Effect of Fe–Fe interactions and X-site vacancy ordering on the OH-stretching spectrum of foitite

Abstract. The OH-stretching infrared absorption spectrum of a tourmaline sample close to the foitite end-member is interpreted in the light of the density functional theory (DFT) modeling of iron-bearing Y3Z6 clusters in tourmaline. The iron-bearing clusters reflect the Al-rich and Na-deficient character of foitite and contain either two Fe2+ and one Al3+ or one Fe2+ and two Al3+ ions at the Y sites. The clusters are embedded in a tourmaline host structure with dravite composition. For the iron dimer models, the structural and vibrational properties corresponding to the ferromagnetic (FM) or anti-ferromagnetic (AFM) arrangement of the iron spins and the effect of vacancy ordering along the [001] axis are considered. A significant difference in the relaxed structure of the FM and AFM clusters is observed, stemming from the electron delocalization and Fe–Fe bonding interactions in the FM cluster. These bonding interactions are not allowed in the AFM cluster. In this case, the valence electrons with opposite spins remain separately localized on the two Fe atoms. The AFM configuration is more stable than the FM one in the theoretical models, provided that the description of the on-site Coulomb repulsion in Fe(3d) orbitals is improved within the DFT + U framework. Based on the theoretical results, the two bands at 3630 and 3644 cm−1 in the vibrational spectra of iron-rich and Na-deficient tourmalines are assigned to WOH groups associated with YFe22+YAl3+ environments with an AFM coupling of Fe ions and surrounded by one and two vacant X sites, respectively. The two major VOH bands of the experimental spectrum are interpreted on the same basis, and these interpretations are extrapolated to Mn-bearing tourmalines.