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Cristobalite is a silica polymorph, which occurs in nature as white octahedra in volcanic rocks and as spherulites in converted diatomaceous earth deposits. It has a high silicon content, crystalline phase (tetragonal), high hardness, density of 2.45 g/cm3, and large specific surface area. These characteristics allow it to have a wide range of applications, such as high-grade paints and special silicone rubber.
The crystal structure of cristobalite is identical to that of quartz, but differs in the way that the tetrahedra are packed together. In b-cristobalite, tetrahedra are linked in a honeycomb structure, whereas in a-quartz, the tetrahedra are packed into a grid or lattice. As a result of this difference, the b-cristobalite crystal structure is less stable than the a-quartz.
As a consequence, the b-cristobalite framework will undergo a phase transition to either a-quartz or HP tridymite on cooling. During the phase transition, the corner-sharing SiO4 tetrahedra will shift their position in the pore space by static tilting, with a-quartz forming tetrahedra that are more tightly packed than b-cristobalite.
As such, a-cristobalite is a more unstable form and will rapidly convert to HP tridymite in the presence of aqueous acid. This means that a-cristobalite is unlikely to be the dominant crystalline phase in volcanic ash particles that reach the human respiratory tract. In addition, it has been observed that the cristobalite surfaces in the Soufriere Hills ash are occluded in a thin layer of annealed glass (known as the Beilby layer), and that heterogenous RCS surfaces will be produced during dome collapse fragmentation processes, resulting in a mixture of vapour-phase cristobalite and adhered feldspar or glass (theory 2). This could reduce exposure to cristobalite and thus the risk of respiratory hazards such as lung cancer mortality, non-malignant respiratory disease morbidity and silicosis.