Contents lists available at ScienceDirect Chemie der Erde journal homepage: www.elsevier.com/locate/chemer Thermal behavior of ferric selenite hydrates (Fe 2 (SeO 3 ) 3 ·3H 2 O, Fe 2 (SeO 3 ) 3 ·5H 2 O) and the water content in the natural ferric selenite mandarinoite Astrid Holzheid a, ⁎ , Marina V. Charykova b , Vladimir G. Krivovichev b , Brendan Ledwig a , Elena L. Fokina b , Ksenia L. Poroshina b , Natalia V. Platonova b , Vladislav V. Gurzhiy b a Institute of Geosciences, Kiel University, 24098 Kiel, Germany b Saint Petersburg State University, 7/9 Universitetskaya nab., St. Petersburg, 199034 Russia ARTICLE INFO Handling Editor: Juraj Majzlan Keywords: Selenites Mandarinoite Ferric selenite hydrates Thermogravimetry Differential scanning calorimetry ABSTRACT Any progress in our understanding of low-temperature mineral assemblages and of quantitative physico-che- mical modeling of stability conditions of mineral phases, especially those containing toxic elements like sele- nium, 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 mi- nerals 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 re- garding 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 re- maining 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 Fe 2 (SeO 3 ) 3 ·(6-x)H 2 O(x = 0.0–1.0) as formula of the hydrous ferric selenite mandarinoite. The total amount of water apparently affects the crystal- linity, and possibly the stability of crystals: the less the x value, the higher crystallinity could be expected. 1. Introduction Interest in selenium behavior under supergene conditions has in- creased owing to needs of environmental protection. Near the surface, selenium occurs in oxidized states as selenites (M-[SeO 3 ] 2− ) and much less frequently as selenates (M-[SeO 4 ] 2− ). The basic sources of sele- nium in near-surface environments are oxidation zones of selenides (M- [Se] 2− ) or selenium-bearing sulfide deposits and technogenic waste products. The Earth's surface and near-surface regions are dominated by the interaction between minerals, aqueous solutions, and atmospheric gases. Such interfaces play an important role in a number of geological and geochemical processes. In an environmental context interfacial processes such as mineral dissolution and mineral precipitation are responsible for the release and/or confinement as well as the seques- tration of selenium and heavy metals that may eventually become pollutants in soils and groundwater. Depending on its concentration, selenium has a narrow concentration range between deficient and toxic levels in the diets of humans and animals (see for more details, e.g., review papers by Plant et al., 2003, 2014). Since the selenium species control Se mobility, bioavailability, and toxicity, it is essential to un- derstand which of the common oxidation states of selenium prevail in mineral phases under certain conditions and to disentangle the influ- ence of parameters such as pH, redox potential, microbial activity and the long-term impact of complexing and precipitating agents on the release of Se to the environment. Recently we have characterized the selenium oxysalts, selenites and selenates, as natural mineral systems, reviewed and evaluated the accuracy of thermodynamic values of the selenites formed during oxidation of selenides and selenium-bearing sulfide minerals by examining the various experimental parameters used, and proposed data on the thermodynamics of selenites at the https://doi.org/10.1016/j.chemer.2018.01.002 Received 27 September 2017; Received in revised form 1 January 2018; Accepted 16 January 2018 ⁎ Corresponding author. E-mail address: holzheid@min.uni-kiel.de (A. Holzheid). Chemie der Erde 78 (2018) 228–240 0009-2819/ © 2018 Elsevier GmbH. All rights reserved. T