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Structural studies of tabasheer, an opal of plant origin

1998; Taylor & Francis; Volume: 77; Issue: 1 Linguagem: Inglês

10.1080/01418619808214238

0141-8610

Jacek Klinowski, Chi‐Feng Cheng, J. Sanz, J. M. Rojo, Alan L. Mackay,

Clay minerals and soil interactions

Abstract Two samples of hydrated silica from the bamboo plant ('tabasheer' and 'bambusa') were examined by X-ray fluorescence and 1H, 27Al and 29Si solidstate NMR, X-ray diffraction and thermogravimetric analysis. Silicon is the major elemental component of both samples, with much smaller amounts of aluminium (about 1·2 wt%), iron (0·3—0·4 wt%), calcium and magnesium, and traces of phosphorus. The maximum theoretical water loss calculated from the OH/Si ratio derived from the 29Si NMR spectra (4·92 and 3·82 wt% for the two samples) agrees with the weight loss measured by thermogravimetric analysis. Adsorbed water is evolved at temperatures below about 250°C, which is followed by gradual dehydroxylation. The water content of tabasheer is greater than that of bambusa, so that the initial rate of weight loss in both in tabasheer is higher. However, above about 250°C the rate of weight loss in both samples is very similar and the thermogravimetric curves are virtually parallel. The weight loss on dehydroxylation agrees with that calcualted from the OH content obtained by 29Si magic-angle spinning NMR. The presence of exclusively 4-coordinate (tetrahedral) aluminium in tabasheer shows that Al is part of the silicate network. Combined with the almost complete absence of aluminium from intracellular silica, this leads us to suggest that the role of extracellular silicon in bamboo is to exclude aluminium from the plant cells. 1H NMR spectra allow us to identify two kinds of hydroxyl group and to estimate the change of the proton—proton distance during dehydroxylation as a function of temperature for each kind of hydroxyl.