The Structure and Thermal Transformations of Allophanes Studied by ²⁹Si and ²⁷Al High Resolution Solid-State NMR

Abstract

Abstract--Examination of two volcanic and two precipitated allophanes by solid-state NMR, thermal analysis and X-ray powder diffraction shows three of the samples to contain structural features similar to both tubular imogolite and defect layer-lattice aluminosilicates such as kaolinite. The fourth allophane, a precipitated sample from New Zealand, had no imogolite-like features and contained tetrahedral as well as octahedral aluminum. The imogolite-like units in allophane are less stable thermally than tubular imogolite. The NMR spectra and their changes on heating can be accounted for by a structural model in which a two-sheet, kaolinite-like structure containing defects (boles in the tetrahedral sheet) is curved into a sphere in which imogolite-like orthosilicate units are anchored into the octahedral sheet and fit into the tetrahedral defects. Computer simulation shows that the model is crystallographically sound, and accounts for all the known facts, including the spherical morphology, the solid-state NMR spectra and the thermal dehydroxylation behavior of all except the New Zealand aUophane, which is of a different structural type.

27Al Mas NMR and Aluminum X-Ray Absorption Near Edge Structure Study of Imogolite and Allophanes

“…Because of this difficulty, NMR gives more accurate estimates of the AFV/A1 t~ ratio at low A1TM contents. However, the experimental conditions used in recording the NMR spectra are important: At lower magnetic field and MAS spinning frequency than in this study, Mackenzie et al, (1991) did not unequivocally determine the presence of A1TM in Db, KiP and VA allophanes, due to the overlap of NMR AF v resonance with AlVX-spinning sidebands.…”

Mineral control of carbon pools in a volcanic soil horizon

“…However, by decomposition of the spectrum, an additional resonance centred at about −90 ppm was observed. It could correspond to a small amount of silicon tetrahedra in the allophane structure (Goodman et al, 1985;MacKenzie et al, 1991;Denaix et al, 1999). In fractions 2.3-2.4 and 2.4-2.5, the spectra exhibited the same resonances, with the sharp peak at −78.7 ppm corresponding to an imogolite-like structure and the broad peak at around −96 ppm ascribed to feldspar Si.…”

“…This shift corresponds to Si in an imogolite-like structure, i.e. imogolite or proto-imogolite (SiO 4 4− unit coordinated with three Al and one OH) (Barron et al, 1982;Goodman et al, 1985;MacKenzie et al, 1991;Denaix et al, 1999;Hiradate and Wada, 2005). The broad higher field hump of low intensity around − 95 was ascribed to more polymerized silicon species.…”

Five-Coordinate Aluminum in Allophane

“…Five-coordinate A1 has not been reported in allophanes previously, but most of the relevant literature has been confined to descriptions of NMR spectra obtained with a medium magnetic field strength (Childs et al 1990;MacKenzie et al 1991), for which the signals from tetrahedral and octahedral sites are so broad that they overlap, or with MAS at 3 to 4 kHz (Goodman et al 1985), which is too slow for a clear distinction between centerband and spinning-sideband signals. A recent study combined a high magnetic field strength with fast MAS (He et al 1995), but the study was confined to a single sample of allophane that did not show 5-coordinate A1.…”