Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (5) Citing Articles via Google Scholar Google Scholar Articles by Hiradate, S. Articles by Wada, S.-I. Search for Related Content GeoRef GeoRef Citation

WEATHERING PROCESS OF VOLCANIC GLASS TO ALLOPHANE DETERMINED BY ²⁷Al AND ²⁹Si SOLID-STATE NMR

¹ Department of Biological Safety Science, National Institute for Agro-Environmental Sciences (NIAES), 3-1-3 Kan-nondai, Tsukuba, Ibaraki 305-8604, Japan
² Faculty of Agriculture, Kyushu University, Fukuoka 812-8581, Japan

^* E-mail address of corresponding author: hiradate{at}affrc.go.jp

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 ²⁷Al NMR) and imogolite-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 ²⁹Si and 55 ppm for ²⁷Al 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 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^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.

Key Words: Allophane • Imogolite • Solid-state MAS NMR • Speciation of ²⁷Al and ²⁹Si • Volcanic Glass

This article has been cited by other articles:

				L. De Pablo, M. Doval, and A. La Iglesia Geochemistry of reversible hydratable tephra from the Trans Mexican Volcanic Belt American Mineralogist, November 1, 2009; 94(11-12): 1603 - 1615. [Abstract] [Full Text] [PDF]

				S. Hiradate STRUCTURAL CHANGES OF ALLOPHANE DURING PURIFICATION PROCEDURES AS DETERMINED BY SOLID-STATE 27Al AND 29Si NMR Clays and Clay Minerals, December 1, 2005; 53(6): 653 - 658. [Abstract] [Full Text] [PDF]