Yasuhiro Kudoh

Hydrous modified spinel, Mg1.75SiH0.5O4: A new water reservoir in the mantle transition region

The water content in modified spinel, synthesized at 15.5 GPa under hydrous conditions, has been measured by means of secondary ion mass spectrometry (SIMSrpar;. We found that the modified spinel crystals contain up to 3.1±0.4 wt % H2O, which is consistent with the amounts estimated from the deficits in the oxide totals of the microprobe analysis. X-ray diffraction analyses for a single crystal showed that the sample containing 2.5±0.3 wt % H2O is of the modified spinel structure with the lattice parameters a=5.663(1), b=11.546(2), c=8.247(4)A and V=539.2(5)A³. The present results suggest that a considerable amount of H2O may exist as hydrous modified spinel in the mantle transition zone, which could have important implications for the constitution and dynamics of the mantle.

A new hydrous silicate, a water reservoir, in the upper part of the lower mantle

We synthesized a new hydrous silicate in the pressure range from 20 GPa to 24 GPa at 800–1300°C. This phase, named tentatively as phase G, has a hexagonal unit cell, a=4.790 (3) A and c=4.344 (3) A, and V=86.3 (2) A³ and the atomic ratio Mg/Si=0.66±0.03. SIMS analysis revealed that it contains 14.5±2.0wt% water. Phase G has a chemical formula of Mg1.14Si1.73H2.81O6 and a density of 3.37g/cm³. Phase G coexists with periclase and Mg-perovskite under the lower mantle conditions, and thus it can be a reservoir of water in cold slabs penetrating into the lower mantle.

Structure and crystal chemistry of hydrous wadsleyite, Mg1.75SiH0.5O4: possible hydrous magnesium silicate in the mantle transition zone

The crystal structure of hydrous wadsleyite, Mg1.75SiH0.5O4 synthesized in an MA 8-type apparatus at conditions of 1300°C and 15.5 GPa, has been analyzed and refined in space group Imma, using the X-ray intensities measured on a 60X60X10 μm single crystal. The composition (Z=8) and unit cell are Mg1.74Si0.97H0.65O4 by E.P.M.A. analysis and a=5.663(1) Å, b= 11.546(2) Å, c=8.247(4) Å, V=539.2(5) Å3. The partial M-site occupancies were determined; vacancies associated with the incorporation of water are strongly concentrated on the Mg 3 site. The OH in the structure was confirmed by Raman and FTIR spectroscopies. The result of valence sum calculation based on the refined bond lengths indicates that O1 is a hydroxyl. The formula of hydrous wadsleyite can be expressed as Mg2-xSiH2xO4, where 0≤x≤0.25. When x=0.25, all of the O1 site is hydroxyl and the maximum solubility of 3.3 wt% H2O is realized. Structural relations to other dense hydrous phases are discussed.

Structure and crystal chemistry of Phase G, A new hydrous magnesium silicate synthesized at 22 GPa and 1050°C

A single crystal of phase G, Mg1.24Si1.76H2.48O6 synthesized at conditions of 1050°C and 22 GPa, using a multi-anvil apparatus was studied at the Photon Factory BL-10A beamline at the National Laboratory for High Energy Physics, Tukuba, Japan. With a (111) Si double-crystal monochromator and a single crystal measuring 47 × 35 × 12 µm, intensities of 95 independent reflections were collected with sin θ/λ 1.5 σ Io at a wave length of 0.6990 A. The unit cell parameters obtained through the refinement of 23 reflections are: Trigonal, a=4.790 (3) A, c=4.344 (3) A, V=86.3 (2) A³. The result of structure analysis in space group P 1m (No. 162) indicates that the structure of phase G has close structural similarity to that of stishovite,SiO2. The calculated density of phase G is 3.43 g/cm³, which is larger than any other known dense hydrous magnesium silicate, suggesting that phase G might be stable even under lower mantle conditions.

