A calorimetric and thermodynamic investigation of the synthetic analogue of chalcomenite, CuSeO₃·2H₂O

Thermophysical and thermochemical calorimetric investigations were carried out on the synthetic analogue of chalcomenite. The synthesis of CuSeO₃·2H₂O was realized from copper nitrate and sodium selenite solutions heated to 80 °C for 2 to 3 h. The precipitate was characterized by X-ray powder diffraction, X-ray fluorescence, IR spectroscopy, and Raman spectroscopy. The low-temperature heat capacity of CuSeO₃·2H₂O was measured using adiabatic calorimetry between 5 and 323 K, and the third-law entropy was determined. A value of S°(298 K, CuSeO₃·2H₂O, cr) = 181.0 ± 1.0 J/(K mol) was obtained with an uncertainty of 0.5%. The enthalpy of formation of CuSeO₃·2H₂O was determined by solution calorimetry with H₂SO₄ solution as the solvent, giving Δ_fH°(298 K, CuSeO₃·2H₂O, cr) = −1035.3 ± 4.9 kJ/mol. The Gibbs energy of formation for CuSeO₃·2H₂O at T = 298 K, 1 atm can be calculated on the basis on Δ_fH° (298 K) and Δ_fS°(298 K): Δ_fG°(298 K, CuSeO₃·2H₂O, cr) = −835.3 ± 5.3 kJ/mol. Smoothed C_p°(T) values between T = 0 K and T = 320 K for CuSeO₃·2H₂O (cr) are presented along with values for S° and the functions [H°(T) – H°(0)]and[G°(T) – H°(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 Δ_fG° for CuSeO₃·2H₂O was 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.