Carbonate mineral dissolution and precipitation play essential roles in aqueous and soil geochemistry as they regulate the availability of critical and toxic elements. Despite the wealth of recent knowledge on multistep growth pathways, how and to what extent a given growth mechanism affects the uptake of trace elements by growing minerals remain elusive, even for extensively studied minerals such as calcite.
Here, the mechanisms by which manganese—a potential archive related to riverine discharge or redox conditions—is incorporated into calcite was investigated using a custom-designed chemostat reactor, in situ small-angle (SAXS) and wide-angle (WAXS) X-ray scattering, in situ Raman spectroscopy, and scanning transmission electron microscopy at cryogenic temperatures (cryoSTEM). Seeded calcite growth experiments were performed in the presence aqueous Mn²⁺ while maintaining a quasi-constant solution chemistry (SI_calcite = 0.8). SAXS intensities arising from calcite interfaces captured the evolution of interfacial growth. Raman scattering confirmed the presence of both amorphous and crystalline carbonates while WAXS identified the crystalline phases as calcite-rhodochrosite solid solutions. Both the interfacial amorphous and crystalline carbonates were observed in cryoSTEM. This combination of bulk scattering techniques overcomes limitations of surface-sensitive scattering methods that rely on the reflection from smooth surfaces and are therefore not able to capture the broad range of processes observed here.
In the first 21 minutes, ionic dendrimers approximately Ø15-nm form and transform into Ø20-nm amorphous carbonates that aggregate into clustered networks on the surface of growing calcite. Amorphous carbonates undergo intermittent crystallization into solid solutions of varying composition while amorphous solids are continuously being formed. The resulting solid solutions at the growing front of calcite exhibits significantly higher Mn partitioning than expected from the equilibrium partitioning of manganese from bulk solution. Our results point that amorphous intermediates control the trace element substitution, providing a useful basis to inform predictive models for growth kinetics and trace element partitioning. These results also illustrate that multimodal in situ time-resolved scattering techniques enable the characterization of interfacial growth on bulk minerals, thus opening the door to nanoscale interfacial studies beyond carbonate minerals.

Ayumi Koishi

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