High-temperature and high-pressure equation of state for the hexagonal phase in the system NaAlSiO4 – MgAl2O4
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Thermal equation of state of an Al-rich phase with Na1.13Mg1.51Al4.47Si1.62O12 composition has been derived from in situ X-ray diffraction experiments using synchrotron radiation and a multianvil apparatus at pressures up to 24 GPa and temperatures up to 1,900 K. The Al-rich phase exhibited a hexagonal symmetry throughout the present pressure–temperature conditions and the refined unit-cell parameters at ambient condition were: a=8.729(1) Å, c=2.7695(5) Å, V 0=182.77(6) Å3 (Z=1; formula weight=420.78 g/mol), yielding the zero-pressure density ρ0=3.823(1) g/cm3 . A least-square fitting of the pressure-volume-temperature data based on Anderson’s pressure scale of gold (Anderson et al. in J Appl Phys 65:1534–543, 1989) to high-temperature Birch-Murnaghan equation of state yielded the isothermal bulk modulus K 0=176(2) GPa, its pressure derivative K 0 ′ =4.9(3), temperature derivative (∂K T /∂T) P =−0.030(3) GPa K−1 and thermal expansivity α(T)=3.36(6)×10−5+7.2(1.9)×10−9 T, while those values of K 0=181.7(4) GPa, (∂K T /∂T) P =−0.020(2) GPa K−1 and α(T)=3.28(7)×10−5+3.0(9)×10−9 T were obtained when K 0 ′ was assumed to be 4.0. The estimated bulk density of subducting MORB becomes denser with increasing depth as compared with earlier estimates (Ono et al. in Phys Chem Miner 29:527–531 2002; Vanpeteghem et al. in Phys Earth Planet Inter 138:223–230 2003; Guignot and Andrault in Phys Earth Planet Inter 143–44:107–128 2004), although the difference is insignificant (<0.6%) when the proportions of the hexagonal phase in the MORB compositions (∼20%) are taken into account.
KeywordsAl-rich phase Hexagonal phase In situ X-ray diffraction High-temperature equation of state MORB density
We are grateful to S. Ono, M. Matsui, F. Brunet and the reviewers for valuable comments on the manuscript. We also thank T. Kunimoto, A. Yamada and Y. Sueda for the help with synchrotron radiation experiments. The synchrotron radiation experiments were performed at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2003B0582-ND2b-np).
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