Takahiro Kuribayashi

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.

Neutron diffraction study of hydrous phase G: Hydrogen in the lower mantle hydrous silicate, phase G

A neutron powder diffraction study was performed to determine the sites occupied by hydrogen in the structure of DHMS phase G, which is stable at the pressure-temperature conditions of the transition zone and lower mantle. The diffraction data from small samples (about 3 mg) were collected by using a highly sensitive imaging plate detector at the BIX-3 beamline at JRR-3M nuclear plant in JAERI Tokai Laboratory, Japan. The present neutron diffraction data reveal that hydrogen (and deuterium) is located in the 6k site of Wykoff letter in the MO 6 layer of the phase G structure with the space group P31m. The bond lengths of O-H and O-D are 1.1(4) and 0.8(3) A, respectively. This result is consistent with previous Raman spectroscopic studies of this phase.

Triclinic liddicoatite and elbaite in growth sectors of tourmaline from Madagascar

Abstract Crystals of liddicoatite-elbaite tourmaline from a pegmatite in Jochy, Madagascar are composed of o{021̄1}, r{101̄1}, c{0001}, a{112̄̄0}, and m{101̄0} sectors, which correspond to the prominent crystal faces, respectively. Therefore, the sectors were produced during growth, not by strain after growth. The o, m, and r sectors of one specimen are biaxial between crossed polars [2V(-) = 30°, 20°, and 15°, respectively] and triclinic, as indicated by X-ray diffraction. The a sector is optically biaxial and orthorhombic. The c sector is optically uniaxial and essentially trigonal as indicated by single-crystal X-ray diffraction. The o, r, and c sectors are of liddicoatite component, whereas the a sector of the one specimen corresponds to fluor-elbaite. Another crystal specimen comprises a and m sectors, which are polysynthetically twinned, resulting in striations parallel to the c axis on the prism faces, and of liddicoatite. All five sectors have vacancies in the X-site (Ca, Na, ⃞ ).

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 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.

Synthesis of large and homogeneous single crystals of water-bearing minerals by slow cooling at deep-mantle pressures

Abstract The presence of water in the Earth’s deep mantle is an issue of increasing interest in the field of highpressure mineralogy. An important task for further advancing research in the field is to create homogeneous single crystals of candidate deep-mantle water-bearing minerals of 1 mm or larger in size, which is required for applying them for the time-of-flight (TOF) single-crystal Laue diffraction method with a third-generation neutron instrument. In this study, we perform several experiments to demonstrate an improved methodology for growing hydrous crystals of such large sizes at relevant transition zone and lower-mantle conditions via very slow cooling over a maximum period of 1 day. Successfully synthesized crystals using this methodology include dense hydrous magnesium silicate (DHMS) phase E, hydrous wadsleyite, hydrous ringwoodite, and bridgmanite (silicate perovskite). It is also demonstrated that these hydrous crystals can be grown from deuterium enriched starting materials in addition to those having a natural hydrogen isotope ratio. Magnitudes of chemical and crystallographic heterogeneities of the product crystals were characterized by comprehensive analysis of X‑ray precession photography, single-crystal X‑ray diffraction (SCXRD), field-emission scanning electron microscope (FE-SEM), electron probe microanalyzer (EPMA), secondary ion mass spectroscopy (SIMS), powder X‑ray diffraction (PXRD), and TOF neutron powder diffraction (TOF-NPD). The product crystals were confirmed to be inclusion free and crystallographically homogeneous. Compositional and isotopic differences of major elements and hydrogen isotope abundances were lower than 1 and 3%, respectively, among intracrystals and intercrystals within each recovered sample capsule. Phase E crystals up to 600 μm in the largest dimension were grown at a constant temperature of 1100 °C kept for 3 h. Using a lattice parameter-to-temperature relation of phase E, the thermal gradient in the sample capsules for the phase E synthesis has been evaluated to be 20 °C/mm. Hydrous wadsleyite crystals up to 1100 μm in the largest dimension were grown at 1390 °C with a temperature reduction of 70 °C during heating for 10 h. Hydrous ringwoodite crystals up to 1000 μm in the largest dimension were grown at around 1400 °C with a temperature reduction of 110 °C during heating for 12 h. Bridgmanite crystals up to 600 μm in the largest dimension were grown at 1700 °C with a temperature reduction of 30 °C during heating for 12 h. A TOF single-crystal diffraction instrument has been successfully used for analyzing one of the hydrous wadsleyite crystals, which demonstrated that single crystals appropriate for their expected usage are created using the method proposed in the present study.

