Zapiski RMO (Proceedings of the Russian Mineralogical Society)Zapiski RMO (Proceedings of the Russian Mineralogical Society)0869-60552658-4352The Russian Academy of Sciences1130510.30695/zrmo/2019.1481.08Research ArticleTschermigite from thermal fields of Southern Kamchatka: high-temperature transformation and peculiarities of IR-spectrumZhitovaE. S.zhitova_es@mail.comSergeevaA. V.zhitova_es@mail.comNuzhdaevA. A.zhitova_es@mail.comKrzhizhanovskayaM. G.zhitova_es@mail.comChubarovV. M.zhitova_es@mail.comInstitute of Volcanology and Seismology FEB RASSaint Petersburg State University0503201914811001160403201904032019Copyright © 2019,2019<p>Tschermigite, (NH<sub>4</sub>)Al(SO<sub>4</sub>)<sub>2</sub> ∙ 12H<sub>2</sub>O, is a widespread mineral in efflorescence at geothermal fields associated with Kambalny and Koshelevsky volcanoes (Southern Kamchatka, Russia). The determined temperature ranges of stability of tschermigite and products of its high-temperature evolution by <em>in</em> <em>situ</em> powder X-ray diffraction: tschermigite (2360C) X-ray amorphous phase (70200 C) godovikovite (210390C) millosevichite (380620C). This implies decomposition of tschermigite (2360C) first to the interim X-ray amorphous phase (probably melt) from which godovikovite is then formed, without direct transformation of tschermigite to godovikovite, as it was suggested in some papers previously. Deammonization of godovikovite to millosevichite occurs at 380390C. Thermal expansion coefficients are given for tschermigite, godovikovite and millosevichite with discussion on some peculiarities on their formation. Regarding IR bands that refer to ammonium ion: <sub>3</sub> (~1430 cm<sup>1</sup>) split, <sub>1</sub> (~3100 cm<sup>1</sup>) active, what attests about at least partial shift of N (NH<sub>4</sub><sup>+</sup>) in tschermigite crystal structure from site 4<em>b</em> (<em>S</em><sub>6</sub>) to site 8<em>c</em> (<em>С</em><sub>3</sub>). In OH stretching region, higher wavenumber band (~3380 cm<sup>1</sup>) corresponds to extraframework water, while the observed band ~2950 cm<sup>1</sup> corresponds to water coordinating Al.</p>tschermigitegodovikovitemillosevichitehigh-temperature crystal chemistryIR-spectroscopyчермигитгодовиковитмиллозевичитвысокотемпературная кристаллохимияИК-спектроскопия[Abdeen A. M., Will G., Schäfer W., Kirfel A., Bargouth M. O., Recker K. X-ray and neutron diffraction study of alums. Z. Krist. 1981. Vol. 157. P. 147-166.][Ballirano P. 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