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3.2 g. High- pressure behaviour of lawsonite (T. Boffa-Ballaran and R.J. Angel)

Water has a significant effect on the physical and chemical properties of materials. Large amounts of water are carried into the mantle at subduction zones in hydrous minerals formed in oceanic crust. The stability of these minerals is a strong control over the processes that might occur during subduction. Results from multi-anvil experiments have revealed that lawsonite, CaAl2Si2O7(OH)2H2O, is stable up to ~ 12 GPa at 1240 K, and could therefore be a potential host for water in the basaltic part of subducting slabs to depths as great as 250 km. Hence, this mineral has recently been a focus of interest for petrologists and mineralogists, and much effort has been paid in determining the equation of state of lawsonite as a function of temperature and pressure. However, whilst the various measurements of thermal expansivity at ambient pressure are in good agreement, the values of the bulk modulus KT determined in different studies differ considerably. In particular, some results from in situ simultaneous high-pressure high-temperature diffraction measurements suggest that there is an anomalous temperature dependence of the bulk modulus. Unfortunately the various high-P, high-T measurements are hampered by poor precision and poor calibration of both temperature and pressure.

X-ray single-crystal diffraction using a diamond anvil cell (DAC) is an ideal tool for obtaining accurate measurement of lattice parameters at room temperature, and can therefore be used to obtain good constraints on the room-temperature value of K0. For this purpose, a series of X-ray single-crystal diffraction experiments have been undertaken at room temperature in order to determine the evolution of the lattice parameters of lawsonite as a function of pressure. Preliminary volume data collected at different pressures up to 6 GPa can be fitted using a Birch-Murnagham equation of state with parameters: V0 = 676.17(4) Å3, K0 = 122.1(9) GPa and K' = 5.4(4). A plot of the "normalised stress" defined as FE = P/[3fE (1+2fE)5/2] versus the finite strain fE = [(V0/V)2/3-1]/2 obtained from the P-V data is linear, confirming that the 3rd-order truncation of the EoS describes adequately the measured data.

It has also been shown by recent X-ray powder diffraction, Raman and IR experiments that a phase transition occurs on increasing pressure above ~ 9 GPa to a phase tentatively identified as having monoclinic symmetry. However, the space group cannot be unambiguously determined from the available data. The single-crystal X-ray diffraction experiments will therefore be extended to higher pressures in order to obtain a complete space-group determination of the high-pressure phase of lawsonite and its orientational relationship with the low-pressure polymorph.

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