Structural studies of tabasheer, an opal of plant origin

Two samples of hydrated silica from the bamboo plant ( t̀abasheer’ and `bambusa’ ) were examined by X-ray ̄ uorescence and H, Al and Si solidstate NMR, X-ray di€ raction and thermogravimetric analysis. Silicon is the major elemental component of both samples, with much smaller amounts of aluminium (about 1 2́wt%), iron (0 3́± 0 4́wt%), calcium and magnesium, and traces of phosphorus . The maximum theoretical water loss calculated from the OH/Si ratio derived from the Si NMR spectra (4 9́2 and 3 8́2wt% for the two samples) agrees with the weight loss measured by thermogravimetric analysis. Adsorbed water is evolved at temperatures below about 250C, 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 tabasheer is higher. However, above about 250C 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 calculated from the OH content obtained by Si 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. H 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. §