A calorimetric and thermodynamic investigation of the synthetic analogue of chalcomenite, CuSeO3 center dot 2H(2)O

作者：Charykova Marina V; Lelet Maxim I; Krivovichev Vladimir G^*; Ivanova Natalya M; Suleimanov Evgeny V

来源：European Journal of Mineralogy, 2017, 29(2): 269-277.

DOI：10.1127/ejm/2017/0029-2595

• chalcomenite
• copper selenite hydrate
• thermodynamic properties
• adiabatic calorimetry
• heat capacity
• entropy
• enthalpy of formation
• Gibbs energy of formation

Thermophysical and thermochemical calorimetric investigations were carried out on the synthetic analogue of chalcomenite. The synthesis of CuSeO3.2H(2)O was realized from copper nitrate and sodium selenite solutions heated to 80 degrees C for 2 to 3 h. The precipitate was characterized by X-ray powder diffraction, X-ray fluorescence, IR spectroscopy, and Ramanspectroscopy. The low-temperature heat capacity of CuSeO3.2H(2)O was measured using adiabatic calorimetry between 5 and 323 K, and the third-law entropy was determined. A value of S degrees(298 K, CuSeO3.2H(2)O, cr) = 181.0 +/- 1.0 J/(Kmol) was obtained with an uncertainty of 0.5%. The enthalpy of formation of CuSeO3.2H(2)O was determined by solution calorimetry with H2SO4 solution as the solvent, giving Delta H-f degrees(298 K, CuSeO3.2H(2)O, cr)=-1035.3 +/- 4.9 kJ/mol. The Gibbs energy of formation for CuSeO3.2H(2)Oat T=298 K, 1 atmcan be calculated on the basis on Delta H-f degrees (298 K) and Delta S-f degrees(298 K): Delta(f)G degrees(298 K, CuSeO3.2H(2)O, cr) =-835.3 +/- 5.3 kJ/mol. SmoothedC(p)degrees(T) values betweenT=0 K and T=320K for CuSeO3.2H(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 value of Delta(f)G degrees for CuSeO3.2H(2)Owas used to calculate Eh-pH diagrams of the Cu-Se-CO2-H2O system. The behaviour of selenium and copper in the surface environment has 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 their precipitation in the form of various solid phases.

• 出版日期2017-4