A calorimetric and thermodynamic investigation of the synthetic analogs of cobaltomenite, CoSeO3{middle dot}2H2O, and ahlfeldite, NiSeO3{middle dot}2H2O

Abstract

Thermophysical and thermochemical calorimetric investigations were carried out on synthetic analogs of two minerals: cobaltomenite (CoSeO3·2H2O) and ahlfeldite (NiSeO3·2H2O). 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 CoSeO3·2H2O and NiSeO3·2H2O were measured using adiabatic calorimetry between 8 and 340 K, and the third-law entropies were determined. Values of S° (298 K, CoSeO3·2H2O, cr.) = 183.2 ± 1.0 J/(mol·K) and S° (298 K, NiSeO3·2H2O, cr.) = 172.9 ± 1.0 J/(mol·K) are obtained with an uncertainty of 0.5%. The enthalpies of formation for CoSeO3·2H2O and NiSeO3·2H2O were determined by solution calorimetry with H2SO4 solution as the solvent and giving ΔfH° (298 K, CoSeO3·2H2O, cr.) = -1135.3 kJ/mol, ΔfH° (298 K, NiSeO3·2H2O, cr.) = -1133.3 kJ/mol. The Gibbs energy of formation for CoSeO3·2H2O and NiSeO3·2H2O at T = 298 K, 1 atm can be calculated on the basis on ΔfH° and ΔfS°:ΔfG° (298 K, CoSeO3·2H2O, cr.) = -937.4 kJ/mol and ΔfG° (298 K, NiSeO3·2H2O, cr.) = -932.4 kJ/mol. Smoothed CP°(T) values between T = 0 K and T = 320 K for CoSeO3·2H2O (cr.) and NiSeO3·2H2O (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 values of ΔfG° for CoSeO3·2H2O and NiSeO3·2H2O 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.