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Sphere Packing Array Structure of Precious Opal

Project Member(s): Thomas, P.

Funding or Partner Organisation: The Australian Nuclear Science and Technology Organisation (Bragg Institute)

Start year: 2015

Summary: Precious opal is a natural hydrous silica (SiO2.nH2O) which is formed by a dissolution-precipitation process [1]. The process of formation is a multi-stage process which initially requires the dissolution of silica usually considered to be through the weathering of alkaline silicates such as feldspars. The silica solution produced increases in concentration and alkalinity until a supersaturated solution is produced which precipitates the silica through a homogeneous process of silicic acid polymerisation to produce a silica colloid. The colloidal particles grow in size with the continuous delivery of silica to the solution and, through a mechanism of Ostwald ripening, a monodispersed colloid of silica spheres is produced with particle diameters in the size range 150 to 300 nm in diameter. The next stage in the process is the concentration of the monodispersed silica colloid so that silica spheres can begin to aggregate. Aggregation can result in a random closed packed system yielding ┬┐potch┬┐ opal (non-precious opal with no play-of-colour (POC)) or in an ordered close packed array displaying POC. The final step in the process is the cementing of the opal microstructure through the introduction of a cementing silica which polymerises in the interstices and bonds the array of spheres into a rigid structure.

Keywords: Opal, USANS

FOR Codes: Chemical Sciences not elsewhere classified, Ceramics, Glass and Industrial Mineral Products not elsewhere classified