Features of phenacite mineralization from the Ural emerald mines

The authors consider the problems of development of phenacite mineralization at the Ural Emerald Mines, which is rather well developed and described in the Mariinsky (Malyshevsky) and Sretensky (Sverdlovsk) emerald-beryllium deposits. Phenacite is widespread in many beryllium deposits, but crystals of jewelry quality, with such large sizes as at the Emerald Mines, form rarely. Despite the prescription of the discovery (1833), and because of the rare occurrence of jewelry quality of crystals, and the presence of more expensive and valuable stones – emeralds and alexandrites – in deposits of the Emerald mines, phenacite remains almost unknown in the precious stones market, and especially abroad. Phenacite mineralization mostly occurs in the micaceous veins represented by gray and greenish-gray phlogopite. Distribution of phenacite in the micaceous veins is extremely uneven. Mineralization is typically nesting. High content of phenacite appears in the micaceous veins, mineral composition of which is mostly phlogopite, veins and concretions of beryllium-containing margarite (B-margarite) and chlorite. Content of phenacite is low in the micaceous veins that include phlogopite, plagioclase, beryl, fluorite, smoky quartz. At the Sretensky deposit is located a vein that refers to a new type of ore bodies of the chrysoberyl-phenacite composition lying in ultrabasic rocks. Unlike emerald-bearing micaceous veins that have a northwestern spread, the chrysoberyl-phenacite ore bodies are oriented in the near-latitudinal direction and have a northern incidence at an angle of 75°–80°. The most common form of phenacite crystals on the Emerald Mines is rhombohedral and short columned. Crystals have a large number of faces. The usual shapes are a hexagonal prism and rhombohedrons. Twin crystals are common, druses, columnar aggregates, and spherulites are characteristic. Phenacite can be colorless or slightly colored in wine yellow, sometimes pinkish, light gray, white, and rarely brown. The color in the crystal can spread non-uniformly. Wine-yellow coloration is not stable; it completely disappears in the light. Most common inclusions in phenacite are clinochlor, ilmenite and pyrrhotite. The work presents spectra of infrared and optical spectroscopy for phenacites of various colors. In addition, the authors show the similarity of gemological properties of the Ural phenacites and crystals from Sri Lanka