Static compressibility of magnesite to 20 GPa: Implications for MgCO₃ in the lower mantle

The bulk modulus of MgCO₃ magnesite has been measured to 20 GPa at room temperature using X‐ray powder diffraction. Since the response to pressure is primarily compression of the MO₆ octahedron, magnesite is substantially less compressible than Ca‐carbonates of similar structure type. Using the Birch equation of state, with K_T′ of 4.0, we obtain K_T of 142 (±9) GPa at room temperature. The full Birch‐Murnaghan equation of state yields K_T of 151 (±7) GPa and K_T′ of 2.5. These results, together with other thermodynamic data, have been applied to calculate the stability of magnesite under lower mantle conditions. Assuming lower‐mantle saturation in (Mg,Fe)O, magnesite should be stable to temperatures well in excess of 6000 K at the core‐mantle boundary, i.e. at least up to the solidus of peridotitic or pyroxenitic compositions. This confirms that magnesite is a stable host for carbon in the mantle.