Near-Infrared Spectroscopy of Stitchtite, Iowaite, Desautelsite and Arsenate Exchanged Takovite and Hydrotalcite

The hydrotalcite minerals stitchtite, iowaite and desautelsite together with the arsenate exchanged takovite and arsenate exchanged hydrotalcite have been studied using near-IR reflectance spectroscopy. Each mineral has its own characteristic NIR spectrum enabling recognition of the particular hydrotalcite. As such the technique has application in the field for the analysis and identification of hydrotalcites. Hydrotalcites have proven useful as an anion exchange material. Takovite and hydrotalcite were used to exchange carbonate anions by arsenate. Three Near–IR spectral regions are identified: (a) the high wavenumber region between 6400 and 7400 cm-1 attributed to the first overtone of the fundamental hydroxyl stretching mode (b) the 4800-5400 cm-1 region attributed to water combination modes of the hydroxyl fundamentals of water, and (c) the 4000-4800 cm-1 region attributed to the combination of the stretching and deformation modes of the MOH units of the hydrotalcites. NIR spectroscopy enables the separation of the hydroxyl bands of the water and M-OH units for the hydrotalcites. Compared with the NIR spectroscopy of the structural units of the hydrotalcites namely gibbsite and brucite, the bands are broad.