HIGH-PRESSURE X-RAY-DIFFRACTION STUDY OF CARBONATES - MGCO3, CAMG(CO3)2, AND CACO3

High-pressure X-ray powder diffraction data on MgCO3, CaMg(CO,)2, and CaCO3 have been collected up to 53, 11, and 18 GPa, respectively. Patt erns were recorded in a diamond-anvil cell in an energy-dispersive con figuration at the storage ring DCI at the LURE (Orsay, France). Anisot ropic compression of the unit-cell parameters is observed for dolomite and magnesite, with the c axis three times more compressible than the a axis. The greater compressibility of the c axis can also be observe d in calcite I. Assuming K0' = 4, we obtain bulk moduli from fits of t he pressure-volume data to a second-order Eulerian finite strain equat ion of state equal to K0 = 112.9 +/- 2.2 GPa for dolomite and K0 = 13 7.5 +/- 3.2 GPa for magnesite. At 25 GPa, the appearance of new diffra ction peaks and the presence of anomalies in the compression curve sug gest the occurrence of a phase change in magnesite. The high-pressure polymorphs of calcite coexist in reduced pressure ranges, suggesting a n important hysteresis for phase changes of calcite at high pressure a nd ambient temperature. In single-phase domains, it is possible to det ermine the isothermal bulk moduli of calcite I and calcite III. Assumi ng K0' = 4, one obtains K0 = 69.5 +/- 2.1 GPa for calcite I and K0 = 9 3.2 +/- 1.9 GPa, associated with an ambient pressure volume V0 = 31.87 +/- 0.17 cm3/mol, for calcite III. The relative volume change at the transition between calcite I and calcite II is very small (DELTAV/V = -1%), whereas it is much larger between calcite Il and calcite III (DE LTAV/V = -15%).