X-ray diffraction and ²⁹ Si magic-angle-spinning NMR of opals; incoherent long- and short-range order in opal-CT

X-ray diffraction and ²⁹Si masnmr experiments have been carried out to assess the structure of amorphous and quasi-crystalline natural and synthetic opals. We collected ²⁹Si masnmr spectra for nine opals, which, according to their X-ray signature, belong to either category A (amorphous) or CT (cristobalite-tridymite). All nmr spectra of natural opals have chemical shifts of about – 112 ppm, characteristic of amorphous silica, and resemble in peak shape amorphous silica most and, for different reasons, silica gel and quartz least. The full width at half maximum (FWHM) of the nmr spectra increases in the order opal-CT, opal-A, and amorphous silica. None of the opal nmr spectra can be mimicked by combining those of the crystalline-silica polymorphs. Comparisons between X-ray and ²⁹Si masnmr spectra indicate that long-range order suggestive of cristobalite-tridymite intergrowth is not strongly coupled to short-range ordering. There is a distinct difference in ²⁹Si chemical shift between the eight natural opals and the synthetic one. Synthetic opal has a maximum chemical-shift peak intensity at -107.0 ppm, close to that for quartz (–107.4 ppm), in contrast to natural opals, whose maximum chemical-shift peak intensity is around -112 ppm, close to the average chemical-shift value of tridymite (–111.0). Because of this quartz-like character, synthetic opal has a higher density and refractive index in comparison to natural samples. It is an important characteristic of nmr spectroscopy that it can distinguish between amorphous substances with similar X-ray patterns.