Natural coarse-grained precious opal structures result from the concentration of suspensions of monodisperse silica spheres and their subsequent supramolecular crystallization at low pH values. These processes occur in different media and at different times. Nucleation is determined by the diffusion of structural units at the interface, their large volume, and the insignificant incorporation of the system into the metastable zone. On synthesis of precious opal, the rate of supramolecular crystallization is determined by sedimentation of structural units, and the frequency of crystal seeding depends mainly on the viscosity of the dispersed medium.
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