Chemical, crystallographic and stable isotopic properties of alunite and jarosite from acid-Hypersaline Australian lakes

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, K_0.87Na_0.04(H₃O)_0.09(Al_0.92Fe_0.08)₃(SO₄)₂(OH)₆ and for jarosite, K_0.89Na_0.07(H₃O)_0.04(Fe_0.80Al_0.20)₃(SO₄)₂(OH)₆.

The δD-values of alunite are notably larger than those for jarosite from Lake Tyrrell and it appears that the minerals have closely approached hydrogen isotope equilibrium with the acidic regional groundwaters. The δD results are consistent with a fractionation ∼60–70‰ between alunite and jarosite observed in other areas. However, interpretation of δD results is complicated by large variability in fluid δD_H2O from evaporation, mixing and possible ion hydration effects in the brine. δD-values of water derived from jarosite by step-wise heating tend to be smaller at 250°C, at which temperature hydronium and other non-hydroxyl water is liberated, than at 550°C, where water is derived from the hydroxyl site, but the differences are not sufficiently different to invalidate measurements of total δD obtained by conventional, single-step heating methods.

δ³⁴S-values for alunite and jarosite from the four lakes (+19.7 to +21.2‰ CDT) and for aqueous sulfate from Lake Tyrrell (+18.3 to +19.8‰) are close to the values for modern evaporites (+21.5 ±0.3‰">‰+21.5 ±0.3‰) and seawater (+20±0.5‰">‰+20±0.5‰) and are probably typical of seawater-derived aerosols in arid coastal environments. δ³⁴-S-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 ³⁴S.

δ¹⁸O_SO4-values for alunite from three Western Australia lakes (+17.8 to +18.3‰ V-SMOW), for alunite and jarosite from Lake Tyrrell (+22.6 to +24.9‰) and for aqueous sulfate from Lake Tyrrell (+17.3 to +19.0‰) are much larger than the average value for seawater (+9.6‰). The data suggest an approach to ¹⁸O-¹⁶O 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 ∼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.