Abstract

Journal of Near Infrared Spectroscopy
Volume 18 Issue 3, Pages 177–189 (2010)
doi: 10.1255/jnirs.877

An application of near infrared and mid-infrared spectroscopy to the study of natural halotrichites: halotrichite, apjohnite and wupatkiite

Ray L. Frost*, B. Jagannadha Reddy and Eloise C. Keeffe
Chemistry Discipline, Faculty of Science and Technology, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001, Australia. E-mail: r.frost@qut.edu.au

Near infrared (NIR) and Fourier transform infrared (FTIR) spectroscopy have been used to determine the mineralogical character of isomorphic substitutions for Mg2+ 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 to 7500 cm–1 with a splitting of two bands resulting from ferrous ion transition 5T2g → 5Eg. 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 ν32 water bending modes. The influence of cations like Mg2+, Fe2+, Mn2+, Co2+, Ni2+ shows on the spectra of halotrichites. Especially wupatkiite-OH stretching vibrations in which bands are distorted conspicuously to low wavenumbers at 3270 cm–1, 2904 cm–1 and 2454 cm–1. The observation of high frequency ν2 mode in the infrared spectrum at 1640 cm–1 indicates coordination of water molecules is strongly hydrogen bonded in natural halotrichites. The splittings of bands in ν3 and ν4 (SO4)2– stretching regions may be attributed to the reduction of symmetry from Td to C2v for sulphate ion. This work has shown the usefulness of NIR spectroscopy for the rapid identification and classification of the halotrichite minerals.

Keywords: halotrichite, apjohnite, wupatkiite, near IR and mid-infrared spectroscopy, cation substitutional effects, OH-overtones, vibrational modes of sulphate ion

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