Amorphous carbonates, in both their liquid and solid (glassy) forms, have been identified to play an important role in biomineralization, volcanism, and deep element cycling. Anhydrous amorphous calcium and calcium-magnesium carbonate (ACC and ACMC05) are structural glasses which exhibit a glass transition upon heating. Here we report a significant effect of water content on glass formation. The results yield a parameterization enabling prediction of the stability of their liquid and solid amorphous phases as a function of temperature and water content. These results, obtained through novel fast differential scanning calorimetry, demonstrate that hydrous ACC and ACMC05 do indeed exhibit the behavior of structural glasses and that dehydration of these materials by lyophilization is a route that can be used to cross the glass transition isothermally. This presents a viable process for a significantly wider range of geo- and biomaterials. Dehydration controlled formation of glassy ACC therefore constitutes the missing link in the transformation from supersaturated aqueous solutions through an intermediate amorphous glassy state to crystalline CaCO3 polymorphs. These results yield direct implications for the mechanistic interpretation of geological processes and biomineralization.

Which mechanisms underlie the formation of amorphous calcium carbonate, and how do organic compounds and foreign ions — such as biomineralization proteins or Magnesium — impact these processes? In addressing these outstanding questions, we conducted a comprehensive set of studies, adopting a molecular perspective. A particular emphasis has been placed on so-called prenucleation clusters and their as-yet elusive impact on calcium carbonate formation. These dynamic clusters, composed of interconnected chains of carbonato-calcium units, have been interpreted as proto-nucleation germs, a nonclassical crystallization species that remains debated. The present study demonstrates their role as compositional gatekeepers of the later ACC phase for organic inclusions and foreign elements, e.g., giving rise to notably low Mg distribution coefficients. In addition, we show distinct interaction with characteristic low-complexity repeat regions typical for biomineralization proteins, revealing clusters as putative binding partners, thereby deorphanising a wide range of biomineralization proteins. From a synthetic perspective, these coordination polymers can thus undergo supramolecular interactions, which breaks new ground for further expansion towards phase separation pathways uncommon for ionic and sparingly soluble mineral systems. With controlled fast flow chemistry, which ensures turbulent mixing, we even achieve spinodal demixing on larger scales, forming characteristic nanosized bicontinuous networks in various cluster-forming inorganic systems. The presented results emphasize the chemical nature of these pre-critical solution species, particularly their specific molecular interaction with organic compounds and their capability to generate supramolecular assemblies.

Carbonates, in both their crystalline and amorphous forms, have been shown to play multifaceted roles across geological and synthetic environments. In this contribution, recent insights are presented into high-temperature interfacial carbonates (HT-CO₃) within CuO-ZnO-based catalyst systems, where carbonate species are retained after calcination and act as molecular anchors at metal oxide interfaces. Through the combination of vibrational spectroscopy and first-principles modelling, their strong binding energies have been determined, and their essential role in suppressing thermal sintering has been demonstrated. The concept of carbonate functionality is further extended to layered double hydroxides (LDHs) — synthetic analogues to natural hydrotalcite-like phases — in which carbonate ions are incorporated as interlayer anions to modulate material stability, reactivity, and ion-exchange capacity. By comparing these two chemically and structurally distinct systems, the versatility of carbonate species is emphasized, both in dynamic catalytic settings and in tuneable layered solids. A broader understanding of carbonate behaviour at the molecular scale is thereby contributed, with implications for geochemistry, nanomaterials design, and carbon management strategies.

Calcite, the most abundant carbonate mineral in Earth’s crust, plays an important role in processes such as the global carbon cycle and biomineralization. To get a deeper understanding of these processes the interaction between water and the most stable calcite (10.4) surface on the nanometer scale is of central importance.

Previously, water coverages ranging from single molecules up to one monolayer were imaged with atomic force microscopy in an ultra-high-vacuum environment. [1] [2] These studies are expanded by several studies of bulk water at ambient conditions. [3] However, the intermediate coverages region of a few multilayers was poorly studied.

We employ atomic force microscopy to follow the growth of water layers at low temperatures. High-resolution imaging reveals, that the first few layers have different superstructures. Depending on temperature and pressure, we observe the formation of amorphous or crystalline ice clusters on top of these layers.

[1] J. Heggemann, Y.S. Ranawat, O. Krejčí, A.S. Foster, P. Rahe, Differences in Molecular Adsorption Emanating from the (2 × 1) Reconstruction of Calcite(104), Journal of Physical Chemistry Letters, 14 (2023) 1983–1989.

[2] L. Klausfering, F. Schneider, R. Bechstein, A. Kühnle, Reconstruction of calcite (10.4) manifests itself in the tip-assisted diffusion of water, Surface Science, 751 (2025) 122598.

[3] H. Söngen, S.J. Schlegel, Y. Morais Jaques, J. Tracey, S. Hosseinpour, D. Hwang, R. Bechstein, M. Bonn, A.S. Foster, A. Kühnle, E.H.G. Backus, Water Orientation at the Calcite-Water Interface, Journal of Physical Chemistry Letters, 12 (2021) 7605–7611.

Carbonates and carbonatite melts are integral to the geodynamics of the upper mantle and transition zone, influencing processes such as subduction, magma generation, mantle metasomatism, and diamond formation. The laboratory synthesis of many carbonate minerals is challenging due to the high pressures and temperatures required for their formation. Understanding the synthesis and reaction pathways of alkali and alkaline-earth metal carbonates is essential for advancing knowledge of their stability, transformations, geological relevance and potential applications. They exhibit diverse crystallographic structures and variable stability conditions, often necessitating specific environmental factors for their formation. Therefore, the development of alternative approaches is essential to enable controlled carbonate formation under more feasible laboratory conditions.¹

Mechanochemical synthesis presents a promising alternative, facilitating carbonate formation without the need for extreme pressures or temperatures. This technique utilizes high-energy ball milling to induce structural and physicochemical modifications, promoting chemical reactions through shear forces, or impact.² This study systematically investigates the mechanochemical synthesis of various carbonate systems, including K₂Ca(CO₃)₂ polymorphs³, dolomite, and sodium-rich natrocarbonatites such as nyerereite and pirssonite. Employing a Retsch MM400 shaker mill in combination with ex situ laboratory analysis and in situ XRPD, we explore reaction pathway, phase transitions, and crystallographic transformations to understand their phase stability and polymorphic behavior.^4,5

References

1.Manning; C. E., et al. Carbon in Earth’s Interior. (2020).

2.James; S. L., et. al. Chem. Soc. Rev. 41, 413 (2012).

3.Kahlenberg, V., et al. RSC mechanochem, 2, 152, (2025).

4.Rathmann;, T. et al. Rev. Sci. Instrum. 92, 114102 (2021).

5.Weidenthaler, C. Crystals 12, 345 (2022).