Abstract
Abstract. The theoretical vibrational properties of a series of Fe- and Al-bearing lizardite models have been determined at the density functional theory level. Each periodic model displays a single cationic impurity substituted at an octahedral or tetrahedral site of a supercell of lizardite (Mg3Si2O5(OH)4) containing 162 atoms. The isovalent Fe2+ for Mg2+ substitution has been considered, as well as the heterovalent substitution of Fe3+ or Al3+ for Mg2+ or Si4+. Comparison of the theoretical absorption spectra with previously reported experimental spectra of natural and laboratory-grown lizardite samples allows us to propose an interpretation for most of the observed bands. Although the identification of specific bands related to octahedral Fe2+ in FTIR spectra is challenging, broad bands at 3584 and 3566 cm−1 reflect the occurrence of octahedral Al3+ and Fe3+, respectively, in the natural samples. These broad bands likely overlap with potential contribution related to tetrahedral Al3+. It is suggested that the modification of the H-bonding pattern related to the incorporation of trivalent ions at tetrahedral sites has an overall broadening effect on the interlayer-OH stretching bands of lizardite.
Highlights
The stretching vibrations of OH groups are widely used to probe the local order of hydrous phyllosilicates
We theoretically investigate the effect of Fe and Al substitutions at low concentration on the vibrational and infrared absorption properties of lizardite, the more symmetric planar member of serpentine-group minerals
As usually observed in density functional theory (DFT) modeling performed at the generalized gradient approximation (GGA) level, they are overestimated with respect to their experimental counterparts (e.g., a = b = 5.3267 Å, c = 7.2539 Å; Gregorkiewitz et al, 1996)
Summary
The stretching vibrations of OH groups are widely used to probe the local order of hydrous phyllosilicates. The potential occurrence of multivalent elements, such as iron, the simultaneous substitution at octahedral and tetrahedral sites by trivalent cations (e.g., Serna et al, 1979), and variations of the cationic ordering and potential intermediate di-tri-octahedral character of extended solid solutions (Bailey, 1988) may affect the interpretation of the vibrational spectra of chemically complex synthetic or natural samples This is true for the serpentine-group minerals, which can display significant variations of their chemical composition, with a frequent departing from the ideal Mg3Si2O5(OH) formula related to the occurrence of iron and aluminum (e.g., Viti and Mellini, 1997). The focus is on the OH stretching modes, the effect of substitutions on the lower-frequency region of the spectra is discussed
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