Spherulite-shaped cristobalite by fused silica devitrification

A new form of cristobalite, collected in the form of spheroidal particles as the result of hot corrosion of clear fused silica ampoules, has been studied by X-ray diffraction, scanning and transmission electron microscopy. The transparent or translucent spherulites have a round but flattened shape, one vortex and a core made of nanosheets, perpendicular to the surface, organized in a nearly radial pattern. The electron diffraction study evidenced a single phase material with a pseudo-cubic supercell (asp≈bsp≈csp≈14.1 Å and a F-type centred lattice) corresponding to an eight-fold multiplicity of the β cristobalite cell. The TEM study evidenced twinning domains and intergrown polytypes, similar to those usually observed in the β→α cristobalite-type transition and cristobalite→tridymite modifications. A formation mechanism of these cristobalite spherulites is proposed on the basis of a salt attack by a doping effect and the formation of droplets from a liquid state of silica.