Weathering process of volcanic glass to allophane determined by ²⁷Al and ²⁹Si solid-state NMR

To clarify the weathering process of volcanic glass to allophane, solid-state ²⁹Si and ²⁷Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) signals of four Japanese volcanic glasses and two (Al- and Si-rich) allophanes were assigned. The volcanic glasses showed a broad ²⁹Si NMR signal between -80 and -120 ppm with the peak centered at ∼ -104 ppm, indicating that they were rich in Si-O-Si bridging structure (silica gel-like polymer Si). Aluminum was present in tetrahedral form in the four volcanic glass samples. In both Al- and Si-rich allophanes, octahedral Al (3 ppm by ²⁷ Al NMR) and imogol ite-like Si (Q³3^VIAl, -78 ppm by ²⁹Si NMR) were the major components. In a Si-rich allophane, NMR signals centered at around -85 ppm for ²⁹ Si and 55 ppm for ²⁷ Al were also observed, although it is possible that those signals were derived from impurities. Impurities could have originated from the soils and/or been unexpectedly synthesized during the purification procedures, e.g. during hot 2% Na₂CO₃ treatments. Based on the NMR spectra of size-fractionated soil samples, the weathering process of volcanic glass to allophane was proposed as follows: (1) dissolution of Al from volcanic glass accompanied by the transformation of ^IVAl to ^VIAl; (2) formation of a gibbsite-like sheet resulting from the hydrolysis of the dissolved A1; (3) dissolution of silica gel-like polymer Si in volcanic glass resulting in the formation of monosilicic acid; and (4) formation of Si(OH)(O^VIAl)₃ structure (Q³3^VIAl) as a result of the reaction between the gibbsite-like sheet and the monosilicic acid. These formation reactions of allophane could occur in solution as well as on the surface of volcanic glass.