First-principles modeling of the infrared spectrum of antigorite

The infrared absorption spectrum of a natural antigorite sample from New Caledonia is compared to its theoretical counterpart computed for the pristine antigorite m = 17 polysome within the density functional perturbation theory framework. The theoretical model reproduces most of the bands related to Si-O stretching in the 800-1300 cm(-1) range, OH libration, hindered OH translation and SiO4 bending in the 400-800 cm(-1) range, and OH stretching in the 3500-3700 cm(-1) range. Most of the observed bands have a composite nature involving several vibrational modes contributing to their intensity, except the apical and one of the basal Si-O stretching bands whose intensity is carried by a single mode. The peculiarity of the antigorite structure favors a localization of the Si-O and OH stretching modes in specific regions of the unit cell. Weaker Si-O stretching bands experimentally observed at 1205 and 1130 cm(-1) are related to the occurrence of 6- and 8-reversals in the antigorite structure, respectively. The distribution of OH bond lengths leads to an asymmetric distribution of frequencies consistent with the width and the shape of the experimentally observed OH stretching band. It also leads to a strong distribution of OH libration frequencies ranging from 600 to 830 cm(-1) explaining the asymmetry of the band observed at 648 cm(-1) in the antigorite spectrum.