The spectroscopic search for water in barite group sulfates.

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) and ca 1.4  μm (7140 cm) assigned to the (HO) v (antisymmetric stretch) + v (bending vibration) combination band and overtones of the fundamental OH stretching modes respectively. Peak-fitting of the ca 4660-6000 cm region of interest reveals four shoulders to the (HO) (v + v) combination which are systematically ordered with respect to interstitial metal covalent radius: [PbSO] (lowest wave number) → [SrSO] → [BaSO] (highest wave number). These bands are assigned to (M-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 (M - O - H) bond lengths accounts for the existence of the four bands and explain why absorption strength at ca 1.91  μm (5236 cm) 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) is further complicated by the presence of a higher wavenumber (v + 4v) sulfate combination and the appearance of a band interpreted as the (5v) overtone of [SO]. The results are discussed in the light of these findings.