Jarosite dissolution I - Trace cation flux in acid sulfate soils

In order to determine trace metal release, dissolution experiments were conducted with a natural jarosite-group sample under a range of conditions relevant to acid sulfate soils. The reaction is incongruent with respect to the Fe³⁺-dominant octahedral cation layer and other elements that substitute for ferric iron. Transition metals that substitute into the octahedral site are almost entirely retained in the solid phase, although when Fe³⁺ becomes increasingly soluble, Cr³⁺ concentration also increases. The solubility behaviour of Rb⁺ and Sr²⁺ generally follows that of K⁺, although the heavier cations are liberated faster in the early stages of the reaction. The REE are also large enough to substitute for K⁺, but their behaviour is more complex. The REE distribution of the solutions, when normalized to Post-Archean Average Australian Shale (PAAS), show enrichments in the MREE in the dissolution experiments, despite the fact that the initial starting material is enriched in the LREE. Mechanisms for LREE and HREE depletion in solution are discussed.