Micro-FTIR Analysis and First-Principle Calculation of Structural Water in Coesite from NAMs

The study of the distribution of structural water at microscopic scale can provide important evidences for the formation environment and tectonic evolution dynamics of UHP metamorphic rocks.In order to investigate the distribution characteristics and the relationship between occurrence state and microstructure defects of "nominal anhydrous minerals" (NAMs) structure water in ultrahigh pressure metamorphic rocks from Dabie Mountains, the NAMs such as coesite in eclogites of the Shima area from Dabie Mountains were studied by FTIR analysis and first-principle calculations. FTIR studies show that the main absorption peaks of coesite are (Ⅰ) 3 561-3 580 cm^-1, (Ⅱ) 3 433-3 462 cm^-1 and (Ⅲ) 3 412-3 425 cm^-1 respectively. The structural water content of the coesites in Shima is 15×10^-6-52×10^-6, with an average of 32×10^-6. The vacancy formation energies of the (4H)_Si and (AlH)_Si complex defect coesite supercells (2×1×1) calculated by the first principle are -4.92 eV and -3.10 eV respectively. The Raman peaks at 3 526 and 3 198 cm^-1 in the hydrogen-containing defect models of coesite are consistent with the experimental results. The vacancy defect formation energy of the (4H)_Si complex defect model is lower, which is the more stable structure than (AlH)_Si. Moreover, the (OH)₄$ \Leftrightarrow $Si hydrogen bonding mechanism is a preferential model, which provides the theoretical basis for the experimental research.