THERMOCHEMISTRY OF JAROSITE-ALUNITE AND NATROJAROSITE-NATROALUNITE SOLID SOLUTIONS

Abstract

The thermochemistry of jarosite-alunite and natrojarosite-natroalunite solid solutions was investigated. Members of these series were either coprecipitated or synthesized hydrothermally and were characterized by XRD, FTIR, electron microprobe analysis, ICP-MS, and thermal analysis. Partial alkali substitution and vacancies on the Fe/Al sites were observed in all cases, and the solids studied can be described by the general formula K1-x-yNay(H3O)xFezAlw(SO4)2(OH)6–3(3-z-w)(H2O)3(3-z-w). A strong preferential incorporation of Fe over Al in the jarosite/alunite structure was observed. Heats of formation from the elements, ΔH°f, were determined by high-temperature oxide melt solution calorimetry. The solid solutions deviate slightly from thermodynamic ideality by exhibiting positive enthalpies of mixing in the range 0 to +11 kJ/mol. The heats of formation of the end members of both solid solutions were derived. The values ΔH°f = −3773.6 ± 9.4 kJ/mol, ΔH°f = −4912.2 ± 24.2 kJ/mol, ΔH°f = −3734.6 ± 9.7 kJ/mol and ΔH°f = −4979.7 ± 7.5kJ/mol were found for K0.85(H3O)0.15Fe2.5(SO4)2(OH)4.5(H2O)1.5, K0.85(H3O)0.15Al2.5(SO4)2(OH)4.5(H2O)1.5, Na0.7(H3O)0.3Fe2.7(SO4)2(OH)5.1(H2O)0.9, and Na0.7(H3O)0.3Al2.7(SO4)2(OH)5.1(H2O)0.9 respectively. To our knowledge, this is the first experimentally-based report of ΔH°f for such nonstoichiometric alunite and natroalunite samples. These thermodynamic data should prove helpful to study, under given conditions, the partitioning of Fe and Al between the solids and aqueous solution.