Conference Agenda

Despite the geological ubiquity of dolomite (CaMg(CO₃)₂) in ancient sedimentary rock formations, its low-temperature formation pathway remains a significant geochemical enigma, as the mineral is difficult to replicate in laboratory settings due to severe kinetic inhibition. Traditional high-temperature synthesis routes are energy-intensive and often struggle with scalability and kinetic control. This study addresses these challenges by exploring a mechanochemical approach, utilizing high-energy ball milling of calcite (CaCO₃) and magnesite (MgCO₃) to drive dolomite formation under ambient conditions.

The mechanical energy applied during milling facilitates solid-state reactions by creating fresh crystal surfaces, generating lattice defects and cation rearrangement, effectively bypassing the kinetic barriers typically associated with magnesium dehydration and ordering. Systematic investigation of milling parameters specifically frequency, time and reactant stoichiometry reveal that dolomitization is achievable at ambient temperatures. By elucidating the physicochemical relationships between reactant stoichiometry, defect-assisted diffusion, and structural evolution, this work advances the fundamental understanding of dolomite formation mechanisms. Furthermore, it establishes a viable, low-temperature synthetic route for the production of dolomite, offering promising implications for both industrial applications and environmental mineralization strategies.

Between room temperature and the freezing point of water, the exchange rate of water molecules between the cation hydration shell and the bulk aqueous solution is much slower for Mg²⁺ than for Ca²⁺. Consequently, the number of magnesium carbonate hydrate phases occurring in this temperature range outlasts the calcium carbonate hydrate phases by far. Disregarding amorphous phases, the calcium carbonate hydrate phases known so far are the hemi, mono, and hexahydrate, whereas in the case of magnesium eleven basic and neutral carbonate hydrate phases were reported in literature. Some of them, however, still lack proper structural and compositional characterization. The most recently reported magnesium carbonate hydrate phase is MgCO₃·6H₂O (Rincke et al., 2020: doi.org/10.1107/S2053229620001540). As this phase decomposes at room temperature within minutes, it serves as a transient phase for the formation of more stable phases. In this way, exposure of hexahydrate to air at low humidity and T = 20-50 °C leads to a new phase: magnesium carbonate tetrahydrate MgCO₃·4H₂O (Jordan et al., 2026: doi.org/10.1007/s00269-026-01349-9).

Magnesium carbonate tetrahydrate adds to the line of neutral magnesium carbonate hydrate phases comprising important minerals such as lansfordite and nesquehonite. Here, we report on the formation and the compositional and structural characterization of MgCO₃·4H₂O and, thus, contribute to an improved understanding of magnesium carbonate hydrate phases. An improved knowledge about these phases can support attempts towards important technical developments such as magnesium-based cement binders or CO₂-storage in magnesium rich settings.

Carbonate melts are the key oxidized carbon-transport agents into Earth’s deep interior [1]. Compared with silicate melts, carbonate melts exhibit anomalously low density under upper‑mantle conditions, yet their behavior across lower‑mantle pressures remains poorly constrained. Recent studies suggest that carbon in carbonate melts undergoes an sp²‑to‑sp³ transformation at extreme pressures (>35 GPa) [2]. This transition is predicted to drive dramatic changes in the physical properties of carbonate melts; however, direct experimental measurements of carbonate‑melt physical properties remain limited to ~10 GPa, largely due to the associated technical challenges of high‑pressure experiments on liquids.

Here we report the first refractive‑index and volumetric measurements of the K₂CO₃-MgCO₃ glass in a diamond anvil cell up to 70 GPa. Constraining the compression behavior of this glass can provide a crucial proxy for predicting the physical properties of associated carbonate melts at extreme pressures.

1. Jones, A. P., Genge, M. & Carmody, L. Carbonate Melts and Carbonatites. Rev. Mineral. Geochem. 75, 289–322 (2013).

2. Cerantola, V. et al. Tetracarbonates in silicate melts may be at the origin of a deep carbon reservoir in the deep Earth. Commun. Earth Environ. 4, 67 (2023).

