Structural studies of Silica Springs allophane

DSIR, Div. Land and Soil Sci., Lower Hutt, New Zealand

The structure of Silica Springs allophane from New Zealand has been studied by high-resolution ²⁷ Al and ²⁹ Si NMR spectroscopy, IR spectroscopy and electron microscopy. Experimental NMR conditions were chosen to give a quantitative relationship between signal strength and Al content. Aluminium is present in both octahedral (Al ^vi ) and tetrahedral (Al ^iv ) coordination, and the ratio Al ^iv :Al ^iv was obtained for three samples with different total Al:Si ratios. In all cases the Al ^iv was apparently balanced by positive charge at adjacent Al ^vi sites. Both NMR and IR spectra indicate that Si exists in a condensed network, probably a sheet, with Si atoms linked (through oxygen) to one, two and, possibly, three other Si atoms, but not four. The spectra are consistent with a mixture of Si-O-Si, Si-O-Al, and Si-OH linkages. A structure based on fragments of single curved 1:1 aluminosilicate layers, with the tetrahedral sheet on the outer surfaces, is suggested, the diameter of curvature being 2-3 nm. The tetrahedral sheet is more or less complete and has a Al ^iv :Si ratio of 1:3; the octahedral A1 ^vi sheet is incomplete. Electron micrographs suggest that, at least in some cases, the fragments form more or less complete spherules. This proposal seems to accord with Farmer's concept of "aluminous hydrous feldspathoids". The minimum pH at which Silica Springs allophane has precipitated corresponds to the initial presence of tetrahedrally coordinated Al species in solution.

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