Hydrothermal precious opals of the Raduzhnoe deposit, north Primorye: The nature of the opalescence

Abstract: This paper reports the results of the study of hydrothermal precious opals. It was established that, in terms of their nanostructure and formation conditions, the hydrothermal opals differ from exogenic precious opals. They are made up of small globules and do not show any structuring of their nanoparticles on the basis of closest packing, which is typical of exogenic precious opals. During their formation, these opals were subjected to pneumatolytic annealing—they experienced the impact of a high-temperature vapor under elevated pressure. The influence of the thermal effect led to the formation of two-dimensional photonic band gaps in the chaotic opal matrix. These bands are composed of sheets whose cells were formed owing to the thermal effects according to the principle of “Benard cells.” Precisely these structured blocks and thin films are responsible for the spectral dispersion of light and iridescence.