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 μm 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 μm regions, while the presence of Fe²⁺ results in absorption features in the 0.9-1.2 μm interval. Fundamental S{single bond}O bending and stretching vibration absorption bands occur in the 8-10, 13-18, and 19-24 μm 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 μm (2000-2500 cm^-1) region. Absorption features seen in the 1.7-1.85 μm interval are attributable to H{single bond}O{single bond}H/O{single bond}H bending and translation/rotation combinations, while bands in the 2.1-2.7 μm regions can be attributed to H₂O- and OH-combinations as well as overtones of S{single bond}O bending fundamentals. OH- and H₂O-bearing sulfate spectra are fundamentally different from each other at wavelengths below ∼6 μm. Changes in H₂O/OH content can shift S{single bond}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 μm region seems best for identifying and discriminating sulfates in the presence of other major rock-forming minerals.