A calorimetric and thermodynamic investigation of the synthetic analogs of cobaltomenite, CoSeO3 center dot 2H(2)O, and ahlfeldite, NiSeO3 center dot 2H(2)O

Charykova, Marina V., Mrivovichev, Vladimir G., Lelet, Maksim I., Yakovenko, Oxana S., Suleimanov, Evgeny V., Depmeier, Wulf, Semenova, Viktorina V. and Zorina, Maina L. (2014) A calorimetric and thermodynamic investigation of the synthetic analogs of cobaltomenite, CoSeO3 center dot 2H(2)O, and ahlfeldite, NiSeO3 center dot 2H(2)O American Mineralogist, 99 (4). pp. 742-748.

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Abstract

Thermophysical and thermochemical calorimetric investigations were carried out on synthetic analogs of two minerals: cobaltomenite (CoSeO3 center dot 2H(2)O) and ahlfeldite (NiSeO3 center dot 2H(2)O). The synthesis was realized by mixing of aqueous solutions of cobalt and nickel nitrates, accordingly, and sodium selenite, acidified with the help of a solution of nitric acid and characterized by X-ray powder diffraction and FTIR spectroscopy methods. The low-temperature heat capacities of CoSeO3 center dot 2H(2)O and NiSeO3 center dot 2H(2)O were measured using adiabatic calorimetry between 8 and 340 K, and the third-law entropies were determined. Values of S degrees (298 K, CoSeO3 center dot 2H(2)O, cr.) = 183.2 +/- 1.0 J/(mol.K) and S degrees (298 K, NiSeO3 center dot 2H(2)O, cr.) = 172.9 +/- 1.0 J/(mol.K) are obtained with an uncertainty of 0.5%. The enthalpies of formation for CoSeO3 center dot 2H(2)O and NiSeO3 center dot 2H(2)O were determined by solution calorimetry with H2SO4 solution as the solvent and giving Delta H-f degrees (298 K, CoSeO3 center dot 2H(2)O, cr.) = -1135.3 kJ/mol, Delta H-f degrees (298 K, NiSeO3 center dot 2H(2)O, cr.) = -1133.3 kJ/mol. The Gibbs energy of formation for CoSeO3 center dot 2H(2)O and NiSeO3 center dot 2H(2)O at T = 298 K, 1 atm can be calculated on the basis on Delta fH degrees and Delta S-f degrees:Delta(f)G degrees (298 K, CoSeO3 center dot 2H(2)O, cr.) = -937.4 kJ/mol and Delta(f)G degrees (298 K, NiSeO3 center dot 2H(2)O, cr.) = -932.4 kJ/mol. Smoothed C-P degrees(T) values between T = 0 K and T = 320 K for CoSeO3 center dot 2H(2)O (cr.) and NiSeO3 center dot 2H(2)O (cr.) are presented along with values for S degrees and the functions [H degrees (T)-H degrees (0)] and [G degrees(T)-H degrees(0)]. These results motivate a re-evaluation of the natural conditions under which selenites, and selenates replace selenides, and sulfides in the oxidation zones of sulfide ore deposits or upon weathering of technologic waste. The values of Delta(f)G degrees for CoSeO3 center dot 2H(2)O and NiSeO3 center dot 2H(2)O were used to calculate the Eh-pH diagrams of the Co-Se-H2O and Ni-Se-H2O systems. These diagrams have been constructed for the average contents of these elements in acidic waters of the oxidation zones of sulfide deposits. The behavior of selenium, cobalt, and nickel in surface environments have been quantitatively explained by variations of the redox potential and the acidity-basicity of the mineral-forming medium. Precisely these parameters determine the migration ability of selenium compounds and its precipitation in the form of various solid phases.

Document Type: Article
Additional Information: Times Cited: 0 Lelet, Maxim/J-8031-2012; Charykova, Marina/I-4305-2014 0
Research affiliation: OceanRep > The Future Ocean - Cluster of Excellence
Kiel University
Refereed: Yes
ISSN: 0003-004X
Projects: Future Ocean
Date Deposited: 30 Mar 2015 12:04
Last Modified: 28 Mar 2018 10:28
URI: http://eprints.uni-kiel.de/id/eprint/27234

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