CRYSTAL CHEMICAL FEATURES OF GREEN AND LIGHT-BLUE AMAZONITE AND GEOCHEMICAL ASPECTS OF THEIR FORMATION PROCESSES

Abstract

A study was carried out in order to identify the aspects of geochemical processes occurring during the formation of green and light-blue amazonite crystals. Green, light-blue and greenish-blue amazonites from rare-metals deposits of Ukraine (Perzhanske, Ukrainian Shield), rf (Gora Parusna, Ploskogirske, Kola Peninsula; Orlovske, Western Transbaikalia), and USA (Rutherford, Virginia) were investigated using X-ray luminescence (XRL), infrared (IR) spectroscopy and X-ray diffraction. The rock-forming microcline from the Perzhanske, Orlovske and Katuginske (rf) deposits was also studied. It was found that the multi-colored crystals of amazonite have similar degrees of Si/Al ordering. They are represented by the maximum microcline with 2t1 = 0.959-1.0. The various samples only differ significantly in their lead contents that range between 2000 and 10000 ppm in green amazonite and 200 ppm in light-blue colored crystals. Differences between untreated crystals and those annealed in air (1173 K) or under a stream of He (923 K) samples were observed in terms of the composition, ratio of optically active centers and oxygen-hydrogen defects. Their composition and concentration in the untreated natural crystals reflect the genesis conditions of parent rocks, and, first of all, the redox and fO2 conditions of water-containing fluids, which affect the isomorphism of plumbum in the structures of the green and light-blue crystals, mechanisms of crystal chemical compensation of Pb2+ ions and correlates with different degree of ferum oxidation (Fe3+ → Fe2+). Changes in redox and fO2 parameters of the mineral-forming fluid are the most important factors affecting the activity and acid-base properties in the residual water fluids, the process of reduction following 2H2O + 2e– → H2 + 2OH– or oxidation via 2H2O – 4e– → O2 + 4H+. A complex combination of these factors allows the formation of certain nanosized defects in the structures of the amazonite crystals. These include impurity ([Pb2+ – Pb+]3+) and impurity-vacancy (Pb2+ – VK) clusters that serve as chromophore centers for green and light-blue color, respectively.