EOS of Carbonates EOS of Carbonates
Anisotropic thermal expansion of calcite as a function of pressure and temperature
Carbonates are an important group of minerals for their abundance in sedimentary rocks on the surface of the Earth and for their existence in the organic material in the ocean, which constitute the most important reservoir for carbon dioxide on the Earth¹s surface. Since there is so much carbonate material in ocean basin sediments, one natural question to ask is what is the destiny of those carbonates in the sediments. Following the subduction of oceanic floor, most of the carbonates are subducted underneath the continental plate and deeper into the mantle. Carbonatite magma and CO2 volcanic gas are the primary evidence that carbon is an active member of the igneous process, where the carbon is cycled back to the surface of the Earth. The occurrence of diamond and carbonate inclusion in the mantle xenoliths, on the other hand, have long been held as evidence of carbon existing in a solid form in the deeper mantle. Recent experimental and field evidence demonstrated that magnesite is a possible host for carbon in the mantle. Therefore, the properties of carbonates at high pressure and temperature, starting from the most basic pressure (P) - volume (V) - temperature (T) relationships, are important keys to a better understanding of this relatively unknown part of the Earth¹s carbon cycle.
The anisotropic thermal expansion and P-V-T equation of state of calcite(CaCO3) and magnesite(MgCO3) were studied in the hydrothermal DAC between 0-1 Gpa (0-10 kbar) and 25-500 C. The thermal expansions of the a axis and c axis of calcite were found to have different signs throughout the experimental P-T range, negative for the a axis and positive for the c axis. The simultaneous effects of pressure and temperature on lattice parameters are fitted to a second order polynomial. The variation of unit-cell volume with the pressure and temperature was also fitted to high temperature Murnghahan equation of state. The temperature dependencies of isothermal bulk modulus were found to be -0.065 (± 0.019) GPa/deg for calcite and -0.16 (± 0.04) GPa/deg for magnesite.