Phase Stability of Natural Ni0.75Mg0.22Ca0.03CO3 Gaspeite Mineral at High Pressure and Temperature

Divalent metal carbonates play an important role in Earth's carbon cycle, but the effect of chemical substitution is still poorly known. In this work, we have studied the structural and vibrational properties of natural mineral gaspeite (Ni0.75Mg0.22Ca0.03CO3) under high pressure and temperature using in situ synchrotron X-ray diffraction and Raman spectroscopy in diamond-anvil cells. These experiments have been complemented by ab initio simulations. Synchrotron high-pressure XRD measurements at room temperature using He as the pressure transmitting medium have shown that the calcite-type structure is stable up to 23.3 GPa. A bulk modulus at zero pressure of B-0 = 105(2) GPa with B-0' = 7.4(3) has been obtained from the experimental equation of state. This result indicates that gaspeite is the most incompressible of all the divalent metal carbonates. The axial compressibilities in gaspeite have a high anisotropy, with the c axis about 3 times more compressible than the a axis. We have followed under pressure the Raman modes of gaspeite up to 19 GPa. Moreover, Ni0.75Mg0.22Ca0.03CO3 compressed at 5.5 GPa has been heated up to 840 K, revealing that this carbonate is stable in these conditions. The equation of state of magnesian gaspeite at this temperature has been determined. Our ab initio calculations on NiCO3 at zero pressure and 5.5 GPa have allowed us to estimate the thermal expansion coefficients. The obtained values show that the anisotropy in the thermal expansion is comparable to that found in axial compressibility.