Adun-Cholon Range, Nerchinsk Gem mines, Nerchinsk (Nertschinsk), Chitinskaya Oblast, Zabaykalsky Krai, Russia

Small quantities of gem beryl were extracted from the miarolitic pegmatites in the Adun-Cholon field, 45 km NW of the town of Borzya. This field is in the late Jurassic Adun-Cholon massif, which intrudes sandy schist rocks of the Lower Carboniferous Urtuiskaya series.

The Adun-Cholon massif is a porphyritic, coarse-grained granite, which grades into a central fine-grained porphyritic phase. There is a zonation of the pegmatite field, with differentiated vugless pegmatites closer to the massif margin, well zoned beryl-morion pegmatites with mineralized cavities in the middle zone, and small pegmatite schlieren in the central part of the massif.

Mineralized cavities of lenticular, equidimensional, and tube-like shape are located in the blocky zone near the quartz core. They contain euhedral crystals of morion and smoky quartz, beryl, schorl, muscovite, and clevelandite, along with sporadic topaz and fluorite.

Beryl crystals have columnar habits. Light green varieties with bluish or yellowish tint dominate. Heliodor and aquamarine are rare. The average crystal size is 5 x 1.5 cm, with a maximum of 20 x 8 cm. Refractive index is 1.570 - 1.579 and the density is 2.69 - 2.72 g/cm³. Typical solid inclusions are beryl, quartz, biotite, muscovite, and hematite. Primary gas-liquid inclusions with tube-like or negative crystal forms contain up to 40% gas phase.

Ref: E.Ya. Kievlenko, Geology of gems, 2003, p. 109 - 110

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