Weathering process of volcanic glass to allophane determined by (super 27) Al and (super 29) Si solid-state nmr

To clarify the weathering process of volcanic glass to allophane, solid-state (super 29) Si and (super 27) Al 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 (super 29) Si NMR signal between -80 and -120 ppm with the peak centered at approximately -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 (super 27) Al NMR) and imogolite-like Si (Q (super 3) 3 (super VI) Al, -78 ppm by (super 29) Si NMR) were the major components. In a Si-rich allophane, NMR signals centered at around -85 ppm for (super 29) Si and 55 ppm for (super 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% Na (sub 2) CO (sub 3) 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 (super IV) Al to (super VI) Al; (2) formation of a gibbsite-like sheet resulting from the hydrolysis of the dissolved Al; (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 (super VI) Al) (sub 3) structure (Q (super 3) 3 (super VI) Al) 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.