A high-pressure infrared and X-ray study of FeCO3 and MnCO3:: comparison with CaMg(CO3)2-dolomite

We have measured the infrared spectra of two carbonates, FeCO3-siderite and MnCO3-rhodochrosite to pressures of 50 GPa at 300 K, and taken X-ray diffraction patterns of these phases before and after in-situ laser heating at pressures to 47 GPa. Our infrared measurements primarily sample the vibrations of the CO3-2 carbonate unit, particularly its asymmetric stretching vibration and out-of-plane and in-plane bends. In particular, we wish to determine whether any high-pressure polymorphism (metastable or otherwise) is present in these phases. Our X-ray results for FeCO3 and MnCO3 demonstrate that the calcite structure is stable for each of these materials to 50 GPa at 300 K, and following heating to 2000 K at pressures to 47 GPa; similarly, within the infrared spectra, all vibrations are observed to shift monotonically with pressure, with no splitting observed. This behavior is in marked contrast to that of dolomite, in which splitting of the infrared vibrations is observed above similar to20-30 GPa. Our results demonstrate that carbonates are stable in the calcite structure at lower mantle pressures if the carbonates' divalent cations have zero pressure radii of less than 1 Angstrom. Accordingly, the crystal structure of carbonates within the lower mantle will critically depend on the amount of calcium that enters into carbonates within lower mantle assemblages. (C) 2004 Elsevier B.V. All rights reserved.