Structural refinements of magnesite at very high pressure

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 V₀ = 279.2(2) ų, in agreement with previously reported values. Combining this result with previous measurements, we show that magnesite with R3̅c structure is stable compared to the assemblage periclase + carbon dioxide at pressures and temperatures corresponding to the coremantle boundary. Crystal structure refinements have also been carried out up to 80 GPa. The main structural change is a strong compression of the MgO₆ octahedra with increasing pressure, largely reflected in the anisotropic compression of the c axis. This compression, however, tends to level off at around 50–60 GPa. On the other hand, the CO₃ 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 MgO₆ octahedron compensates in a given pressure range for the energy cost of the small expansion of the CO₃ carbonate unit.