An Application of near Infrared and Mid-Infrared Spectroscopy to the Study of Natural Halotrichites: Halotrichite,Apjohnite and Wupatkiite

Near infrared (NIR) and Fourier transform infrared (FT-IR) spectroscopy have been used to determine the mineralogical character of isomorphic substitutions for Mg^2* by divalent transition metals Fe, Mn, Co and Ni in natural halotrichite series. The minerals are characterised by d→d transitions in the NIR region 12,000–7500 cm⁻¹. The NIR spectrum of halotrichite reveals broad features from 12,000 cm⁻¹ to 7500 cm⁻¹ with a splitting of two bands resulting from ferrous ion transition ⁵T_2g→⁵E_g. The presence of overtones of OH⁻ fundamentals near 7000 cm⁻¹ confirms molecular water in the mineral structure of the halotrichite series. The appearance of the most intense peak at around 5132 cm⁻¹ is a common feature in the three minerals and is derived from a combination of OH⁻ vibrations of water molecules and v₂ water-bending modes. The influence of cations such as Mg²⁺, Fe²⁺, Mn²⁺, Co²⁺ and Ni²⁺ shows on the spectra of halotrichites, especially for wupatkiite-OH stretching vibrations in which bands are distorted conspicuously to low wave numbers at 3270 cm⁻¹, 2904 cm⁻¹ and 2454 cm⁻¹. The observation of a high-frequency v₂ mode in the infrared spectrum at 1640 cm⁻¹ indicates that the coordination of water molecules is strongly hydrogen bonded in natural halotrichites. The splitting of bands in v₃ and v₄ (SO₄)²⁻ stretching regions may be attributed to the reduction of symmetry from T _d to C_2v for sulphate ion. This work has shown the usefulness of NIR spectroscopy for the rapid identification and classification of the halotrichite minerals.