Spectral and Hydration Properties of Allophane and Imogolite

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 OH2v, a broad band near 5220 cm−1 (1.92 μm) due to H2Ov+δ, and a band near 4560 cm−1 (2.19 μm) due to OHv+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 OH2v, a broad band near 5200 cm−1 (1.92 μm) due to H2Ov+δ, and a band near 4565 cm−1 (2.19 μm) due to OHv+δ. A strong broad band was also observed near 3200–3700 cm−1 (~2.8–3.1 μm) which is a composite of OHv, H2Ov, and H2O2δ vibrations. Visible/near-infrared spectra were also collected under two relative humidity (RH) conditions. High-RH conditions resulted in increasing band strength for the H2O 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 H2O molecules. Variation in adsorbed H2O content caused an apparent shift in the bands near 1.4 and 1.9 μm. A doublet H2Oδ 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 H2O molecules and the curved allophane and imogolite structures.

Recognizing sulfate and phosphate complexes chemisorbed onto nanophase weathering products on Mars using in-situ and remote observationsk

“…6a) stretching and bending vibrations). The band positions reported here are consistent with those previously reported for natural and synthetic allophane (e.g., Bishop et al 2013). The band near 1.4 mm is a doublet with minima near 1.38 and 1.40 mm (from the OH and H 2 O overtone, respectively), as was reported by Bishop et al (2013), with greater contributions from the band at 1.38 mm as the samples became more desiccated.…”

“…The allophane pattern also displays a low-angle peak at ~24 Å, which has been attributed to diffraction off of closely packed spheres of allophane (van der Gaast et al 1985). Previous studies of natural and synthetic allophane and imogolite have noted that this peak becomes more intense with dehydration (van der Gaast et al 1985;Bishop et al 2013) because as adsorbed water is removed, the spherules become more tightly packed. This phenomenon occurs in chemisorbed allophane because the chemisorption of ions to the allophane surface results in the removal of H 2 O adsorbed to the surface (Fig.…”

“…This increase in low-angle scattering intensity has been attributed to the regular packing of nanophase particles (van der Gaast et al 1985;Bishop et al 2013). However, the low-angle rise is not present in patterns of all amorphous materials (e.g., igneous glasses do not display this feature; Morris et al 2015b).…”

“…We suggest that this variation is the result of different relative H 2 O abundances in the samples. The band near 1.4 mm has a contribution from H 2 O, and the band near 1.9 mm is from H 2 O. Chemisorption causes the removal of adsorbed H 2 O, and as H 2 O is removed from allophane, the bands at 1.4 and 1.9 mm shift to shorter wavelengths (Bishop et al 2013). More phosphate is chemisorbed to allophane than sulfate in our samples, causing the 1.4 and 1.9 mm band minima in the phosphate-chemisorbed allophane to be at shorter wavelengths than the bands of sulfatechemisorbed allophane.…”

Stratigraphy and formation of clays, sulfates, and hydrated silica within a depression in Coprates Catena, Mars

“…Al‐rich phyllosilicates exhibit Al‐OH stretch + bend combination bands near 2.2 µm, while these bands are observed near 2.29–2.31, 2.33–2.34, and 2.35–2.37 µm for Fe 3+ ‐rich, Mg‐rich, and Fe 2+ ‐rich phyllosilicates, respectively [e.g., Bishop et al ., ]. Related nanophase hydrated alluminosilicates such as allophane and imogolite exhibit spectral features near 1.37–1.41, 1.92, and 2.18–2.20 µm that vary depending on the degree of hydration and the Al/Si ratio [ Bishop et al ., ]. Spectral absorptions for hydrated silica occur at 1.38–1.46 and 2.20–2.26 µm that vary depending upon the hydration state, form, and composition of the material, with opal representing one form of SiO 2 · n H 2 0 [ Anderson and Wickersheim , ; Milliken et al ., ].…”