High-temperature transformation of tridymite single crystals to cristobalite*

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Abstract
The reconstructive high-temperature transformation of tridymite to cristobalite has been studied with X-ray precession photographs on single crystals. For this purpose inclusion-free and optically untwinned tridymite crystals, separated from a used refractory-grade silica brick were heat-treated between 1500 and 1620°C. The X-ray patterns of the heat-treated tridymite crystals show additional though weak reflections corresponding to those of the pseudo-orthorhombic tridymite TPsOrh-5. Diffuse intensity parallel [001]* (pseudo-orthohexagonal setting) indicates reconstructive stacking disorder of the tetrahedra layers of tridymite. According to the X-ray pictures the newly formed cristobalite crystallites are in a definite orientation to tridymite, with [112]c//[100]T, [[unk]l0]c//[010]T, and [111]C//[001]T [orthohexagonal setting of tridymite (T), and cubic setting of cristobalite (C)]. At the beginning of the transformation a one-sided structural orientation of cristobalite with respect to tridymite is observed. With the progress of transformation a second generation of cristobalite with alternate orientation occurs. Both orientations are convertible by rotation around [111]* of cristobalite (i.e. [001]* of tridymite) by 180°. The X-ray patterns of tridymite, being completely transformed to cristobalite exhibit alternate orientations of cristobalite again. The close structural relationship between tridymite and cristobalite probably causes a transformation with preservation of structural units. The reconstructive rearrangement of the “antiparallel” tetrahedral layer sequence in tridymite to the “parallel” layer sequence in cristobalite requires rotation and shifting of the tetrahedral layers which occurs by cooperative movements of layer fragments. Complete decomposition of the tridymite tetrahedral framework, and subsequent nucleation and growth of cristobalite is less probable.

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