Photon Factory Activity Report 2018 #36 (2019)

BL-10A/2018G118

Structural determination of the Pmcn lawsonite above 2 GPa
K. Okamoto1, S. Kakizawa2, T. Inoue3, T. Kuribayashi1 and T. Nagase4

Department of Earth Science, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
Geochemical Research Center, Graduate School of Science, The University of Tokyo, Tokyo, 113-0033,
Japan
3
Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University,
Higashi-Hiroshima 739-8526, Japan4
The Tohoku University Museum, Tohoku University, Sendai 980-8578, Japan1

21 IntroductionLawsonite, CaAl2 [Si2O7] (OH)2·H2O, can transport waterfrom subduction zone into upper mantle on the benefit of
its abundant water content (11.47 wt.%) and its efficientresistance to pressure [1].
Two high-pressure phases are known for lawsonite [2].
Whereas the monoclinic phase (P21/m) in the higherpressure range (>10 GPa) is well-understood by diffraction
studies [3], the primitive orthorhombic phase (Pmcn orP21cn) in the lower-pressure range remains to be elucidated[2,4]. A single-crystal X-ray diffraction study on
synthesized lawsonite reported lower phase boundary (~2GPa) [4] than the pressure of ~4 GPa obtained from theexperiments on a natural specimen [2].
The primal aim of this experiment is to determine thespace group of the P-orthorhombic phase by structuralrefinements. We also refined the structures of natural
sample above 2 GPa to discuss the vast gap of the phaseboundary between natural and synthesized lawsonites.
2 Experiment
Synthesized samples were obtained by using a 2,000-ton
multi-anvil press in Geodynamics Research Center, EhimeUniversity. The powdered mixture of Ca (OH)2, Al (OH)3and SiO2 was compressed up to 10 GPa and was kept
heated at 1,000 ˚C (1 h) and 950 ˚C (1 h). The run productcontained single-crystals of ~300 µm in diameter.
Lawsonite from Reed Ranch, California was used as anatural specimen. The chemical formula of the natural
sample is Ca1.00 (Al1.95Fe0.05) [Si2.00O7] (OH)2·H2O [5].
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 wasdetermined from the ruby fluorescence line shift.
Diffraction data were measured with an automated fourcircle diffractometer installed in the beam line BL-10A,
Photon Factory, High Energy Accelerator Research
Organization, KEK. The wavelength of synchrotron
radiation (λ = 0.7012 Å) was calibrated by the unit-cellvolume of a NIST ruby at room condition. Structuralrefinements except for H-sites were carried out by using
the SHELXL97 [6]. Neutral atom scattering factors andisotropic atomic displacement parameters were applied forthe structure model. The inconsistent equivalents andinconsistent reflections were omitted before the final
refinement. The conditions of the data collections and theresults of the refinements are listed in Table 1.

3 Results and Discussion
The structural refinement with Pmcn model provides
much better accordance with the diffraction data. The final
structure model shows that displacement along c-axis ofO1 (bridging two SiO4 tetrahedra) and O5 (constitutingH2O) are opposite. Since both oxygens do not coordinate
with Al-sites, the driving force of this phase transition may
be [001] shear motion between the AlO6 chains, which isdifferent from the shearing of AlO6 chain in the [100]direction occurring at the orthorhombic-to-monoclinic
phase transition [3]. Above 2 GPa, the natural sample also
underwent the same phase transition.

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]O'Bannon, E., Beavers, C. M., Kunz, M., & Williams,Q. (2017). J. Geophys. Res. Sol. Ea., 122(8), 6294-6305.

[4] K. Okamoto, S. Kakizawa, T. Inoue, T. Kuribayashi
and T. Nagase (2018). Photon Factory Activity Report, 35,
293. [5] J. M. Martín-Olalla, S. A. Hayward, H. W. Meyer,
S. Ramos, J. D. Cerro and M. A. Carpenter, Eur. J.
Mineral., 13(1), 5-14 (2003). [6] G. M. Sheldrick, and T.
R. Schneider., Methods Enzymol., 277, 319–343 (1997).
* keitaro.okamoto.p3@dc.tohoku.ac.jp