Redetermination of the high-pressure modification of AlOOH from single-crystal synchrotron data

The single-crystal synchrotron study of the high-pressure modification of aluminium oxide hydroxide, δ-AlOOH, confirms the previous structure determination in the space group P21nm, which was based on X-ray powder data [Suzuki, Ohtani & Kamada (2000). Phys. Chem. Miner. 27, 689–693]. The present study includes the determination of the H-atom parameters, which revealed a strong asymmetric hydrogen bond with an O⋯O distance of 2.5479 (12) A. The δ-AlOOH structure is isotypic with that of β-CrOOH and may be considered as a distorted rutile type with all atoms located on mirror planes.

Precise determination of elastic constants by high-resolution inelastic X-ray scattering.

Inelastic X-ray scattering (IXS) measurements have been performed on an MgO single crystal in order to evaluate IXS as a methodology for accurate and precise determination of elastic constants and sound velocities. By performing the IXS experiment using a 12-analyzer array, the complete set of single-crystal elastic constants of MgO were determined to a precision better than 0.8% (sound velocities to better than 0.2%). The results are consistent with values in the literature. The precision and accuracy of this work, which is significantly better than other published work to date, demonstrates the potential of IXS in determining elastic properties.

Crystal structure and compressibility of superhydrous phase B, Mg20Si6H8O36

Crystal structure of superhydrous phase B, Mg20Si6H8O36, synthesized at 17 GPa and 1000 °C using the uniaxial, split‐sphere, multi‐anvil apparatus, was analyzed by the single‐crystal X‐ray diffraction method. Compressibility of the unit cell was measured up to 6.5 GPa with single crystal X‐ray method using a diamond anvil high pressure cell and synchrotron radiation at the BL‐10 A beam line at the National Laboratory for High Energy Physics. The observed bulk modulus K0T=145 GPa is 12% larger than that of α‐Mg2SiO4 and smaller than that of β‐Mg2SiO4.

Pressure dependence of the OH-stretching mode in F-rich natural topaz and topaz-OH

Abstract OH stretching vibration modes for F-rich natural topaz (F-topaz) and for fully hydrated topaz (topaz-OH) synthesized at high pressure, were observed using IR and Raman spectroscopies at pressures up to 30.4 GPa and 17.3 GPa, respectively. In F-topaz, the pressure derivative of the frequency of the OH stretching band observed at 3650 cm-1 at ambient pressure was 0.91(3) cm-1/GPa, which was consistent with the value recently reported by Bradbury and Williams (2003). On the other hand, in topaz-OH, the pressure derivatives of the bands initially at 3599 and 3522 cm-1 were -5.2(2) and -2.56(6) cm-1/GPa, respectively. This contrasting behavior between the two forms of topaz at high pressures suggests that the OH substitution for F in topaz affects the hydrogen-bonding behavior under high pressure.

Crystal structures of high-pressure phases in the alumina-water system: I. Single crystal X-ray diffraction and molecular dynamics simulation of η-Al(OH)3

The high-pressure phase of gibbsite has been studied by in situ single crystal X-ray diffraction method and molecular dynamics (MD) simulation at 3.0 GPa. The crystal structure of the high-pressure phase, η-Al(OH)3, was successfully determined by direct methods based on the intensities of X-ray diffraction. The space group and lattice constants for η-Al(OH)3 are P21/b11 (#14), a = 8.612(3) Å, b = 5.013(2) Å, c = 9.194(5) Å and α = 90.34(6)°, respectively. The crystal structure of η-Al(OH)3 consists of an Al octahedral layer, and the layers are connected via H-bonds. The mechanism of the phase transition from gibbsite to η-Al(OH)3 is also discussed in the context of previously reported powder X-ray diffraction data.

Crystal structures of high-pressure phases in the alumina-water system: II. Powder X-ray diffraction study of a new dense aluminum deuteroxide, δ-Al(OD)3

In this study we report the solution of the structure of δ-Al(OD)3. The crystal structure was determined using powder X-ray diffraction and direct methods, while the positions of deuterium atoms were estimated by difference-Fourier methods and MEM/Rietveld analysis. In the A-site deficient hydroxy-perovskite-type structure of δ-Al(OD)3, we find two types of one-dimensional H-bonding network.