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Five-Coordinate Aluminum in Allophane

Published online by Cambridge University Press:  28 February 2024

Cyril W. Childs
Affiliation:
School of Chemical and Physical Sciences, Victoria University, PO Box 600, Wellington, New Zealand
Shigenobu Hayashi
Affiliation:
National Institute of Materials and Chemical Research, Tsukuba 305, Japan
Roger H. Newman
Affiliation:
Industrial Research Ltd, PO Box 31-310, Lower Hutt, New Zealand
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Abstract

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Samples of Silica Springs allophane from Tongariro National Park, New Zealand, having Al/Si atomic ratios in the range 1.1-1.9, were studied by 27Al nuclear magnetic resonance (NMR) spectroscopy with high field strength (9.4 and 11.7 T) and fast magic-angle spinning (MAS) (9-13 kHz). Spectra for all samples show peaks for 6- and 4-coordinate Al and also for 5-coordinate Al. For 1 sample, the peak for 5-coordinate Al is dominant. Use of 2 instruments and 2 field strengths allowed the integrity of the spectra and the assignment of 5-coordinate Al to be verified. The “true” chemical shift (after a small correction for quadrupolar shift) observed for 5-coordinate Al in Silica Springs allophane is 36 ± 1 ppm, which is consistent with shifts reported for 5-coordination in well-characterized crystalline structures. We suggest that 5-coordination in Silica Springs allophane is associated with the edges of fragments of incomplete octahedral sheets that are bonded to disordered, though more complete, curved tetrahedral sheets in the primary particles of this allophane. Other allophanes with Al/Si < 2, and which are poor in octahedra relative to tetrahedra, may also have significant Al in 5-coordinate sites.

Information

Type
Research Article
Copyright
Copyright © 1999, The Clay Minerals Society