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 (Q33VIAl, -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)(OVIAl)3 structure (Q33VIAl) 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.
All Science Journal Classification (ASJC) codes
- Water Science and Technology
- Soil Science
- Geochemistry and Petrology
- Earth and Planetary Sciences (miscellaneous)