Enthalpy of formation of natural hydrous iron phosphate: Vivianite

•The enthalpy of dehydration and oxidation of natural vivianite was measured using Tian-Calvet microcalorimeter.•The enthalpies of formation of natural vivianite and vivianite Fe2+3(PO4)2·8H2O were determined by melt solution calorimetry.

This paper presents the new data on thermodynamic properties of hydrous iron phosphate, vivianite. The thermochemical study of natural vivianite Fe2.322+Fe0.333+ Mg0.35(PO4)2(OH)0.33·7.67H2O (Kerch iron ore basin, Republic of Crimea, Russia) was carried out on a high-temperature heat-flux Tian-Calvet microcalorimeter “Setaram” (France). The mineral was characterized by X-ray microprobe analysis, powder and single-crystal X-ray diffraction, thermal analysis, FTIR, Raman and Mössbauer spectroscopy. The value of ΔfHm0(T = 298.15 K) for natural vivianite was found to be −(5217 ± 11 kJ mol−1). The standard values of the entropy, enthalpy and Gibbs energy of formation at T = 298.15 K for vivianite of theoretical composition Fe32+(PO4)2·8H2O were calculated as (571.0) J (K mol)−1, −(5119 ± 19) kJ mol−1 and −(4439 ± 19) kJ mol−1 respectively.