First experimental synthesis of mg orthocarbonate by the MgCO$_3$ + MgO = Mg$_2$CO$_4$ reaction at pressures of the Earth's lower mantle

Mg-carbonate is one of the main carbonates subducting from the Earth's surface into the mantle and possible reactions of this compound with the main mantle minerals determine the stability of carbonates into the Earth's mantle. Recently, we have theoretically shown that at pressures and temperatures of the Earth's lower mantle MgCO$_3$ should react with MgO, producing Mg2CO$_4$. Here, using diamond anvil cell technique, we have performed in situ investigation of the product of MgCO$_3$ + MgO reaction at pressures around 50 GPa and temperatures above 2000 K. Investigation of Raman and X-ray diffraction patterns unambiguously indicates that reaction has been realized and as the reaction product we suggest the modification similar to Mg$_2$CO$_4$-$P2_1/c$. Using the transmission electron microscopy technique, we show the presence of the new Mg-orthocarbonate/carbonate phase in the sample extracted from high-pressure environment.