The spectroscopic search for water in barite group sulfates.

Abstract

This study examines the mid and near infrared spectra of a suite of natural and synthetic powdered minerals of the isostructural barite mineral group. The mid-infrared (MIR) spectra are wholly compliant with anhydrous high purity anglesite, celestine and barite. The complementary near-infrared (NIR) spectra contain bands at ca 1.9 μm (5263 cm-1) and ca 1.4 μm (7140 cm-1) assigned to the (H2O) v2 (antisymmetric stretch)+v3 (bending vibration) combination band and overtones of the fundamental OH stretching modes respectively. Peak-fitting of the ca 4660-6000 cm-1 region of interest reveals four shoulders to the (H2O) (v2+v3) combination which are systematically ordered with respect to interstitial metal covalent radius: [PbSO4] (lowest wave number) → [SrSO4] → [BaSO4] (highest wave number). These bands are assigned to (M2+-O-H) surface complexation of water with the sulfates. It is further interpreted that the two good-to-perfect cleavages in the mineral group with resultant variation in (M2+ - O - H) bond lengths accounts for the existence of the four bands and explain why absorption strength at ca 1.91 μm (5236 cm-1) is not a reliable indication of the overall abundance of water in sulfate minerals. Spectral interpretation within the vicinity of the water combination ca 1.91 μm (5236 cm-1) is further complicated by the presence of a higher wavenumber (v1+4v3) sulfate combination and the appearance of a band interpreted as the (5v3) overtone of [SO42-]. The results are discussed in the light of these findings.