The mid-infrared spectrum of a new double carbonate, BaFe[CO3

Abstract

BaFe[CO3]2, a new double carbonate from the norsethite-type family of carbonates, was recently discovered and synthesized at high-pressure-temperature (high-PT) conditions. Two polymorphs of the new double carbonate were identified belonging to the R3¯m and R3¯c space groups, respectively1. In the present communication we present for the first time the mid-infrared (IR) spectra (measured with transmission infrared spectroscopy and attenuated total reflectance infrared spectroscopy) and assign the observed bands to the respective fundamental and combined IR-active modes. This study, therefore, adds new spectroscopic results to the previous crystal chemical characterization of the new double carbonate. Results are compared to those of other Ba-bearing double carbonates with the norsethite-type structures. With substitution of Ba2+ in the double carbonate lattice by increasingly smaller cations a decrease in wavenumbers of absorption bands is observed. Due to similar crystal structures, and masses and radii of the cations, the IR spectra of both BaFe[CO3]2 carbonates are similar to the ones observed for BaMn[CO3]2 and BaMg[CO3]2, with some characteristic deviations. The carbon isotope shift coefficient of 0.969 for ʋ2 of the carbonate ion group agrees with previous theoretical predictions.