Anomalies in thermal properties of ferromagnesite (Mg_1−<i>x</i>Fe_<i>x</i>)CO₃ across the spin transition

Ferromagneiste (Mg,Fe)CO₃ is considered as a major carbon carrier in the Earth’s lower mantle. Thorough knowledge of this mineral at high pressure (P) and temperature (T) can thus provide valuable insights to the Earth's deep carbon cycle. With Fe²⁺ substituting Mg²⁺ in the octahedral site, (Mg,Fe)CO₃ undergoes a spin transition from the high-spin (HS, S = 2) to the low-spin (LS, S = 0) state at 45–50 GPa. Previous static calculations [1] adopting the local density approximation + self-consistent Hubbard U (LDA+U_sc) method have succesfully explained the spin transition and accompanying volume/elastic anomlies observed in room-temperature experiments. Here, by combining LDA+U_sc with lattice dynamics, we compute the thermal properties of (Mg,Fe)CO₃ at high-(P,T) conditions. Our results indicate that nearly all thermal properties, including thermal expansion, Grüneisen parameter, heat capacity, and thus thermal conductivity, are significanlty changed by spin transition. Geophysical and geochemical consequence of spin transition in (Mg,Fe)CO₃ can thus be expected.

[1] H. Hsu and S.-C. Huang, Phys. Rev. B 94, 060404(R) (2016).