Thermal behavior of ferric selenite hydrates (Fe2(SeO3)3·3H2O, Fe2(SeO3)3·5H2O) and the water content in the natural ferric selenite mandarinoite.

Holzheid, Astrid, Charykova, Marina V., Krivovichev, Vladimir G., Ledwig, Brendan, Fokina, Elena L., Poroshina, Ksenia L., Platonova, Natalia V. and Gurzhiy, Vladislav V. (2018) Thermal behavior of ferric selenite hydrates (Fe2(SeO3)3·3H2O, Fe2(SeO3)3·5H2O) and the water content in the natural ferric selenite mandarinoite. Geochemistry, 78 (2). pp. 228-240. DOI 10.1016/j.chemer.2018.01.002.

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Abstract

Any progress in our understanding of low-temperature mineral assemblages and of quantitative physico-chemical modeling of stability conditions of mineral phases, especially those containing toxic elements like selenium, strongly depends on the knowledge of structural and thermodynamic properties of coexisting mineral phases. Interrelation of crystal chemistry/structure and thermodynamic properties of selenium-containing minerals is not systematically studied so far and thus any essential generalization might be difficult, inaccurate or even impossible and erroneous. Disagreement even exists regarding the crystal chemistry of some natural and synthetic selenium-containing phases. Hence, a systematic study was performed by synthesizing ferric selenite hydrates and subsequent thermal analysis to examine the thermal stability of synthetic analogues of the natural hydrous ferric selenite mandarinoite and its dehydration and dissociation to unravel controversial issues regarding the crystal chemistry. Dehydration of synthesized analogues of mandarinoite starts at 56–87 °C and ends at 226–237 °C. The dehydration happens in two stages and two possible schemes of dehydration exist: (a) mandarinoite loses three molecules of water in the first stage of the dehydration (up to 180 °C) and the remaining two molecules of water will be lost in the second stage (>180 °C) or (b) four molecules of water will be lost in the first stage up to 180 °C and the last molecule of water will be lost at a temperature above 180 °C. Based on XRD measurements and thermal analyses we were able to deduce Fe2(SeO3)3·(6-x)H2O (x = 0.0–1.0) as formula of the hydrous ferric selenite mandarinoite. The total amount of water apparently affects the crystallinity, and possibly the stability of crystals: the less the x value, the higher crystallinity could be expected.

Document Type: Article
Keywords: Selenites, Mandarinoite, Ferric selenite hydrates, Thermogravimetry, Differential scanning calorimetry
Research affiliation: Kiel University > Kiel Marine Science
OceanRep > The Future Ocean - Cluster of Excellence
Kiel University
Refereed: Yes
Open Access Journal?: No
DOI etc.: 10.1016/j.chemer.2018.01.002
ISSN: 0009-2819
Projects: Future Ocean
Date Deposited: 01 Aug 2018 08:57
Last Modified: 27 Mar 2019 11:31
URI: http://oceanrep.geomar.de/id/eprint/43883

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