Phys Chem Minerals (1997) 24:122–130 C Springer-Verlag 1997 ORIGINAL PAPER J. Zhang  I. Martinez  F. Guyot  P. Gillet S.K. Saxena X-ray diffraction study of magnesite at high pressure and high temperature Received September 28, 1995Revised, accepted May 22, 1996 Abstract P – V – T measurements on magnesite MgCO 3 have been carried out at high pressure and high tempera- ture 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 K 0 =103 (1) GPa, a (K -1 )=3.15 (17)10 -5 +2.32 (28)10 -8 T, (d K T d T) P =-0.021 (2) GPa K -1 , (dad P) T =-1.8110 -6 GPa -1 K -1 and (d K T d T) V = -0.007 (1) GPa K -1 ; whereas the third-order Birch- Murnaghan equation of state with K' 0 as an adjustable parameter yields the following values: K 0 =108 (3) GPa, K' 0 =2.33 (94), a (K -1 )=3.08 (16)10 -5 +2.05 (27) 10 -8 T, (d K T d T) P =-0.017 (1) GPa K -1 , (dad P) T = -1.4110 -6 GPa -1 K -1 and (d K T d T) V =-0.008 (1) GPa K -1 . Within the investigated P–T range, thermal pressure for magnesite increases linearly with tempera- ture and is pressure (or volume) dependent. The present measurements of room-temperature bulk modulus, of its pressure derivative, and of the extrapolated zero-pres- sure volumes at high temperatures, are in agreement with previous single-crystal study and ultrasonic measure- ments, whereas (d K T d T) P ,(d ad P) T and (d K T d 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+CO 2 and (2) magnesite+enstatite=forsterite+CO 2 are consistent with existing experimental phase equilibrium data. Introduction It has been recognized that magnesite might be a major reservoir for carbon in the Earth’s mantle. A number of experimental studies have demonstrated that in peri- dotites, carbonates and kimberlites, the carbonate in equilibrium with silicates changed from calcite at low pressure to dolomite and magnesite with increasing pres- sure (e.g., Eggler et al. 1976; Wyllie and Huang 1976). Above 3.5 GPa, magnesite was found to be the only sta- ble carbonate in peridotite (Brey et al. 1983). These ear- lier observations have been recently confirmed by Kat- sura et al. (1991), who showed that magnesite by itself did not breakdown up to 2000 K at 50 GPa, and by Biell- man et al. (1993), who observed that magnesite re- mained stable in the presence of majorite garnet at 20 GPa and (Mg, Fe)SiO 3 perovskite and magnesio- wustite at 50 GPa in the temperature range 1500– 2500 K. Magnesite was also found as an inclusion along with forsterite in a natural diamond from kimberlite (Wang et al. 1994). Knowledge of the stability of magnesite at conditions of the Earth’s mantle is thus important in understanding the carbon cycle on a global sale. In the simplest way, this is determined by the decarbonation reaction: magne- site=periclase+CO 2 . Since precise experimental phase equilibrium data on this reaction at the pressures and temperatures of the lower mantle are missing, thermody- namic calculations would in this regard provide impor- tant information. For this approach to be successful, it is J. Zhang ( ✉ )  I. Martinez 1  F. Guyot 2 CHiPR and Department of Earth and Space Sciences, University of New York at Stony Brook, Stony Brook, NY 11794, USA P. Gillet Laboratoire de Geologie, Ecole Normal Superieure de Lyon, 46, allee d’Italie, 69346 Lyon cedex 07, France S.K. Saxena Theoretical Geochemistry Program, Institute of Earth Sciences, Uppsala University, S-75236 Uppsala, Sweden Present addresses: 1 Laboratoire de Geologie, Ecole Normal Superieure de Lyon, 46, alle d’Italie, 69364 Lyon cedex 07, France 2 IPGP, LMCP, Case 115 Tour 16, 4, Place Jussieu, 75252 Paris Cedex 05, France