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 SO 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 HOH/OH 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 SO 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 SO 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.