Photon Factory Activity Report 2016 #34 (2017)

Structural Properties of Al-replaced Phase D

BL-10A/2015G503

T. Kuribayashi1,*, T. Inoue2, 3 and T. Nagase4
1

Department of Earth Science, Graduate School of Science, Tohoku University, Sendai 980-8578
2GRC, Ehime University, Matsuyama 790-8577, Japan3
Department of, Earth and Planetary System Science, Graduate School of Science, Hiroshima
University, Higashi-Hiroshima 739-8526, Japan
4
The Tohoku University Museum, Tohoku University, Sendai 980-8578, Japan

1. Introduction
Phase D, ideally MgSi2H2O6, is one of dense hydrous
magnesium silicate (DHMS) minerals. DHMS phases arestable under cold slab conditions, and are important tounderstand the transportation of H into deep Earth’sinterior. Two research groups first synthesized phase D at
the almost same time [1, 2]. Although one of the groups
described the new phase as “phase G” because the name“phase D” was already used for other phase, “phase D”
was adopted. Phase D is generally synthesized with nonstoichiometric chemical formula like phase E, which isother DHMS phase. Crystal structure of phase D has the_trigonal symmetry with the space group P 3 1m (#162).
One of the important structural properties of the phase isthat Si atom occupies in six-coordinated site like in thestishovite structure [1, 3]. Although the chemical formula
is non-stoichiometric, the crystal structure of the phase
does not show any modulated structures. Recently, phase
D including Al was synthesized at the conditions of 25-26
GPa and ~1600 ºC coexisting bridgmanite [4, 5]. Also,
Al-phase D, which is perfectly replaced Mg by Al, wasreported [5], but structural information remains unclear.
Although the behavior of Al in phase D structure is paid
attention to, the structural information on Al-containing
phase D is insufficient.
In this report, we conducted single-crystal X-ray
diffraction measurements to investigate Al-site in phase Dstructure, and to clarify the replacement mechanism in thestructure.
2. Experimental Procedure
The samples used for this study were synthesized by[4] at the conditions of 26 GPa and 1200 ºC for sample #1
and 25 GPa and 1600 ºC for sample #2, respectively. The
chemical compositions of the synthesized phases werebriefly checked by [4] with EDS-SEM. The chemicalformulae of the samples (#1 and #2) wereandMg0.91Si1.28Al0.73H2.85O6,
Mg0.99Si1.41Al0.47H3.00O6
respectively Hydrogen contents were estimated from total
deficit. After we checked the quality of crystals by taking
oscillation photos, single crystal of each run were selected.
Single-crystal X-ray diffraction experiments wereperformed using an automated four-circle X-ray
diffractometer installed at the beam line BL-10A, PF,

KEK. The wavelength (λ = 0.7012 Å for #1 and λ =0.7123 Å for #2) of synchrotron radiation in each run wascalibrated by the unit cell volume of the NIST standard
ruby at ambient temperature. The unit cell parameters of
the samples were determined from the centeredreflections in the 2θ range between 35º and 69º.
The X-ray diffraction intensity data were collected up
to 2θmax=116º for both experiments (sinθ/λ = 1.18 for #1
and sinθ/λ = 1.16 for #2) by using ω-scan method.
Structural refinement for #2 was conducted using
SHELX97 with WINGX software [6, 7]. The latest R andwR2 values are 2.04 %and 5.51 %, respectively.
3. Results and DiscussionThe lattice constants of each sample are determined asfollows: a = 4.7892(20) Å and c = 4.3145(9) Å for #1; a= 4.7987(15) Å and c = 4.3106(4) Å for #2. The a-axis
length is increasing with an increase of Al content. On theother hand, the a-axis length of a perfectly Al-replaced
phase D is 4.7114(6) Å [5]. This finding indicates theexistence of the local maximum of the a-axis length in theAl incorporation stage, and suggests that the Al
replacement mechanism into phase D structure should bechanged beyond the local maximum point.
As the results from structure refinement of the sample#2, the mean M-O distances of two octahedral sites are2.1026(6) Å for Mg-octahedral site and 1.8279(5) Å forSi-octahedral site. These averaged M-O distances showsthat the Al replacement mechanism of phase D structure
preferably occurs at Si-octahedral site (Si4+ ⇔ Al3+ + H+)
up to the local maximum point (half of Si is probably
replaced). Beyond the point, 3Mg2+ ⇔ 2Al3+ and/or2Mg2+ ⇔ Al3+ + H+ replacement would be predicted.
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, 10511054 (1997); [4] Inoue et al., in prep. (2016); [5] Pamatoet al. Nat. GeoSci., DOI:10.1038/NGEO/2306 (2015); [6]Sheldrick, G.M. and Schneider T.R. Methods in Enzy.,
277, 319-343, (1997); [7] Farrugia, L.J. J. Appl.
Crystallogra., 32, 837-838, (1999).
* t-kuri@m.tohoku.ac.jp