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

13A/2003G187

New high-pressure phase of CaCO3
Shigeaki ONO*1
1
Institute 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 highpressure apparatus. Synthetic powdered CaCO3 wasloaded into a 100 µm diameter hole that was drilled into arhenium gasket. Platinum powder was mixed with thesample to absorb the laser radiation to provide a heatsource, and was also used as an internal pressure calibrant.
The samples were heated with a YAG laser to overcome
any potential kinetic effects on possible phase transitions.
The samples were probed 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 toa diameter of 30 µm, and the angle-dispersive X-ray
diffraction patterns were obtained on an imaging plate(Rigaku).
Results and Discussion
In the first set of experiments using the pressuretransmitting medium, the pressure was increased directly
to about 40 GPa at room temperature, and an X-ray
diffraction pattern of the sample was recorded. A strain–
broadening of the diffraction peaks occurred, because alarge differential stress was induced in the diamond anvil
cell experiments as pressure increased. After the desiredpressure was achieved, the sample was heated to about
1500-2000 K to relax the differential stress and toovercome potential kinetic effects on possible phasetransitions. After the heating, some new peaks appeared

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Users' Reportin the diffraction pattern. This implies that the starting
material transformed to a new high-pressure phase (postaragonite phase). The experimental pressures of this study
were from 11 to 86 GPa. The transition pressure fromaragonite to the post-aragonite phase was located at about
35 GPa and room temperature after the heating (Fig. 1).
The crystal structure of the post-aragonite phase is likely
to have an orthorhombic symmetry. The number ofmolecules of this phase in a unit cell (Z) is 2. The lattice
parameters at 66.4 GPa and 300 K, for example, are a =4.101(12) Å, b = 4.561(4) Å and c = 3.964(3) Å, with aunit cell volume of 74.1(2) Å3 for the orthorhombic cell[2]. Recently, the phase transition of SrCO3, which seems
to be an analog material of CaCO3, from aragonite-type toorthorhombic structure (P21212) was reported at 10 GPa[3]. The phase transition sequence of CaCO3 observed inthis study agrees with that of SrCO3. As seen in Fig. 1, adiscontinuity in volume between aragonite and the postaragonite phase indicates that this is a first-order phasetransformation. At 35 GPa, the relative volume change isabout 5%.
References[1] Ono et al., J. Appl. Phys. 97, 073523 (2005).
[2] Ono et al., Am. Mineral. 90, 667 (2005).
[3] Ono et al., Phys. Chem. Mineral. 32, 8 (2005).
* sono@jamstec.go.jp
40

CalciteMolar volume (cm 3/mole)

Introduction
Calcite is the dominant carbon-bearing phase in theEarth’s crust, and acts as a buffer for the long-term
cycling of CO2 between the atmosphere, oceans, and solid
Earth. It is unsurprising, therefore, that the high-pressurestability and behaviour of CaCO3 and related phases has
attracted considerable interest. In order to understandcarbon recycling in the mantle, there have been a numberof investigations into the pressure response of thestructure of CaCO3 polymorphs. It is known that calcite
transforms to aragonite, which often occurs in highpressure metamorphic rocks, at high P-T that correspondto the lower crust and the uppermost upper mantle.
However, it is unknown whether aragonite transforms to a
new high-pressure phase at the deep mantle. In this study,
therefore, the high-pressure stability limit of aragonite
was investigated using a laser-heated diamond anvil cellcombined with a synchrotron X-ray diffraction method.

7.4% volume change

35Aragonite

Post-aragonite

30~5 % volume change
25

20
020

40

60

80

Pressure

Figure 1. Pressure-volume data for the CaCO3 phases at
300 K. Dashed curves are Birch-Murnaghan equation fits.
Blue, aragonite; red, post-aragonite.