Detection and discrimination of sulfate minerals using reflectance spectroscopy

A suite of sulfate minerals were characterized spectrally, compositionally, and structurally in order to develop spectral reflectance-compositional-structural relations for this group of minerals. Sulfates exhibit diverse spectral properties, and absorption-band assignments have been developed for the 0.3-26 mu m range. Sulfate absorption features can be related to the presence of transition elements, OH, H2O, and SO4 groups. The number, wavelength position, and intensity of these bands are a function of both composition and structure. Cation substitutions can affect the wavelength positions of all major absorption bands. Hydroxo-bridged Fe3+ results in absorption bands in the 0.43, 0.5, and 0.9 mu m regions, while the presence of Fe2+ results in absorption features in the 0.9-1.2 mu m interval. Fundamental S-O bending and stretching vibration absorption bands occur in the 8-10, 13-18, and 19-24 mu m regions (1000-1250, 550-770, and 420-530 cm(-1)). The most intense combinations and overtones of these fundamentals are found in the 4-5 mu m (2000-2500 cm(-1)) region. Absorption features seen in the 1.7-1.85 mu m interval are attributable to H-O-H/O-H bending and translation/rotation combinations, while bands in the 2.1-2.7 mu m regions can be attributed to H2O- and OH-combinations as well as overtones of S-O bending fundamentals. OH- and H2O-bearing sulfate spectra are fundamentally different from each other at wavelengths below similar to 6 mu m. Changes in H2O/OH content can shift S-O band positions due to change in bond lengths and structural rearrangement. Differences in absorption band wavelength positions enable discrimination of all the sulfate minerals used in this study in a number of wavelength intervals. Of the major absorption band regions, the 4-5 mu m region seems best for identifying and discriminating sulfates in the presence of other major rock-forming minerals. (c) 2006 Elsevier Inc. All rights reserved.