Spectral and Hydration Properties of Allophane and Imogolite

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Abstract

Abstract Allophane and imogolite are common alteration products of volcanic materials. Natural and synthetic allophanes and imogolites were characterized in the present study in order to clarify the short-range order of these materials and to gain an understanding of their spectral properties. Spectral analyses included visible/near-infrared (VNIR), and infrared (IR) reflectance of particulate samples and thermal-infrared (TIR) emissivity spectra of particulate and pressed pellets. Spectral features were similar but not identical for allophane and imogolite. In the near-infrared (NIR) region, allophane spectra exhibited a doublet near 7265 and 7120 cm −1 (1.38 and 1.40 μm) due to OH 2v , a broad band near 5220 cm −1 (1.92 μm) due to H 2 O v+δ , and a band near 4560 cm −1 (2.19 μm) due to OH v+δ . Reflectance spectra of imogolite in this region included a doublet near 7295 and 7190 cm −1 (1.37 and 1.39 μm) due to OH 2v , a broad band near 5200 cm −1 (1.92 μm) due to H 2 O v+δ , and a band near 4565 cm −1 (2.19 μm) due to OH v+δ . A strong broad band was also observed near 3200–3700 cm −1 (~2.8–3.1 μm) which is a composite of OH v , H 2 O v , and H 2 O 2δ vibrations. Visible/near-infrared spectra were also collected under two relative humidity (RH) conditions. High-RH conditions resulted in increasing band strength for the H 2 O combination modes near 6900–6930 cm −1 (1.45 μm) and 5170–5180 cm −1 (1.93 μm) in the allophane and imogolite spectra due to increased abundances of adsorbed H 2 O molecules. Variation in adsorbed H 2 O content caused an apparent shift in the bands near 1.4 and 1.9 μm. A doublet H 2 O δ vibration was observed at 1600–1670 cm −1 (~6.0–6.2 μm) and a band due to OH bending for O3SiOH was observed at ~1350–1485 cm −1 (~6.7–7.4 μm). The Si-O-Al stretching vibrations occurred near 1030 and 940 cm −1 (~9.7 and 10.6 μm) for allophane and near 1010 and 930 cm −1 (~9.9 and 10.7 μm) for imogolite. OH out-of-plane bending modes occurred near 610 cm −1 (16.4 μm) for allophane and at 595 cm −1 (16.8 μm) for imogolite. Features due to Si-O-Al bending vibrations were observed at 545, 420, and 335 cm −1 (~18, 24, and 30 μm) for allophane and at 495, 415, and 335 cm −1 (~20, 24, and 30 μm) for imogolite. The emissivity spectra were obtained from pressed pellets of the samples, which greatly enhanced the spectral contrast of the TIR absorptions. Predicted NIR bands were calculated from the mid-IR fundamental stretching and bending vibrations and compared with the measured NIR values. Controlled-RH X-ray diffraction (XRD) experiments were also performed in order to investigate changes in the mineral structure with changing RH conditions. Both allophane and imogolite exhibited decreasing low-angle XRD intensity with increasing RH, which was probably a result of interactions between H 2 O molecules and the curved allophane and imogolite structures.

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