Trace element fractionation through halite crystallisation: Geochemical mechanisms and environmental implications

Halite is an important mineral for industry, agriculture and food production. It crystallises after water evaporation, while the progressive growth of dissolved metal ions in brines is occurring. Then, halite exploitation may provide the delivery of metal ions in the environment and the mechanism of this trace element accumulation should be studied. In this work we investigate the distribution of lanthanides and Y (hereafter named Rare Earth Elements, REE), Zr and Hf between crystallising halite and brines in the Dead Sea as geochemical tools for recognising the mechanism of metal ion removal from brines and accumulation in halite. Halite forms cubic crystals where octahedral planes sometimes can occur under particular thermal gradient conditions. Our findings indicate that the crystal morphology is crucial and influences the mechanism of metal ion removal from brines since octahedral surfaces are polar unlike those that are cubic. Accordingly, octahedra preferentially fractionate aqueous charged species, such as [Hf(OH)5]–, compared to neutral species, such as [Zr(OH)4]0. Cubic surfaces do not fractionate aqueous species. In crystal cores, positive Eu anomaly occur suggesting the Eu substitution for Na in the lattice. It is energetically justified by ab-initio calculations. Hf enrichment relative to Zr also occurs in primary halite-rich evaporites. It is not found in cubic halite from saltworks. The results of this study suggest that primary halite kinetically crystallised from brines can concentrate dissolved metal ions onto the crystal surface where their dissolved charged species are scavenged. Accordingly, the dissolution of halite during human activities can release these metal ions to the environment.