American Mineralogist, Volume 87, pages 1261–1265, 2002 0003-004X/02/0809–1261$05.00 1261 INTRODUCTION There have been a considerable number of studies suggest- ing that magnesite should be considered when evaluating the role of carbon in the Earth’s mantle. Magnesite (MgCO 3 ) is indeed a very good candidate for hosting oxidized carbon in the mantle, since experiments or calculations indicate that mag- nesite is, among all carbonates, the most stable at high-pres- sure and high-temperature (e.g., Katsura et al. 1991; Katsura and Ito 1990; Biellmann et al. 1993; Gillet 1993; Martinez et al. 1998). Numerous experiments have been dedicated to de- termining the compression properties (i.e., bulk modulus and its pressure derivative) of magnesite, as these govern to some extent its stability versus decomposition into a mixture of the oxides MgO and CO 2 (Redfern et al. 1993; Fiquet et al. 1994; Zhang et al. 1997; Ross 1997; Fiquet and Reynard 1999). The structural refinement of Ross (1997), however, was only car- ried out to 8 GPa. Here, we extend the knowledge of this crys- tal structure over a larger pressure domain. The high-pressure behavior of magnesite (MgCO 3 ) has been studied by angle dis- persive X-ray diffraction up to 81 GPa. The combination of a bright focused monochromatic beam and a two dimensional detection system (imaging plates) allowed us to carry out a structural study of magnesite above 80 GPa. EXPERIMENTAL DETAILS Clear inclusion-free crystals from Bahia magnesite, similar to those used in Humbert and Plicque’s (1972) ultrasonic study as well as Fiquet and Reynard’s (1999) X-ray diffraction study, were chosen for this work. Samples were powdered in an agate mortar and placed in a diamond-anvil cell (Chervin et al. 1994) equipped with type Ia beveled diamonds with 100 m m inner diameter culets. No pressure transmitting medium was used to avoid any chemical reaction. Magnesite crystals were intimately mixed with powdered platinum (platinum black) used as inter- nal pressure calibrant as well as infrared absorber (Jamieson et al. 1982; Holmes et al. 1989). At each pressure increase, the sample was thoroughly annealed with a multi-mode infrared YAG (cw 280 W from Lee lasers) by focusing the laser beam into a 50 m m hot spot and scanning it across the sample for several minutes. The resulting local heating in the temperature range 2000–2500 K is enough to (1) release deviatoric stresses accumulated during the compression at ambient temperature and (2) promote any possible phase transition or decomposi- tion. This annealing is very important since it results in a dra- matic release of deviatoric stress, from a value of more than 2 GPa after cold compression to about 0.5 GPa after heating at 80 GPa. This is also evidenced by a factor of two reduction of the line widths during laser heating. The diamond-anvil cell was then mounted on the diffractometer at the ESRF high-pres- sure beamline ID09. A focused monochromatic beam (wave- length l = 0.4561 Å) was used in combination with imaging * E-mail: fiquet@lmcp.jussieu.fr Structural refinements of magnesite at very high pressure GUILLAUME FIQUET, 1, * FRANÇOIS GUYOT, 1 MARTIN KUNZ, 2 JAN MATAS, 3 DENIS ANDRAULT, 4 AND MICHAEL HANFLAND 5 1 Laboratoire de Minéralogie Cristallographie,UMR CNRS 7590, Institut de Physique du Globe de Paris, Universités Paris VI et VII, 4 Place Jussieu, 75252 Paris cedex 05 France 2 Naturhistorisches Museum, Augustinergasse 2, CH-4001 Basel 3 Laboratoire de Sciences de la Terre, Ecole Normale Supérieure de Lyon, 46, Allée d’Italie 69364 LYON Cedex 07 France 4 Département des Géomatériaux, Institut de Physique du Globe de Paris, 4 Place Jussieu, 75252 Paris Cedex 05 France 5 ESRF - High-pressure group, 6 rue Jules Horowitz BP220, 38043 Grenoble Cedex France ABSTRACT Unit-cell parameters of magnesite were measured between ambient pressure and 80 GPa using angle dispersive powder X-ray diffraction. The isothermal bulk modulus determined from a third order Birch Murnaghan equation of state is K T = 108(3) GPa with K ' T = 5.0(2), and V 0 = 279.2(2) Å 3 , in agreement with previously reported values. Combining this result with previous measurements, we show that magnesite with R3 – c structure is stable compared to the assemblage periclase + carbon dioxide at pressures and temperatures corresponding to the core-mantle boundary. Crystal structure refinements have also been carried out up to 80 GPa. The main structural change is a strong com- pression of the MgO 6 octahedra with increasing pressure, largely reflected in the anisotropic com- pression of the c axis. This compression, however, tends to level off at around 50–60 GPa. On the other hand, the CO 3 groups do not remain invariant since they undergo first a slight expansion and then a compression above the same threshold pressure of 60 GPa above which Mg-O bonds cannot compress further. Thus, in this structure-type, the energy gain due to a drastic volume reduction of the MgO 6 octahedron compensates in a given pressure range for the energy cost of the small expan- sion of the CO 3 carbonate unit.