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A Calorimetric and Thermodynamic Investigation of the Synthetic Analogue of Mandarinoite, Fe_{2}(SeO_{3})_{3}·5H_{2}O

^{1}

Research Institute for Chemistry, Lobachevsky State University of Nizhny Novgorod, Gagarin Ave. 23, Nizhny Novgorod 603950, Russia

^{2}

Department of Geology, St. Petersburg State University, 7-9 University Embankment, Saint-Petersburg 199034, Russia

^{3}

Institute of Geosciences, Kiel University, 24098 Kiel, Germany

^{*}

Author to whom correspondence should be addressed.

Received: 24 August 2018 / Revised: 9 October 2018 / Accepted: 9 October 2018 / Published: 28 October 2018

(This article belongs to the Special Issue Dynamics and Kinetics of Melt-Fluid-Rock Interactions)

Thermophysical and thermochemical calorimetric investigations were carried out on the synthetic analogue of mandarinoite. The low-temperature heat capacity of ${\mathrm{Fe}}_{2}{({\mathrm{SeO}}_{3})}_{3}\xb75{\mathrm{H}}_{2}\mathrm{O}\left(\mathrm{cr}\right)$ was measured using adiabatic calorimetry between 5.3 and 324.8 K, and the third-law entropy was determined. Using these ${C}_{p,\mathrm{m}}^{\mathrm{o}}\left(T\right)$ data, the third law entropy at T = 298.15 K, ${S}_{\mathrm{m}}^{\mathrm{o}}$ , is calculated as 520.1 ± 1.1 J∙K ${C}_{p,\mathrm{m}}^{\mathrm{o}}\left(T\right)$ values between T → 0 K and 320 K are presented, along with values for ${S}_{\mathrm{m}}^{\mathrm{o}}$ and the functions $[{H}_{\mathrm{m}}^{\mathrm{o}}\left(T\right)-{H}_{\mathrm{m}}^{\mathrm{o}}\left(0\right)]$ and $[{\Phi}_{\mathrm{m}}^{\mathrm{o}}\left(T\right)-{\Phi}_{\mathrm{m}}^{\mathrm{o}}\left(0\right)]$ . The enthalpy of formation of ${\mathrm{Fe}}_{2}{({\mathrm{SeO}}_{3})}_{3}\xb75{\mathrm{H}}_{2}\mathrm{O}\left(\mathrm{cr}\right)$ was determined by solution calorimetry with HF solution as the solvent, giving ${\mathsf{\Delta}}_{\mathrm{f}}{H}_{\mathrm{m}}^{\mathrm{o}}(298\mathrm{K},{\mathrm{Fe}}_{2}\left({\mathrm{SeO}}_{3}{)}_{3}\xb75{\mathrm{H}}_{2}\mathrm{O},\text{}\mathrm{cr}\right)$ = −3124.6 ± 5.3 kJ/mol. The standard Gibbs energy of formation for ${\mathrm{Fe}}_{2}{({\mathrm{SeO}}_{3})}_{3}\xb75{\mathrm{H}}_{2}\mathrm{O}\left(\mathrm{cr}\right)$ at T = 298 K can be calculated on the basis on ${\mathsf{\Delta}}_{\mathrm{f}}{H}_{\mathrm{m}}^{\mathrm{o}}\left(298\mathrm{K}\right)$ and ${\mathsf{\Delta}}_{\mathrm{f}}{S}_{\mathrm{m}}^{\mathrm{o}}\left(298\mathrm{K}\right)$ : ${\mathsf{\Delta}}_{\mathrm{f}}{G}_{\mathrm{m}}^{\mathrm{o}}(298\mathrm{K},{\mathrm{Fe}}_{2}\left({\mathrm{SeO}}_{3}{)}_{3}\xb75{\mathrm{H}}_{2}\mathrm{O},\text{}\mathrm{cr}\right)$ = −2600.8 ± 5.4 kJ/mol. The value of Δ

^{−1}∙mol^{−1}. Smoothed_{f}G_{m}for Fe_{2}(SeO_{3})_{3}·5H_{2}O(cr) was used to calculate the Eh–pH diagram of the Fe–Se–H_{2}O system. This diagram has been constructed for the average contents of these elements in acidic waters of the oxidation zones of sulfide deposits. The behaviors of selenium and iron in the surface environment have been quantitatively explained by variations of the redox potential and the acidity-basicity of the mineral-forming medium. These parameters precisely determine the migration ability of selenium compounds and its precipitation in the form of solid phases. View Full-Text*Keywords:*mandarinoite; adiabatic calorimetry; heat capacity; entropy; enthalpy of formation; the Gibbs energy of formation

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**Supplementary File 1:**PDF-Document (PDF, 206 KB)

**MDPI and ACS Style**

Lelet, M.I.; Charykova, M.V.; Holzheid, A.; Ledwig, B.; Krivovichev, V.G.; Suleimanov, E.V. A Calorimetric and Thermodynamic Investigation of the Synthetic Analogue of Mandarinoite, Fe_{2}(SeO_{3})_{3}·5H_{2}O. *Geosciences* **2018**, *8*, 391.

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