Theoretical Calculations Of Infrared Spectra Of Hydrous Ringwoodite

Water in the earth plays an important role in plate tectonics and the formation and evolution of the earth.Rinwoodite is believed to be a major constituent of the earth's 550-660 km mantle transition zone.Its physical and chemical properties are significantly affected by the structural water?OH?in the mineral lattice.Fourier infrared spectroscopy?FTIR?is one of the main tools to examine the water content in Ringwoodite and its high-pressure phase.The absorption peak will change correspondingly with the temperature and pressure.The water-bearing mechanism corresponding to the infrared absorption peak of hydrous Ringwoodite is believed to be as follows:Mg vacancy mechanism[V?_{M g}+2H^·],Si vacancy mechanism[V_S?_i?+4H^·]and Mg-Si disorder mechanism[Mg?_{S i}+2H^·].The infrared spectra are analyzed by simulating the relative optical absorption coefficients at different frequencies for each water-bearing mechanisms to understand the way in which water occurs in the Ringwoodite.This dissertation has two parts:In the first part,the unit cell and the supercell?2×2×2?structure of Fe-free Rinwoodite were modeled.The unit cell and the supercell respectively stand for high and low water content Ringwoodite.The influence of the change of water content on the optical absorption spectrum was explored.In addition,the influence of temperature and pressure on the absorption spectrum was investigated.The results show that the decrease of water content will affect the intensity of optical absorption and make the position of absorption peak shift to low frequency.The OH absorption peak position of[V?_{M g}+2H^·]is sensitive for temperature and pressure but the other two mechanisms are not.Under the same pressure condition,the peak position of[V?_{M g}+2H^·]move towards the high frequency as the temperature increases,and the change trend is more obvious under the low pressure.Under the same temperature condition,the peak position move towards the low frequency with the increase of pressure,and the change is more obvious at high temperature.With the increase of pressure,temperature effect of absorption peak frequency of[V?_{M g}+2H^·]and[V_S?_i?+4H^·]weakened gradually.This is basically consistent with the conclusion in high temperature and high pressure experiment.Under 18.4 GPa,the influence of temperature change on the infrared spectrum of hydrous Ringwoodite was not obvious.The second part studies the effect of Fe²⁺doping on the infrared absorption spectrum of Ringwoodite.We simulated Ringwoodite cell with 100%,50%,and 25%Fe²⁺at the Mg site respectively,and designed the Fe vacancy mechanism[V_F?_e+2H^·],Si vacancy mechanism[V_S?_i?+4H^·]and Fe-Si disorder mechanism[Fe?_{S i}+2H^·]to get the corresponding optical absorption spectrum.The results show that the increase of iron content make[V_F?_e+2 H^·]absorption peak position toward high frequency direction.[V_S?_i?+4H^·]and[Fe?_{S i}+2H^·]absorption peak intensity is enhanced.We also found that the content of Fe did not change the temperature and pressure effects of the optical absorption spectra of Ringwoodite.It can be seen that the absorption spectrum of the water-bearing mechanism of Ringwoodite simulated by the empirical potential is basically consistent with the infrared spectrum on the experiment.The three water-bearing mechanisms we designed can be used for theoretical reference to explain the water-bearing mechanism in the Ringwoodite structure.In this paper,the absorption spectrum of the water-bearing mechanism of Ringwoodite obtained by the empirical potential simulation method is basically consistent with the infrared spectrum on the experiment.The three water-bearing mechanisms designed can explain the way of water occurrence in Ringwoodite structure,which has certain theoretical reference significance.