Enthalpy of formation of natural hydrous iron phosphate: Vivianite

This paper presents the new data on thermodynamic properties of hydrous iron phosphate, vivianite. The thermochemical study of natural vivianite Fe2.322+Fe0.333+ Mg-0.35(PO4)(2)(OH)(0.33).7.67H(2)O (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, FFIR, Raman and Mossbauer spectroscopy. The value of Delta H-f(m)0(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 Fe-3(2+)(PO4)(2).8H(2)O were calculated as (571.0) J (K moI)(-1), -(5119 19) kJ mol(-1) and -(4439 +/- 19) kJ mol(-1) respectively. (C) 2017 Elsevier Ltd.