Isotopic fractionation of sulfur during formation of basaluminite, alunite, and natroalunite

The extent of fractionation of sulfur (S) isotopes during precipitation of basaluminite and alunite in supersaturated Al2(SO4)3 solutions was determined in a laboratory synthesis experiment. An acid 0.05 M Al2(SO4)3 solution was partly neutralized with Ca(OH)2, NaOH, and KOH to a molar OH/Al ratio of 2.0. Subsequent aging of the precipitates for 15 weeks at temperatures of 20 and 50 °C resulted in the formation of poorly crystalline basaluminite [Al4(SO4)(OH)10·4H2O], of mixtures of basaluminite with crystalline natroalunite [NaAl3(SO4)2(OH)6] or alunite [KAl3(SO4)2(OH)6], and of pure crystalline alunite. No significant isotopic fractionation of S was observed during the precipitation of poorly crystalline basaluminite (Δδ34Sbasaluminite-solution=+0.04±0.34‰). In contrast, the formation of alunite and natroalunite was accompanied by small but significant isotopic fractionation that favored the incorporation of the heavier isotope, 34S, in the minerals. Sulfur-isotope fractionation (Δδ34Smineral-solution) was +0.84±0.34‰ for alunite, and +1.72±0.34‰ for natroalunite. The extent of S-isotope fractionation was found to be dependent on (i) the chemical composition and (ii) the crystal structure of the synthesized minerals, and seems to be related to the physico-chemical properties of the involved ions (charge, radius, mass), the energy balance of the reaction, and the reaction rate. The results indicate that S-isotope mass balances in aerated acid soils are not significantly confounded by S isotopic fractionation effects associated with the formation of Al hydroxysulfates.