Identification of the rosasite group minerals—An application of near infrared spectroscopy

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The ability of near infrared reflectance spectroscopy to classify the rosasite group minerals from spectral characteristics is demonstrated. NIR spectroscopy can be regarded as an alternative tool for structure analysis. The spectra show that rosasite group minerals with different cations can be distinguished. Ni 2+ in nullaginite [Ni 2(CO 3)(OH) 2] is conspicuous through a single broad band absorption feature at 8525 cm −1, extended from 11,000 to 7000 cm −1. The effect of Ni on Cu is seen in the spectrum of glaukosphaerite [(Cu, Ni) 2(CO 3)(OH) 2] both by a red shift of the spectrum and reduction in intensity of bands with variable positions of band maxima for Cu 2+ at 6995 cm −1and Ni 2+ at 7865 cm −1. The spectrum of rosasite [(Cu, Zn) 2(CO) 3(OH) 2] is characterised by Cu 2+ band at 7535 cm −1. Kolwezite [(Cu, Co) 2(CO) 3(OH) 2] is a spectral mixture of Cu and Co but optically separated by Co 2+ and Cu 2+ peaks at 8385 and 7520 cm −1. Vibrational spectra of carbonates show a number of bands in the 7000–4000 cm −1 region attributable to overtones, combination of OH stretching and deformation modes. They appear to be uniform in nature since the structure of rosasite group minerals is identical. The complexity of these features varies between samples because of the variation in composition and hence is useful for discriminating different hydrous carbonates.