Thermodynamic Properties of Coquimbite and Aluminocoquimbite

Abstract

Coquimbite AlFe3+3[SO4]6(H2O)12⋅6H2O (sample from the Javier Mine, Peru) has been studied by thermal and electron microprobe analysis, X-ray powder diffraction, Raman spectroscopy, and Mössbauer spectroscopy. The enthalpy of formation of the coquimbite from elements ∆fH0(298.15 K) = −11 118 ± 40 kJ/mol was determined by the method of solution calorimetry in melt of lead borate 2PbO∙B2O3 on a Setaram (France) Calvet microcalorimeter. The value of its absolute entropy S0(298.15 K) = 1248.3 ± 3.0 J/(mol K) was estimated, the entropy of formation ∆fS0(298.15 K) = − 5714.0 ±3.0 J/mol K), and the Gibbs energy of formation from elements ∆fG0(298.15 K) = −9411 ± 40 kJ/mol were calculated. The values of the enthalpy and Gibbs energy of formation of aluminocoquimbite Al2Fe3+2[SO4]6(H2O)12⋅6H2O from elements were estimated at −11 540 ± 29 and −9830 ± 29 kJ/mol, respectively.