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Annotation The relevance of the work is due to the need to study the Ural gems, including those discovered in the Urals. This is important not only from a scientific point of view, but for the history of science and the development of geological tourism in our region as well. Purpose of the work: study of the chemical composition of demantoid from the Telyansky Klyuch deposit, which is the site of the first find of this gem in the Urals. Research methodology. Demantoid (green jewelry andradite) was washed by the author in the Telyansky Klyuch gold placer; the quantitative analysis of the chemical composition of garnet was performed using the JXA-733 X-ray electron probe microanalyzer. Results. A historical study of the sites of finds of demantoids in the Middle Urals was carried out. It was established that precious garnet was found not in placers of the Bobrovka river located south of Nizhny Tagil, but in the gold placer of the Telyansky Klyuch to the north of Nizhny Tagil. Microprobe analysis of individual demantoid crystals showed that garnets with different green color saturation according to the main components are andradites with average contents: CaO – 33,18%; Fe2O3 – 30,74%; SiO2 – 35,02% (for 6 analyzes). The feature of the chemical composition of Telyansky demantoids is the absence or insignificant amount of aluminum impurity in them – not more than 0.02% Al2O3, which is practically at the level of microprobe analysis sensitivity for this element. The amount of chromium varies from 0.02% Cr2O3 in a light green demantoid to 0.5–0.74% Cr2O3 in green and bright green andradites. Conclusions. The place of the first find of low-impurity and chromium-containing andradites in the Urals with the name of “diamantoite” (proposed by N. G. Nordenskiöld ) is the Telyansky Klyuch gold deposit on the East Tagil dunite-harzburgite massif in the Middle Urals.

Demantoid from the Telyansky Klyuch deposit (Middle Urals) – is the place of the first discovery in the Urals