Photon Factory Activity Report 2014 #32(2015) B

BL-10A/2014G081

Single crystal diffraction study on Al-bearing phase D
1T. Kuribayashi1, *, T. Inoue2 and T. Nagase3
Department of Earth Science, Graduate School of Science, Tohoku University, Sendai 980-8578
2GRC, Ehime University, Matsuyama 790-8577
3
The Tohoku University Museum, Tohoku University, Sendai 980-8578

1. IntroductionDense hydrous magnesium silicate (DHMS) minerals
are important to consider the transportation of H into deep
Earth’s interior. DHMS phases are synthesized under coldslab conditions corresponding to the subduction zone.
Several alphabetical phases belong to DHMS such asphase A, B, D, E and H. Phase D has the ideal chemical
formula of MgSi2H2O6, but generally has nonstoichiometric one. Two reports on the first synthesis ofphase D was opened at the same time by two researchgroups [1, 2]. One of the group described the new phaseas phase G, because the name of “phase D” was alreadyused as another phase at that time.
Crystal structure of phase D _belongs to the trigonal
system with the space group P 31m (#162). One of theimportant properties of the structure is that Si occupies in
six-coordinated site in the structure like stishovite [1, 3].
Recently, phase D including a larger amount of Al and!
H was synthesized at the conditions of 25-26 GPa and~1600ºC coexisting with bridgmanite (MgSiO3perovskite) [4]. Therefore, Al in phase D structure is
expected to play important role in its thermodynamic
stability and H incorporation mechanism. However, thestructural information on Al-bearing phase D is poor, andthe incorporation mechanism of Al and H in phase D isunclear.
Single-crystal X-ray diffraction measurements on Albearing phase D was conducted to obtain its structural
information in detail. In this report, we showed the brief
results of X-ray diffraction experiments, and compared
the lattice parameters of our sample with the previously
reported values of the phase D structure.
2. Experimental Procedure
The sample used for this study was synthesized by [4]at 25-26 GPa and ~1600ºC. The run product included Albearing phase D, bridgemanite, stishovite and quencheddendritic phase, which was liquid under high pressure andtemperature conditions. The Al content in Al-bearing
phase D was briefly checked by [4] with EDS-SEM, andthe content was measured less than 18 wt%Al2O3. Afterchecking the quality of crystals by taking oscillation
photographs, a single crystal of Al-bearing phase D wasselected for synchrotron X-ray diffraction experiments.
Single crystal X-ray diffraction experiments wereperformed using the automated four-circle X-ray
diffractometer installed at the beam line BL-10A, Photon
Factory, High Energy Accelerator Research Organization.

The wavelength (λ = 0.7123 Å) of synchrotron radiation
was calibrated by the unit cell volume of the NIST rubystandard crystal at ambient temperature. The unit cellparameters of Al-bearing phase D at room temperature
were determined from 80 centered reflections in the 2θ
range between 39° and 69°. The X-ray diffraction
intensity data were collected up to sinθ/λ < 1.16 (2θmax =112º) by using ω-scan method.
3. Results and Discussion
The obtained lattice parameters of Al-bearing phase D
are as follows: a = 4.7987(15) Å and c = 4.3106(4) Å.
The lattice parameters of several phase D with minor
components such as Al were summarized in Table 1.
The length of a-axis of our Al-bearing phase D has thelargest value but the length of c-axis is the shortest except
for the Mg-Free Al-phase D. However, since the spacegroup of Mg-free Al-phase D is different from that ofphase D, we need to check these properties carefully.
The axial lengths of phase D structure depend on its
chemical formula, and especially, seem to be affected by
the size of octahedron and the octahedral connection.
These observations implied that the octahedral site is akey to understand the change of cell size and itsincorporation mechanism. Now, the crystal structure ofAl-bearing phase D is still in analysis to clarify Al
distributions in its structure and Al replacement
mechanism.
Table 1. Lattice parameters of phase D with various chemical formulae
Chemical Formula
Methodac

Reference

Al-bearing ph̲D

4.7987 (15)4.3106 (4)SCXRD

this study

Al1.54Si0.98H3.5O6

4.7114 (6)4.3039 (7)SCXRD

Pamato et al. (2015)

Mg1.11Si1.89H2.22O6
Mg1.23Si1.80H2.33O6
Mg1.11Si1.60H3.40O6
Mg1.15Si1.85H2.30O6
Mg1.12Si1.88D2.24O64.7453 (4)4.790 (3)4.7749 (6)4.7590 (6)4.7492 (4)4.3450 (5)4.344 (3)4.3389 (7)4.3403 (4)4.3520 (5)

SCXRD
SCXRD
PXRD
Neutron̲PD
Neutron̲PDYang et al. (1997)
Kudoh et al. (1997)
Frost and Fei (1999)
Suzuki et al. (2001)
Suzuki et al. (2001)

4. References[1] Yang et al., Am. Mineral., 82, 651-654 (1997) [2]
Ohtani et al., Geophys. Res. Lett., 24, 1047- 1050 (1997).
[3] Kudoh et al., Geophys. Res. Lett., 24, 1051- 1054
(1997). [4] Inoue et al., in prep. (2015). [5] Pamato etal., .Nat. GeoSci., (2015) DOI: 10.1038/NGEO2306. [6]Frost and Fei, Phys. Chem. Mineral., 26, 415-418 (1999).
[7] Suzuki et al., Geophys. Res. Lett., 28, 3987-399(2001).
* t-kuri@m.tohoku.ac.jp