Abstract
Abstract. 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.
Highlights
Serpentine-group minerals are hydrous phyllosilicates produced by the hydrothermal alteration of anhydrous (Mg,Fe) silicates
We have theoretically computed the infrared absorption spectrum of the antigorite m = 17 polysome using a firstprinciples quantum mechanical approach. This computation on a much larger system size (291 atoms) than that investigated in our previous study of lizardite (18 atoms; Balan et al, 2002) was rendered possible thanks to the increase in computing power over the years
The comparison of the theoretical spectrum with an experimental one recorded on a natural antigorite sample allowed us to assign most of the observed bands to specific vibrational modes of antigorite, supporting the model proposed by Capitani and Mellini (2004)
Summary
Serpentine-group minerals are hydrous phyllosilicates produced by the hydrothermal alteration of anhydrous (Mg,Fe) silicates. The crystal structures proposed by, for example, Uehara and Shirozu (1985), Uehara (1998), Grobéty (2003) and Capitani and Mellini (2004, 2005, 2006) are consistent with the initial model proposed by Kunze and display 6-reversals alternating with another type of reversal displaying juxtaposed 4- and 8-membered rings along the [010] direction These different reversals are referred to as 8-reversals. Useful insights into fine details of mineral structures can be obtained using vibrational spectroscopic methods (e.g., Farmer, 1974; Madejová et al, 2011) In this respect, the infrared (IR) spectrum of antigorite displays significant differences with that of lizardite, the more symmetric serpen-. Comparison of the theoretical spectrum with that of a representative antigorite sample from New Caledonia (Fritsch et al, 2016) allows us to assign most of the experimental bands to specific vibrational modes of the antigorite structure
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