X-ray diffraction study of magnesite at high pressure and high temperature |
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| Authors: | J Zhang I Martinez F Guyot P Gillet S K Saxena |
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| Institution: | (1) CHiPR and Department of Earth and Space Sciences, University of New York at Stony Brook, Stony Brook, NY 11794, USA, US;(2) Laboratoire de Geologie, Ecole Normal Superieure de Lyon, 46, allee d'Italie, 69346 Lyon cedex 07, France, FR;(3) Theoretical Geochemistry Program, Institute of Earth Sciences, Uppsala University, S-75236 Uppsala, Sweden, SE |
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| Abstract: | P–V–T measurements on magnesite MgCO3 have been carried out at high pressure and high temperature up to 8.6 GPa and 1285 K, using a DIA-type, cubic-anvil apparatus
(SAM-85) in conjunction with in situ synchrotron X-ray powder diffraction. Precise volumes are obtained by the use of data collected above 873 K on heating and
in the entire cooling cycle to minimize non-hydrostatic stress. From these data, the equation-of-state parameters are derived
from various approaches based on the Birch-Murnaghan equation of state and on the relevant thermodynamic relations. With K′0 fixed at 4, we obtain K0=103(1) GPa, α(K−1)=3.15(17)×10−5 +2.32(28)×10−8 T, (∂KT/∂T)P=−0.021(2) GPaK−1, (dα/∂P)T=−1.81×10−6 GPa−1K−1 and (∂KT/∂T)V= −0.007(1) GPaK−1; whereas the third-order Birch-Murnaghan equation of state with K′0 as an adjustable parameter yields the following values: K0=108(3) GPa, K′0=2.33(94), α(K−1)=3.08(16)×10−5+2.05(27) ×10−8 T, (∂KT/∂T)P=−0.017(1) GPaK−1, (dα/∂P)T= −1.41×10−6 GPa−1K−1 and (∂KT/∂T)V=−0.008(1) GPaK−1. Within the investigated P–T range, thermal pressure for magnesite increases linearly with temperature and is pressure (or
volume) dependent. The present measurements of room-temperature bulk modulus, of its pressure derivative, and of the extrapolated
zero-pressure volumes at high temperatures, are in agreement with previous single-crystal study and ultrasonic measurements,
whereas (∂KT/∂T)P, (∂α/∂P)T and (∂KT/∂T)V are determined for the first time in this compound. Using this new equation of state, thermodynamic calculations for the
reactions (1) magnesite=periclase+CO2 and (2) magnesite+enstatite=forsterite+CO2 are consistent with existing experimental phase equilibrium data.
Received September 28, 1995/Revised, accepted May 22, 1996 |
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