Thermochemistry of yavapaiite KFe(SO4)(2): formation and decomposition

摘要

Yavapaiite, KFe(SO4)(2), is a rare mineral in nature, but its structure is considered as a reference for many synthetic compounds in the alum supergroup. Several authors mention the formation of yavapaiite by heating potassium jarosite above ca. 400 degrees C. To understand the thermal decomposition of jarosite, thermodynamic data for phases in the K-Fe-S-O-(H) system, including yavapaiite, are needed. A synthetic sample of yavapaiite was characterized in this work by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and thermal analysis. Based on X-ray diffraction pattern refinement, the unit cell dimensions for this sample were found to be a = 8.152 +/- 0.001 angstrom, b = 5.151 +/- 0.001 angstrom, c = 7.875 +/- 0.001 angstrom, and P = 94.80'. Thermal decomposition indicates that the final breakdown of the yavapaiite structure takes place at 700 degrees C (first major endothermic peak), but the decomposition starts earlier, around 500 degrees C. The enthalpy of formation from the elements of yavapaiite, KFe(SO4)(2), Delta H degrees(f) = -2042.8 +/- 6.2 kJ/mol, was determined by high-temperature oxide melt solution calorimetry. Using literature data for hematite, corundum, and Fe/Al sulfates, the standard entropy and Gibbs free energy of formation of yavapaiite at 25 degrees C (298 K) were calculated as S'(yavapaiite) = 224.7 +/- 2.0 J.mol(-1).K-1 and Delta G degrees(f) = -1818.8 +/- 6.4 kJ/mol. The equilibrium decomposition curve for the reaction jarosite = yavapaiite + Fe2O3 + H2O has been calculated, at pH(2)O = 1 atm, the phase boundary lies at 219 +/- 2 degrees C. Copyright (c) 2005 Elsevier Ltd