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Chemical, crystallographic and isotopic analyses were made on samples containing alunite and jarosite from the sediments of four acid, hypersaline lakes in southeastern and southwestern Australia. The alunite and jarosite are K-rich with relatively low Na contents based on chemical analysis and determination of unit cell dimensions by powder X-ray diffraction. Correcting the chemical analyses of fine-grained mineral concentrates from Lake Tyrrell, Victoria, for the presence of halite, silica and poorly crystalline aluminosilicates, the following formulas indicate best estimates for solid-solution compositions: for alunite, K0.87Na0.04(H3O)0.09(Al0.92Fe0.08)3(SO4)2(OH)6 and for jarosite, K0.89Na0.07(H3O)0.04 (Fe0.80Al0.20)3(SO4)2(OH)6.

The delta-D-values of alunite are notably larger than those for jarosite from Lake Tyrell and it appears that the minerals have closely approached hydrogen isotope equilibrium with the acidic regional groundwaters. The delta-D results are consistent with a fractionation approximately 60-70 parts per thousand between alunite and jarosite observed in other areas. However, interpretation of delta-D results is complicated by large variability in fluid delta-D(H2O) from evaporation, mixing and possible ion hydration effects in the brine. Delta-D-values of water derived from jarosite by step-wise heating tend to be smaller at 250-degrees-C, at which temperature hydronium and other non-hydroxyl water is liberated, than at 550-degrees-C, where water is derived from the hydroxyl site, but the differences are not sufficiently different to invalidate measurements of total delta-D obtained by conventional, single-step heating methods.

Delta-S-34-values for alunite and jarosite from the four lakes (+19.7 to +21.2 parts per thousand CDT) and for aqueous sulfate from Lake Tyrrell (+18.3 to +19.8 parts per thousand) are close to the values for modern evaporites (+21.5 +/- 0.3 parts per thousand) and seawater (+20 +/- 0.5 parts per thousand) and are probably typical of seawater-derived aerosols in arid coastal environments. Delta-S-34-values slightly smaller than that for seawater may reflect a minor contribution of sulfate from pyrite oxidation in the Parilla Sand or a reservoir effect from removal of gypsum enriched in S-34.

Delta-O-18(SO4)-values for alunite from three Western Australia lakes (+17.8 to +18.3 parts per thousand V-SMOW), for alunite and jarosite from Lake Tyrrell (+22.6 to +24.9 parts per thousand) and for aqueous sulfate from Lake Tyrrell (+17.3 to +19.0 parts per thousand) are much larger than the average value for seawater (+9.6 parts per thousand). The data suggest an approach to O-18-O-16 equilibrium between aqueous sulfate and groundwater, which is known from experimental studies to be possible at low pH and low temperatures, but has not been previously documented in nature. A residence time of approximately 0.1-1 kyr for sulfate in acidic water (pH 3-4) is needed to achieve the apparent partial oxygen exchange, using previously published data of R.M. Lloyd.