Photon Factory Activity Report 2016 #34 (2017)

BL-10A/2016G558

High-pressure phase transition of lawsonite at ~4 GPa
K. Okamoto1, T. Kuribayashi1 and T. Nagase2
1Department of Earth Science, Graduate School of Science, Tohoku University, Sendai 980-8578
2The Tohoku University Museum, Tohoku University, Sendai 980-8578
1 Introduction
Lawsonite, CaAl2 [Si2O7] (OH)2 · H2O, contains ~11.45
wt.% H2O, and has space group Cmcm. The wide P-Tstability fields of lawsonite indicate the potential fortransporting water into a depth of more than 200 km [1].
Single-crystal X-ray diffraction experiments at highpressure have been carried out to find two pressureinduced phase transitions of lawsonite [2-4]. The Ccentered lattice of lawsonite is reduced to primitive
orthorhombic above ~4 GPa [2], and subsequently turns
into primitive monoclinic (P21/m) at ~10 GPa [3, 4].
In the meantime, single-crystal X-ray diffraction
experiments on lawsonite at low temperature revealed
two reversible phase transitions at ~273 K and ~155 K [5].
The driving force of the low-temperature phase transition
is rotation of the H2O molecules and the OH groups, andis associated with the lineation of hydrogen bonds [5, 6].
Configurations of the hydrogens at high pressure arestill unknown. Besides, the space group of lawsonite
between ~4 and ~10 GPa is not determined. The phasetransition near 4 GPa is associated with the shift of thehydrogen sites which provides little changes to the X-ray
diffraction data [2]. Therefore, we challenged singlecrystal X-ray diffraction experiments with synchrotron
source to confirm the phase transition around 4 GPa.
2 Experiment
For the present investigation, well-shaped crystals oflawsonite from Novato, California were used. The
average of 23 probe analyses (EDS) yielded the chemical
formula, Ca0.99 (Al1.92Fe0.05Ti0.01) [Si2.03O7] (OH)2·H2O,
which is close to the ideal formula of lawsonite.
X-ray diffraction experiments at ambient and highpressure condition were obtained on fragments measuring
160 × 160 × 140 μm3 and 100 × 60 × 30 μm3, respectively.
Diffraction data were collected with the automated fourcircle diffractometer installed in the beam line BL-10A(PF). The wavelength of the beam (λ = 0.71250 Å) wascalibrated by the unit-cell volume of a NIST rubystandard crystal at room condition.
At the high-pressure experiments, the sample wasmounted into a Merrill-Bassett type diamond anvil cellwith a 4:1 methanol-ethanol fluid mixture, and using a
stainless-steel gasket which has a 200 μm hole. Pressure
was calculated from the 3rd-order Birch-Murnaghan
Equation of State of lawsonite [2].
Intensities of the certain h + k = odd reflections (017 /015 / 234) were measured above 4 GPa to monitor thechange of the lattice type and to discuss the relation
between HP transition and LT transition. All structuralrefinements were carried out by using the SHELXL97 [7]with WIN_GX software [8].
3 Results and Discussion
The unit-cell parameters were determined at pressures
up to 4.7(2) GPa (Table 1). The compressibility of eachaxis is substantially comparable with the value on theprevious study referred to estimate the pressures [2].
At 4.0(1) and 4.7(2) GPa, weak 017 and 015 reflections
were observed. Additionally, other h + k = odd reflections
were detected in the dataset collected at 4.7(2) GPa. they
indicate the phase transition from C-lattice to P-lattice,unless these reflections are multiple diffractions. As 017
and 015 reflections were observed 4.0(1) GPa, diffraction
data between 3 and 4 GPa are required to determine theactual phase transition point.
As the local symmetries of the non-hydrogen atoms
were kept until the other phase transition near 10 GPa [4],the Wyckoff position of H would control the variation of
space group through the HP phase transition. Therefore,
determination of the space group just above 4 GPa
demands the configurations of the hydrogens. However,as long as the phase transition near 4 GPa is alsocharacterized by the spontaneous strain of the lattice [2],structural refinements excluding H would provide us
some information like discontinuous atomic shifts around
4 GPa. Especially, the shifts of oxygens with pressure
would throw light on the prediction of the H positions.
Table 1. Lattice constants of lawsonite at various pressures
Pressure
[GPa]
a [Å]
b [Å]
c [Å]
V [Å3]

RP

1.0(1)

4.0(1)

4.7(2)

5.8534(12)8.797(3)13.126(4)675.9(3)5.8370(11)8.767(3)13.097(9)670.1(5)5.7949(14)8.699(2)13.006(9)655.6(5)5.788(2)8.684(3)12.989(11)652.8(6)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] I.
Daniel et al., Eur. J. Mineral., 12, 721-733 (2000). [4] 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