.