Genesis of Heterostructures of the Brazilian Noble Opal

摘要

Regular structures of noble opal are commonly represented by a close packing of monodisperse spherical silica particles (MSSP) of the same dimensions. The diameter of particles varies from ～0.10 to ～0.35 μm, determining the lattice parameters and prevalent diffraction play of color from blue to red. As a rule, the homogeneity of MSSP dimensions is retained within large samples or fragments of opal-bearing rock cut by opal veinlets. This implies the monodispersivity of the initial MSSP suspension in compliance with the specific conditions of its origin. However, the samples of Brazilian noble opal described in 1978 revealed the regular packing of MSSP of two dimensions, where the large particles (r = 90 nm) are systematically dispersed among the small particles (r = 50 nm), and the small particles also exhibit a complex ordering. The origin of such a complex heterostructure remains ambiguous. It is suggested that MSSPs of two dimensions could initially originate in suspension, then, still in the colloidal state, become ordered due to the interaction between particles, and then jointly precipitate into a crystal-like structure. However, it is impossible to understand how it was that MSSPs of two different dimensions could form together. The experimental attempts to crystallize MSSP mixtures of two dimensions were also unsuccessful.