Photon Factory Activity Report 2004 #22 Part B (2005)
High Pressure Science

13A/2003G187

Equation of state of CAS phase to 44 GPa at 300 K
Shigeaki ONO*1
1Institute for Research on Earth Evolution (IFREE), Japan Agency for Marine-Earth Science andTechnology (JAMSTEC), Yokosuka, Kanagawa 237-0061, Japan

Experimental methods
High-pressure X-ray diffraction experiments wereperformed using a laser-heated diamond anvil cell(LHDAC) high-pressure apparatus. The powdered CAS
phase, CaAl4Si2O11, was loaded into a 100 µm diameterhole that was drilled into a rhenium gasket. Platinum
powder was mixed with the sample to absorb the laser
radiation to provide a heat source, and was also used as
an internal pressure calibrant. Argon was used as apressure transmitting medium to reduce deviatric stressand temperature gradients in the sample. The samples
were heated with a YAG laser to overcome any potential
kinetic effects on possible phase transitions. The pressure
was determined from the observed unit cell volume ofplatinum using the Pt equation of state. The samples wereprobed using an angle-dispersive X-ray diffraction
technique at the synchrotron beam lines BL13A, Photon
Factory in Japan [1]. A monochromatic incident X-ray
beam with a wavelength of about 0.42 angstrom was used.
The X-ray beams were collimated to a diameter of 30 µm,
and the angle-dispersive X-ray diffraction patterns wereobtained on an imaging plate (Rigaku).
Results and Discussion
The powder X-ray diffraction data of the CAS phase atambient pressure revealed that it had a hexagonal barium
ferrite-type structure (P63/mmc), with unit celldimensions of a = 5.428(4) Å and c = 12.728(26)Å. The
volume and density of the CAS phase at ambient pressure
were 324.7(8)Å and 3.888(10) g/cm3. Our sample washeated using the laser after each pressure increment, and
the X-ray diffraction measurements were taken. The laser
heating annealed the sample, and reduced any pressure
gradient occurring across the sample, thus improving thequality of the X-ray diffraction data. The heating wasperformed at pressures > 15 GPa, which corresponds tothe stability field of the CAS phase, because weinvestigated the possibility that the CAS phase would

dissociate into separate phases under metastableconditions. The entire sample was carefully heated byscanning with the laser beam to eliminate any potential
reactions with either the argon pressure transmitting
medium or the platinum pressure calibrant. The effect ofpressure on the volume of the CAS phase is plotted in Fig.
1. The P-V data were fitted to the Birch-Murnaghan
equation of state to determine the elastic parameters:
P = 1.5K0(x-7-x-5)[1+0.75(K0’-4)(x-2-1)]
1/3
where x = (V/V0) ; and V0, K0, and K0’ are the volume,the isothermal bulk modulus, and the first pressure
derivative of the isothermal bulk modulus, respectively.
The isothermal bulk modulus and the first pressure
derivative of the isothermal bulk modulus were
determined to be K0 =229(9) GPa and K0’ = 4.7(7),respectively [2]. When the value of K0‘ was set to K0’ = 4,
then a value of K0 = 239(2) GPa was obtained. We did notobserve any phase transition occurring in the CAS phase
up to a pressure of 44 GPa, which was the maximum
pressure used in this study.
References[1] Ono et al., J. Appl. Phys. 97, 073523 (2005).
[2] Ono et al., Phys. Earth Planet. Inter. 150, 331 (2005).
* sono@jamstec.go.jp
10.98

300 K

0.96
0.94
V/V0Introduction
The calcium aluminosilicate (CAS) phase, CaAl4Si2O11,occurs in rocks formed from sediments at pressurescorresponding to those found in the upper mantle andtransition zone conditions. Previous studies reported that
this CAS phase has a hexagonal barium ferrite-typestructure. The physical properties of a given phase in thesubducted sediments may affect the dynamics of thesubduction and material circulation in the mantle and,
therefore, it is important to determine the compressibilityof the CAS phase.

0.92
0.9
0.88
0.86
0.84
0102030
Pressure (GPa)4050

Figure 1. Pressure-volume data for the CAS phase at 300
K. Dashed curve: third-order Birch-Murnaghan equation
fit with K0 and K0’ are 229 (GPa) and 4.7, respectively.

Users' Report205