Photon Factory Activity Report 2017 #35 (2018)

BL-10A/2016G558

Pressure-induced phase transition of lawsonite into primitive lattice
1K. Okamoto1, S. Kakizawa2, T. Inoue3, T. Kuribayashi1 and T. Nagase4

Department of Earth Science, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
2
GRC, Ehime University, Matsuyama, 790-8577, Japan3
Department of Earth and Planetary Systems 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
A hydrous mineral lawsonite, CaAl2 [Si2O7] (OH)2 · H2O,
is thought to be one of the water carrier into the upper
mantle [1]. The space group of lawsonite under ambientcondition is Cmcm.
Two pressure-induced phase transitions are known for
lawsonite. The C-centered lattice of lawsonite turns into
primitive orthorhombic (space group: unknown) above ~4
GPa [2], and subsequently into primitive monoclinic(space group: P21/m) around ~10 GPa [2,3].
Lawsonite at low-temperature occurs two reversible
phase transitions at ~273 K and ~155 K. Previous singlecrystal diffraction experiments using X-ray/neutron andthe structural refinements revealed that the reduction of
the symmetry is caused by optimization of H-bonds
accompanied by rotation of H2O and OH groups [5, 6].
For the single-crystal X-ray diffraction studies of boththe phase transitions at 4 GPa and 273 K, 017 reflections
were observed with the highest intensity among thereflections who were absent for the C-centered lattice [2,6]. As long as the 4 GPa phase transition is second-order[2], the space group of the phase above 4 GPa is expected
to be the subgroup of Cmcm. As the intensities of thesereflections are weak, we conducted single-crystal X-ray
diffraction experiments synthesized samples withsynchrotron radiation.
2 Experiment
We synthesized the sample by using a 2,000-ton multianvil press in Geodynamics Research Center, Ehime
University. The starting material was the mixture ofCa(OH)2, Al(OH)3 and SiO2. The powder was compressed
up to 10 GPa and was heated at 1,000 and 950 ˚C. Eachtemperature was kept for an hour. The run productcontained several single-crystals of lawsonite.
X-ray diffraction experiments under high-pressurewere obtained on a fragment measuring 50 × 50 × 40 μm3.
The sample was mounted on a Merrill-Bassett type
diamond anvil cell with a 4:1 methanol-ethanol fluid
mixture and a fragment of a natural ruby. A stainless-steel
disk with a 200 μm hole was used as a gasket. Pressure
was calculated from the ruby fluorescence line shift.
Diffraction data were measured with the automatedfour-circle diffractometer installed in the beam line BL10A, Photon Factory, High Energy Accelerator Research
Organization, KEK. The wavelength of the beam (λ =0.70145 Å) was calibrated by the unit-cell volume of aNIST ruby standard crystal at room condition. Intensity
data was collected on the orthorhombic HP phase up to 2θ= 70˚. Structural refinements were carried out by using
the SHELXL97 [7] with WIN_GX software [8].
Moreover, ω-scans with 10 s/step were performed
under various pressure especially on 017 and 3014
reflections who were useful for the reduction of thecandidates for the space group and had relatively highintensities at the LT phase. To distinguish the reflections
from those of the multiple diffraction of the diamonds,
other scans were conducted with the offset angle of thegoniometer head (φ angle) changed.
3 Results and Discussion
The 017 and 3014 reflections were observed above 1.5
GPa instead of 4 GPa. Changing the φ-offset angle by 15˚
did not weaken both of the reflections. Eventually, theintensity data were collected at 2.3 GPa. Table 1 shows
the number of reflections which break the reflection
conditions of Cmcm. Among the subgroups of Cmcm
belonging to primitive orthorhombic, Pmcn and P21cn
satisfy the table. Thus, the space group of orthorhombic
HP phase is restricted to these two.
The structural refinement with Pmcn provided the R1value of 7.51 %, while the insufficient number of uniquereflections disturbs the refinement with P21cn.

References[1] S. Poli and M. W. Schmidt, Ann. Rev. Earth Planet.
Sci., 30, 207-25 (2002). [2] T. Boffa-Ballaran and R. J.
Angel, Eur. J. Mineral., 15, 241-246 (2003). [3] A. R.
Pawley and D. R. Allan, Mineral. Mag., 65(1), 41-58(2001). [5] E. Libowitzly and T. Armbruster Am. Mineral.,80, 1277-1285 (1995). [6] Meyer et al., Am. Mineral., 86,566-577 (2001). [7] G. M. Sheldrick, and T. R. Schneider.,Methods Enzymol., 277, 319–343 (1997). [8] L. J.
Farrugia, J. Appl. Crystallogr., 32, 837-838 (1999).
* keitaro.okamoto.p3@dc.tohoku.ac.jp