Marine organisms such as molluscs or brachiopods employ CaCO₃ for their shells. As crystals of CaCO₃ phases are extremely brittle, the organisms employ elaborate mesocrystal architectures and hybrid composite formation for toughening against propagation of dislocations and cracks. We employ electron back scatter micro diffraction in the SEM to investigate the crystallographic architecture of biogenic carbonate minerals. One of the most sophisticated biogenic mesocrystal structures of calcite that we encountered so far is the foliated calcite microstructure found in oyster shells. The ca. 100 micrometer sized mesocrystals constituting this biogenic material consist of calcite nano-laths arranged into sheets (folia). Nano-laths and folia are subcrystals separated by small-angle boundaries, giving the mesocrystal an overall mosaic spread in the order of 10°. The small-angle crystal lattice rotation between abutting nano-laths is around a <100> type crystallographic axis (hex. setting of calcite unit cell). The flat face of the laths is a {018]-type plane. Across the boundary of abutting folia the lattice rotates around a <8 16 1> type axis, which is the long axis of the laths and which corresponds to one of those space-diagonal directions of the {104}-cleavage rhombohedron which are not parallel to the triad axis <001>. The small-angle lattice rotations in the mosaic structure of the calcite mesocrystal can be understood in terms of spherulite-like split growth and the disclocation glide systems that are known from plastic deformation of calcite. The corresponding stresses occuring during crystal growth are related to impurities like occluded organic matrix molecules and Mg ions.

Over the past three billion years, biological and abiotic carbonates have been a dominant component in shallow marine environments. Carbonate cement minerals can facilitate the formation of microbialites, carbonate buildups, and hardgrounds by filling the pore spaces within sedimentary particles. Additionally, the mineralogy, geochemical data, and petrology of cement may provide valuable insights into Earth's evolution and the paleo-marine history. By utilizing modern shallow carbonate environments at Abu Dhabi beach (Persian/Arabian Gulf) and stratigraphically controlled ageing, this study documented a detailed diagenetic sequence with time resolution. Thin-section observations, scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS), confocal Raman microscopy, transmission electron microscopy (TEM), cathodoluminescence (CL) microscope, electron probe microanalysis (EPMA) and powder X-ray diffraction (pXRD) were used to characterize and confirm the formation and transformation of carbonate cements. The cement succession begins with the earliest amorphous carbonates (ACC), which are significantly associated with microbial organic materials. ACC transform into micrite cement envelope and aragonite cement via a solid-state crystallization pathway (nonclassical crystalization). Only nano- and microscale dissolution is involved in this process; therefore, most geochemical information from seawater will be well preserved in the cement. With time, aragonite needles of early cementation partially dissolved and were overprinted by secondary, replacive high-Mg calcite and dolomite crystals in Pleistocene carbonates. Aragonite needles are typically corroded, with irregular, serrate crystal shapes. Obviously, the diagenesis from aragonite to high-Mg calcite and dolomite is dominated by Ostwald ripening (dissolution and reprecipitation). How much original seawater information remains after the process is uncertain.

Carbonate-intercalated layered double hydroxides (LDHs) are key model systems for understanding how carbonate controls the structure and properties of hydrated layered oxides. Using hybrid density functional theory (DFT) calculations on Zn–Al LDHs, we place the carbonate phase at the centre of a systematic comparison with nitrate and several aromatic carboxylates to derive carbonate-specific microscopic design rules. For Zn–Al–CO₃, we show that the nearly parallel orientation of CO₃²⁻ relative to the brucite-like layers and its higher effective charge lead to reduced basal spacings and strong electrostatic stabilisation, while preserving non‑coordinating host–guest geometries and a well-defined hydrogen‑bond network in the interlayer.

Intercalation energies and effective carbonate charges clarify why CO₃²⁻ stabilises the LDH framework and often suppresses anion exchange, despite significant charge delocalisation into the slab. Electronic‑structure analyses reveal that the valence band maximum remains dominated by layer O 2p states, whereas the conduction band minimum is systematically transferred to intercalate‑derived C/O 2p states, demonstrating how carbonate orientation and local environment can tune band edges and optical gaps. Simulated infrared spectra provide carbonate-specific fingerprints that link mode shifts and splittings to hydrogen-bond motifs, without evidence for direct Zn–CO₃ coordination. These results establish carbonate-intercalated Zn–Al LDHs as benchmark systems for probing carbonate’s structural, electronic, and spectroscopic roles in layered minerals, with implications for anion exchange, CO₂‑derived carbonate uptake, and the design of carbonate-based dielectric or photocatalytic composites.