Abstract:
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 (Δδ34 Sbasaluminite-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 (Δδ34 Smineral-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. © 2004 Elsevier B.V. All rights reserved.
