Improvement of color characteristics of Kedrovka’s Fancy sapphires by thermal treatment

Nezametnoe deposit is the largest occurrence of jewelry-quality sapphires in Russia; it belongs to Kedrovka River Placer in the Primorsky Krai. The minerals are gold and jewelryquality corundums with a wide color range, including Fancy sapphires of various colors, although most colors are plain grey and jade. The experimental research aimed to improve color characteristics of these sapphires by thermal treatment in different gas environments at the temperature of 1600 °С demonstrated the color modification potential. It was found that yellow color reduced and changed into green and blue owing to formation of optically active centers Fe2+–Fe3+ and Fe2+–Ti4+ in the reducing environment. The color change mechanism is proved by the data on chemistry and optical spectra of sapphires. Moreover, the experiments revealed weakened observability of flaws in sapphires as well as their higher clarity, which is connected, among other things, with dissolving and fragmentation of rutile needles in crystal structure of the corundums. The phase transformation of rutile and the formation of characteristics artefacts implies a certain effect and can be used in gemological examination.

Akhmetshin E. A., Savina E. I., Plechov P. Yu., Petrova O. B. Improvement of color characteristics of Kedrovka’s Fancy sapphires by thermal treatment. MIAB. Mining Inf. Anal. Bull. 2022;(2):30-50. [In Russ]. DOI: 10.25018/0236_1493_2022_2_0_30.

E.A. Akhmetshin¹, Assistant Professor, e-mail: 4966945@mail.ru,
E.I. Savina², employee; Student, e-mail: elizaveta.sawina@gmail.com, Lomonosov Moscow State University, Moscow, Russia,
P.Yu. Plechov², Dr. Sci. (Geol. Mineral.), Professor of the Russian Academy of Sciences, Director, e-mail: pplechov@gmail.com,
O.B. Petrova¹, Dr. Sci. (Chem.), Professor, e-mail: petrova@muctr.ru, ORCID ID: 0000-0001-7828-1058,
¹ D. Mendeleev University of Chemical Technology of Russia (MUCTR) 125480, Moscow, Russia,
² Fersman Mineralogical Museum of Russian Academy of Sciences, 119071, Moscow, Russia,

E.I. Savina, e-mail: elizaveta.sawina@gmail.com.

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