Titre du document / Document title

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

Auteur(s) / Author(s)

FORRAY Ferenc Lazar ; DROUET Christophe ; NAVROTSKY Alexandra ;

Résumé / Abstract

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°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 Å, b = 5.151 ′ 0.001 Å, c = 7.875 ′ 0.001 Å, and β = 94.80°. Thermal decomposition indicates that the final breakdown of the yavapaiite structure takes place at 700°C (first major endothermic peak), but the decomposition starts earlier, around 500°C. The enthalpy of formation from the elements of yavapaiite, KFe(SO₄)₂, ΔH°_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°C (298 K) were calculated as S°(yavapaiite) = 224.7 ′ 2.0 J.mol^-1.K^-1 and ΔG°_f = -1818.8 ′ 6.4 kJ/mol. The equilibrium decomposition curve for the reaction jarosite = yavapaiite + Fe₂O₃ + H₂O has been calculated, at pH₂O = 1 atm, the phase boundary lies at 219 ′ 2°C.

Revue / Journal Title

Geochimica et cosmochimica acta   ISSN 0016-7037   CODEN GCACAK 

Source / Source

2005, vol. 69, n^o8, pp. 2133-2140 [8 page(s) (article)]

Langue / Language

Anglais

Editeur / Publisher

Elsevier, Amsterdam, PAYS-BAS  (1950) (Revue)

Localisation / Location

INIST-CNRS, Cote INIST : 5743, 35400012981339.0180