STRUCTURAL STUDIES OF TABASHEER, AN OPAL OF PLANT-ORIGIN

Two samples of hydrated silica from the bamboo plant ('tabasheer' and 'bambusa') were examined by X-ray fluorescence and H-1, Al-27 and Si-2 9 solid-state NMR, X-ray diffraction and thermogravimetric analysis. S ilicon is the major elemental component of both samples, with much sma ller amounts of aluminium (about 1.2 wt%), iron (0.3-0.4 wt%), calcium and magnesium, and traces of phosphorus. The maximum theoretical wate r loss calculated from the OH/Si ratio derived from the Si-29 NMR spec tra (4.92 and 3.82 wt% for the two samples) agrees with the weight los s measured by thermogravimetric analysis. Adsorbed water is evolved at temperatures below about 250 degrees C, which is followed by gradual dehydroxylation. The water content of tabasheer is greater than that o f bambusa, so that the initial rate of weight loss in tabasheer is hig her. However, above about 250 degrees C the rate of weight loss in bot h samples is very similar and the thermogravimetric curves are virtual ly parallel. The weight loss on dehydroxylation agrees with that calcu lated from the OH content obtained by Si-29 magic-angle spinning NMR. The presence of exclusively 4-coordinate (tetrahedral) aluminium in ta basheer shows that Al is part of the silicate network. Combined with t he almost complete absence of aluminium from intracellular silica, thi s leads us to suggest that the role of extracellular silicon in bamboo is to exclude aluminium from the plant cells. H-1 NMR spectra allow u s to identify two kinds of hydroxyl group and to estimate the change o f the proton-proton distance during dehydroxylation as a function of t emperature for each kind of hydroxyl.