First-Principle Calculation and Micro-FTIR Analysis of Structural Water in Andalusite

The structural water of andalusite critically influences the physical and chemical properties, such as the density, composition and elasticity of andalusite. In this study, micro Fourier transform infrared spectroscopy (Micro-FTIR) analysis and first-principle calculation are conducted to investigate the bonding mechanism and the spectra of defects models in andalusite. Micro-FTIR results show that the main absorption peaks of andalusite are3 749, 3 674, 3 659, 3 609, 3 599, 3 525, 3 517, 3 450 cm^-1 and 3 444 cm^-1. The calculation results indicate that the formation energy of (4H)_Si defect model is lower than that of (AlH)_Si and (3H)_Al defect models. Moreover, the (4H)_Si complex defect is a preferential model in andalusite. Raman spectra of hydrogen defects in andalusite were calculated by the first principles, and it is basically consistent with the results of the infrared spectrum experiment. By comparing the results of infrared spectroscopy, it was found that (4H)_Si hydrogen defect almost appeared in all particles, indicating that this bonding mechanism is more stable. The attribution of structural hydroxyl groups by infrared spectrum analysis and first-principle calculation provides a theoretical basis for the experimental study of minerals and a new idea for the study of spectral attribution of other minerals.