Sherlova Gora (Sherlovaya Gora), Adun-Cholon Range, Nerchinsk Gem mines, Nerchinsk (Nertschinsk), Zabaykalsky Krai, Russia

Sherlova Gora granite massif, which hosts aquamarine greisen deposits, is exposed 20 km NE of the Adun-Cholon massif. The Adun-Cholon and Sherlova Gora massifs are part of the Kukul'beisky intrusive complex.

The deposit is famous for its beautiful aquamarine crystals, it was discovered in 1723 by the Nerchinsk Cossack, Ivan Gurkov, about 40 km NW of the town Borzya in the Chita oblast. Apogranite greisens of the deposit contain topaz, beryl, wolframite, cassiterite, arsenopyrite and bismuthinite. Greisen bodies have lenticular forms with bulges, which pinch and swell along their strike. They are grouped into elongated area, extending for several hundred meters with thicknesses of 50 to 80 m. The greisens are zoned.

The high-quality aquamarine is of great economic value. Topaz with few exceptions, is not suitable for jewellery purposes because of small sizes of the crystals and the abundance of inclusions and fractures.

The beryl of Sherlova Gora deposit has blue, bluish green, pale green, greenish yellow and golden colours, it may be colourless. Large crystals are characterized by colour zoning, which changes from a greenish core to blue or colourless rims. The length of the crystals is from several millimeters to 6 cm and rarely reaches 10 cm or more. The length to width ratio varies from 5:1 for aquamarine to 3:1 for yellow beryl (heliodor). The long-prismatic crystals display dominant development the hexagonal prism {10-10} and pinacoid {0001}, which may be flattened parallel to the prism. Transparent deep blue aquamarine crystals, 4 to 5 cm long and 0.5 to 1 cm across, are the most valuable. The RI for beryl is 1.565-1.581, density is 2.68-2.72 g/cm³. Common solid inclusions are goethite, hematite, adularia, siderophyllite, topaz and colourless beryl. Primary gas-liquid inclusions have needle-like and negative prismatic forms.

Topaz is extremely widespread in the deposit and forms sugary granular masses. Euhedral crystals are small in size (0.5 to 2 cm) and are intensely fractured and filled with inclusions. Specific variety of topaz, which contains numerous kaolin inclusions in its outer layer and resembles dental enamel is called "horse's tooth".

Ref: E.Ya. Kievlenko, Geology of gems, 2003, pp. 109, 116

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