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 23 minerals of the isostructural barite mineral group. The mid-infrared (MIR) spectra are wholly 24 compliant with anhydrous high purity anglesite, celestine and barite. The complementary near-25 infrared (NIR) spectra contain bands at ca 1.9μm (5263cm−1) and ca 1.4μm (7140 cm−1) assigned to 26 the (H2O) v2 (antisymmetric stretch) + v3 (bending vibration) combination band and overtones of the 27 fundamental OH stretching modes respectively. Peak-fitting of the ca 4660-6000cm-1 region of 28 interest reveals four shoulders to the (H2O) (v2 + v3) combination which are systematically ordered 29 with respect to interstitial metal covalent radius: [PbSO4] (lowest wave number) → [SrSO4] → 30 [BaSO4] (highest wave number). These bands are assigned to (M2+−O−H) surface complexation of 31 water with the sulfates. It is further interpreted that the two good-to-perfect cleavages in the 32 mineral group with resultant variation in (M2+ - O - H) bond lengths accounts for the existence of the 33 four bands and explain why absorption strength at ca 1.91μm (5236cm-1) is not a reliable indication 34 of the overall abundance of water in sulfate minerals. Spectral interpretation within the vicinity of 35 the water combination ca 1.91μm (5236cm-1) is further complicated by the presence of a higher 36 wavenumber (v1 + 4v3) sulfate combination and the appearance of a band interpreted as the (5v3) 37 overtone of [SO42-]. The results are discussed in the light of these findings.