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

Area of Science:

• Geochemistry
• Mineralogy
• Environmental Science

Background:

• Halite (salt) crystallization concentrates dissolved metals in brines.
• Understanding metal ion accumulation in halite is crucial due to potential environmental release.
• The Dead Sea provides a natural laboratory for studying trace element behavior during halite formation.

Purpose of the Study:

• Investigate rare earth elements (REEs), Zr, and Hf distribution between halite and Dead Sea brines.
• Determine the mechanism of trace element removal from brines and incorporation into halite.
• Assess the role of halite crystal morphology in metal ion fractionation.

Main Methods:

• Analysis of REEs, Zr, and Hf in halite and brine samples from the Dead Sea.
• Microscopic examination of halite crystal morphology (cubic vs. octahedral).
• Computational modeling (ab initio calculations) to assess energetic favorability of ion substitution.

Main Results:

• Octahedral halite surfaces preferentially fractionate charged aqueous species (e.g., [Hf(OH)5]-) over neutral species (e.g., [Zr(OH)4]0).
• Cubic halite surfaces show no significant fractionation of aqueous species.
• Positive Europium (Eu) anomalies in crystal cores suggest substitution of Eu for Na.
• Hf enrichment relative to Zr observed in primary halite, but not in saltworks halite.

Conclusions:

• Halite crystal morphology significantly influences the mechanism of metal ion removal from brines.
• Kinetically formed primary halite can concentrate dissolved metal ions via surface adsorption.
• Dissolution of contaminated halite may release concentrated metal ions into the environment.