Chemical Stability and Thermal Equation of State of Si3N4: Implications for Nitrogen Storage in the Deep Earth

Most nitrogen is predicted to reside in the Earth’s interior and could be in the form of nitrides. In this study, the chemical reaction of Si 3 N 4 with SiO 2 was explored up to 50 GPa and 2 600 K using laser-heated diamond anvil cells combined with synchrotron radiation X-ray diffraction. The experimental results demonstrate that Si 3 N 4 does not react with SiO 2 and cubic γ-Si 3 N 4 remains stable at extreme pressure and temperature. First-principles calculations were performed to investigate the chemical stability of Si 3 N 4 with the SiO 2 -MgO assemblage, showing that Si 3 N 4 can coexist with Mg 2 SiO 4 and MgSiO 3 polymorphs. Based on our thermal equation of state of cubic γ-Si 3 N 4 , we suggest that cubic γ-Si 3 N 4 is denser than the upper mantle but less dense than the lower mantle. The formation of Si 3 N 4 in the early Earth may contribute to the nitrogen preservation in the deep mantle soon after crystallization of the mantle and Si 3 N 4 is likely a precursor of nitrogen hosts in the modern deep Earth.