Mg/Ca partitioning between aqueous solution and aragonite mineral: a molecular dynamics study

Area of Science:

• Geochemistry
• Materials Science
• Paleoclimatology

Background:

• Coral skeletons are primarily composed of aragonite (calcium carbonate).
• Magnesium-to-calcium (Mg/Ca) ratios in coral skeletons are widely used as paleothermometers.
• The precise location and incorporation mechanism of Mg within aragonite are not fully understood.

Purpose of the Study:

• To calculate magnesium concentrations in the bulk and surfaces of aragonite.
• To investigate Mg incorporation in aragonite in equilibrium with aqueous solutions.
• To assess the implications for Mg/Ca paleothermometry in corals.

Main Methods:

• Molecular dynamics simulations.
• Grand-canonical statistical mechanics.
• Thermodynamic calculations of Mg incorporation.

Main Results:

• Magnesium preferentially incorporates into aragonite surfaces, particularly (001) terraces, rather than the bulk.
• The calculated total Mg content in bulk and surface aragonite is insufficient to explain measured coral Mg/Ca ratios.
• This suggests Mg is either highly metastable within the aragonite lattice or located outside the aragonite phase.

Conclusions:

• The study questions the direct correlation between bulk aragonite Mg content and environmental Mg/Ca.
• Findings suggest alternative Mg storage mechanisms in coral skeletons.
• Re-evaluation of Mg/Ca paleothermometry assumptions may be necessary due to Mg metastability or exsolution.