Destinezite: physicochemical and calorimetric study

Destinezite (Fe3+ Al0.02)(PO4)0.99(SO4)0.90(OH)1.20 ⋅ 5.97H2O (Czech Republic) has been studied by thermal and electron-microprobe analyses, X-ray powder diffraction, IR, Raman, and Mössbauer spectroscopy. The enthalpy of formation of destinezite Fe3 +(PO4)(SO4)(OH)⋅6H2O from the elements ∆fH0(298.15 K) = −4258 ± 12 kJ/mol was determined by the method of solution calorimetry in melt lead borate 2PbO ⋅ B2O3 on a Calvet microcalorimeter Setaram (France). The value of its absolute entropy S0(298.15 K) = 462.0 J/(mol⋅K) was estimated, the entropy of formation ∆fS0(298.15 K) = −2054 J/(mol⋅K) and the Gibbs energy of formation from the elements ∆fG0(298.15 K) = −3646 kJ/mol.