Computational simulations of the structure of Japan twin boundaries in quartz

The structures of Japan twin boundaries in quartz are studied through molecular dynamics simulations and energy minimization calculations. Four types of twinning are grouped under the Japan twin law, comprising 10 subtypes of geometrical configurations based on the structural handedness and composition plane. For each subtype, the twin displacement vector is determined and the structures of the twin boundaries with {11 22} or {TT22} composition plane are simulated. It is shown that six subtypes composed of two crystals with same-handed structure have the same type of SiO 4 tetrahedral linkage at twin boundaries. The twin boundary structures of these six subtypes can be further divided into two enantiomorphic structures composed of right- or left-handed quartz. The other four subtypes are composed of combinations of right- and left-handed quartz, and are structurally distinct from the same-handed group. The twin boundary energies of the same-handed group are lower than for the opposite-handed group. The Si-O bond lengths in the region of the twin boundary differ by no more than 0.05 A from those in the bulk quartz for all subtypes. The differences between the Si-O-Si angles at the twin boundaries and in the bulk exceed 20°, whereas the differences in O-Si-O angles are less than 10°. The present calculations demonstrate that SiO 4 tetrahedra at Japan twin boundaries are very stiff in comparison with inter-tetrahedral forces, consistent with previous reports for silica polymorphs, and that structural matching of twin individuals is primarily achieved through the rotation of SiO 4 tetrahedra.

Crystal structures of the {011}, {610}, and {010} growth sectors in brewsterite

Abstract Crystal structures were determined by single-crystal X-ray methods for the {011}, {610}, and {010} growth sectors of brewsterite from Strontian, Scotland. Refinements Rw = 4.3-6.6%) showed that all three growth sectors are triclinic and that their structures differ slightly.

Determination of Al/Si order in sillimanite by high angular resolution electron channeling X-ray spectroscopy, and implications for determining peak temperatures of sillimanite

Abstract Sillimanite is a polymorph of Al2SiO5 that is widely used as an indicator of pressures and temperatures reached during metamorphism. The degree of disorder in the double chains of SiO4 and AlO4 tetrahedra in sillimanite, particularly at high temperatures, is of interest as a factor in the phase relations of the Al2SiO5 polymorphs. We determined the Al/Si order parameter (Q) of sillimanite from Rundvågshetta, Antarctica, by the high angular resolution electron channeling X-ray spectroscopy (HARECXS) method using transmission electron microscopy with energy-dispersive X-ray spectrometry. HARECXS profiles were successfully obtained from regions ~1 μm in diameter by automated control of beam tilting and X-ray detection. The obtained Q value was close to that previously estimated by single-crystal X-ray diffraction. Moreover, the Q values of annealed samples were obtained while avoiding interference from mullite or SiO2-rich glass domains formed by annealing. For quantitative determination of Q, we also performed theoretical calculations of HARECXS profiles and evaluated sample thicknesses by convergent-beam electron diffraction. The experimentally obtained profiles were successfully fitted by a linear combination of simulated profiles of completely ordered and completely disordered sillimanite, which yielded Q values. The Q values obtained from 18 measurements showed no effect from differing sample thicknesses. Moreover, the results from annealed samples showed that Q decreases continuously with increasing annealing temperature. The temperature dependence of Q values, formulated by least-squares fitting on the basis of the Bragg-Williams approximation, yielded a transition temperature from order to disorder at 1727 °C. The obtained curve is more accurate at high temperatures than previous estimates. It indicates that the sample material reached peak temperatures greater than ~1000 °C, which is close to previous estimates of the peak metamorphic temperature of Rundvågshetta sillimanite. This study also implies that the HARECXS method is suitable for accurate analyses of other natural samples with complicated microtextures.

Field Reaction Experiments of Carbonate Minerals in Spring Waters: Natural Analogue of Geologic CO2 Storage

To diminish the uncertainty of the mineral trapping rate during geologic CO2 storage, the growth rate of carbonate minerals was measured in CO2-containing spring waters, which can be regarded as a natural analogue of geologic CO2 storage. The authors’ approach, using nanoscale analysis of seed crystal surfaces after immersion into spring waters, enables rapid and accurate measurement of mineral reaction rates. The results show that calcite growth rates in spring waters were lower by 1–3 orders than the values given in a database of laboratory experiment results. We verified the traditional paradigm that Mg2+ controls carbonate reaction kinetics. An increase of the Mg/Ca ratio to around 5 by adding Mg2+ to spring waters markedly reduced the calcite growth rate. However, even if effects of Mg2+ and flow rate are considered, we were unable to explain satisfactorily the difference of the calcite growth rates between those of spring waters and laboratory experiments. Therefore, other factors might also be related to the slow growth rate in nature. The present results, including the fact such that neither dolomite nor magnesite was formed even at the high Mg/Ca ratio, are expected to provide an important constraint to overestimation of the mineral trapping rate.