The Stability Study Of Sinoite ?Si₂N₂O? At High Temperature And High Pressure

As an important volatile element,nitrogen is not only the main component of the atmosphere,but also a basic element of life.The comparison of the volatile compositions of bulk silicate Earth?BSE?and chondrites shows that the nitrogen in the bulk Earth is depleted by an order of magnitude relative to other volatile elements.This implies that a potential nitrogen reservoir may exist in the deep Earth,although there is still a lot of controversy about the undiscovered amount or the existence form of nitrogen.Sinoite?Si₂N₂O?is a common associate mineral in enstatite chondrites,which are thought to be the building blocks of the Earth.Therefore,the stability of sinoite under high temperature and high pressure directly affects the distribution of nitrogen in the early stages of the Earth's evolution.It is of great significance to restrict the possible deep nitrogen storage and the global nitrogen cycle.In this study,sinoite was synthesized in a piston-cylinder apparatus.The Raman spectra of sinoite at different pressures?<61 GPa?have been systematically studied by combining the in-situ Raman measurements in diamond anvil cells and the first principles calculations based on the density functional theory.The stability of sinoite under the mantle conditions was further investigated by the laser-heating experiment.The results show that the unit cell of sinoite has strong compressibility,and the Raman spectra of sinoite changed regularly with changing bond length and band angle under different pressures.The intensity of various peaks generally decreased with increasing pressure under higher pressure?>35.5 GPa?,which was a characteristics of typical pressure-induced amorphous.When the pressure arrived 55.8 GPa,all the Raman peaks disappeared and the sinoite transformed almost completely amorphized.On the other hand,a series of laser-heating experimental data confirmed that the sinoite is fairly stable within a wide range of temperature and pressure,and the highest stable point is over 25GPa and 3000 K.The experimental results confirmed that the sinoite or its amorphous form is stable in the upper mantle and even in the lower mantle.Sinoite may be a potential form to store nitrogen in the deep Earth,which can provide a new clue to explain the“Missing Nitrogen”in the deep Earth.