[M-IS14] Interface- and nano-phenomena on crystal growth and dissolution

[MIS14-04] A multimodal in situ approach reveals molecular pathways for manganese substitution in growing calcite

*Ayumi Koishi¹, Chaya Weeraratna², Musahid Ahmed², Chenhui Zhu³, Laura N. Lammers^1,4, Michael L. Whittaker¹ (1.Energy Geosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, United States, 2.Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, United States, 3.Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, United States, 4.Department of Environmental Science, Policy, and Management, University of California, Berkeley, CA 94720, United States)

Keywords:Calcite, Manganese uptake, In situ small-angle X-ray scattering, In situ Raman spectroscopy, Chemostat, Nonclassical growth

Carbonate minerals play an essential role in aqueous and soil geochemistry by regulating the availability of critical and toxic elements. Recent advances in the field of crystal growth have led to a new paradigm of growth mechanisms collectively referred to as the multistep growth pathway in which at least one rate-limiting step involves non-monomer species. Surprisingly, however, how and to what extent a given growth pathway affects the chemical composition of growing minerals in the presence of trace element impurities, even for extensively studied minerals such as calcite, remains elusive. Manganese, a representative natural impurity in carbonate minerals is of interest due to its relevance as a potential proxy and archive related to riverine discharge or redox conditions.

In this work, the mechanism by which manganese is incorporated into growing calcite crystals was investigated using a custom-designed chemostat reactor. Seeded growth experiments were performed in the presence and absence of aqueous Mn²⁺ while maintaining a fixed solution pH and temperature with quasi-constant saturation index (SI = 0.8 with respect to calcite) and solution composition. To monitor interfacial growth, in situ time-resolved small-angle X-ray scattering (SAXS) was employed by isolating scattering due to interfacial processes from that due to bulk materials. In situ Raman and static light scattering were used to constrain the relevant SAXS models, revealing growth mechanisms that differed considerably in the presence of Mn²⁺. This combination of bulk scattering techniques overcomes limitations of surface-sensitive scattering techniques that rely on the reflected intensities from atomically smooth surfaces and are therefore not able to capture the broad range of processes observed here.

It is revealed that 15-20 nm Mn-bearing amorphous calcium carbonate (ACC) particles form on the surface of growing calcite at [Mn²⁺]/[Ca²⁺] = 0.1 in the growth solution. These particles quickly reorganize to highly porous and polymeric mass fractal structures that undergo intermittent and likely local crystallization events. These crystallization events can result in local atomic reorganization and hence modifies the chemical composition at the growing front of calcite. Knowledge obtained on how “non-classical” growth units such as Mn-ACC are involved in crystal growth and evolve in time provides new mechanistic insight and perspectives of trace element uptake in different classes of carbonate (bio)minerals. This provides a useful basis to inform predictive models for growth kinetics and trace element partitioning in carbonates under controlled solution conditions. These results 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.