Solubility of Nitrogen in Stishovite: A Possible Storage Mechanism for Nitrogen in Earth's Deep Interior

Though large amounts of nitrogen are allocated to the Earth's mantle, not much is known concerning how it is stored and transported. In this study, stishovite is proposed as a host for nitrogen within the Earth's deep interior. Stishovite was synthesized and heated under nitrogen rich conditions using diamond-anvil cell equipment and double-sided laser heating. Synthesis pressures ranged from 16 to 44 GPa and temperatures centered at ~1800 K. Experimental products were removed from diamond anvil cells and analyzed for nitrogen content via SIMS and SEM/EDX analysis. Unit cell parameters were obtained through XRD analysis. N solubility in stishovite was calculated to be up to 1.54 wt % from SIMS data through the use of an ion implant and a relative sensitivity factor. XRD data indicated a decrease in unit cell volume at higher pressures, with the c-axis length showing larger compressibility than the a-axis length. Through SEM and EDX analysis, a uniformly low level of N was observed throughout the sample indicating that N was uniformly incorporated into the crystal structure of stishovite. The data suggests that, rather than existing separately from stishovite as a silicon or carbon nitride, N has substituted into the crystal structure of stishovite. Both O and N have largely similar atomic radii, with N being slightly smaller, indicating that N can substitute for O. With the levels of N observed in the experiment, it is implicated that the mantle has an extremely large storage capacity for N. Further experimentation, with the addition of TEM analysis, should be conducted in order to determine the effects of pressure and temperature on the solubility of N in stishovite. Additionally, substitution of N as HN into stishovite should be investigated as HN accounts for the charge imbalance seen when substituting N for O.