Tschermigite from thermal fields of Southern Kamchatka: high-temperature transformation and peculiarities of IR-spectrum

Zhitova E. S., Sergeeva A. V., Nuzhdaev A. A., Krzhizhanovskaya M. G., Chubarov V. M.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2019. V. 148. N 1. P.100-116

https://doi.org/10.30695/zrmo/2019.1481.08

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Language: Russian 

Abstract

Tschermigite, (NH₄)Al(SO₄)₂ ∙ 12H₂O, 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 in situ powder X-ray diffraction: tschermigite (23—60°C) → X-ray amorphous phase (70—200 °C) → godovikovite (210—390°C) → millosevichite (380—>620°C). This implies decompositionof tschermigite (23—60°C) 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 millosevichiteoccurs at 380—390°C. Thermal expansion coefficients are given for tschermigite, godovikovite andmillosevichite with discussion on some peculiarities on their formation. Regarding IR bands thatrefer to ammonium ion: ν₃ (~1430 cm^–1) split, ν₁ (~3100 cm^–1) active, what attests about at least partial shift of N (NH₄⁺) in tschermigite crystal structure from site 4b (S₆) to site 8c (С₃). In O—H stretching region, higher wavenumber band (~3380 cm^–1) corresponds to «extraframework» water, while the observed band ~2950 cm^–1 corresponds to water coordinating Al.

Key words: tschermigite, godovikovite, millosevichite, high-temperature crystal chemistry, IR-spectroscopy