Five-Coordinate Aluminum in Allophane

Abstract

Abstract--Samples of Silica Springs allophane from Tongariro National Park, New Zealand, having A1/ Si atomic ratios in the range 1.1-l.9, were studied by 27A1 nuclear magnetic resonance (NMR) spectroscopy with high field strength (9.4 and 1 1.7 T) and fast magic-angle spinning (MAS) (9-13 kHz). Spectra for all samples show peaks for 6-and 4-coordinate A1 and also for 5-coordinate A1. For 1 sample, the peak for 5-coordinate AI is dominant. Use of 2 instruments and 2 field strengths allowed the integrity of the spectra and the assignment of 5-coordinate A1 to be verified. The "true" chemical shift (after a small correction for quadrupolar shift) observed for 5-coordinate A1 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 A1/Si < 2, and which are poor in octahedra relative to tetrahedra, may also have significant Al in 5-coordinate sites.

The Effect of Dissolved Humic Acids on Aluminosilicate Formation and Associated Carbon Sequestration

“…These values are the observed peak positions at 11.7 T. The peaks for 4-and 6-coordinated Al (Al IV and Al VI ) are similar to those reported for natural imogolite and allophane [29,36] and for synthetic amorphous hydroxyaluminosilicates [38]. In addition to Al IV and Al VI , a peak for 5-coordinated Al (Al v ) from allophane similar to that observed here has been reported [40] but would have been difficult to detect at the slower spinning rates used in the earlier studies. The integrated intensities of the three resonances were estimated by fitting the center 29 Si NMR, corresponding to the integrated relative intensity of the peak at −78.3 ppm.…”

Theoretical and 27Al CPMAS NMR investigation of aluminum coordination changes during aluminosilicate dissolution

“…Kubicki and Toplis (2002) found that calculations using HF/6-311ϩG(d,p) reproduced 27 Al NMR shifts more accurately than B3LYP/6-311ϩG(d,p). In an analogous manner, smaller chemical shifts, closer to those reported by Tsomaia et al (2003) for [4] Al and [6] Al and by Childs et al (1999) for [5] Al are calculated in this study with HF/6-311ϩG(d,p) rather than B3LYP/ 6-31G(d).…”

“…Although five-fold Al occurs in ordered aluminosilicate minerals (Burnham and Buerger, 1961;Alemany et al, 1991;Jansen et al, 1998), in disordered allophane, (Childs et al, 1999), and in aluminosilicate glasses (Li et al, 1995;Yarger et al, 1995;Risbud et al, 1987;Schaller and Stebbens, 1998;Stebbins et al, 2000), Tsomaia et al (2003) obtained no evidence for [5] Al on the surfaces of aluminosilicate glasses or minerals using NMR spectroscopy. Therefore, there is no experimental evidence to support the computational results that suggest [5] Al is a stable species on the surface of dissolving feldspars.…”

Accelerated Leach Testing of GLASS (ALTGLASS): I. Informatics approach to high level waste glass gel formation and aging

“…Amorphous or poorly crystalline allophane (Al 2 O 3 Á1.3-2SiO 2 ÁxH 2 O) is a disordered aluminosilicate with Al in 5-fold coordination-recall from section 2.1 that Al coordination in solutions and gels is strongly pH dependent. 83 In situ development of Al in 5-or 6-fold coordination has been identified as one potential form of aluminum formed during leaching of feldspar in acidic environments by Criscenti. 84 Hisingerite (Fe 2 O 3 Á2SiO 2 ÁxH 2 O) is an amorphous or poorly crystalline iron silicate that is isostructural with allophane.…”