High-Pressure Phase Behavior of Carbonate Minerals

Carbonate minerals, characterised by CO3 anions bound to metal cations, play a pivotal role in the deep carbon cycle and influence geophysical processes within Earth’s mantle. Under extreme pressures and temperatures, common carbonates such as calcite, aragonite and siderite undergo a series of structural transformations, often involving changes in coordination number, density and electronic state. These transitions have been probed using diamond anvil cells coupled with in situ spectroscopic and diffraction techniques, revealing novel phases with CO4 tetrahedra, orthorhombic lattices and spin-state changes in iron-bearing compounds. The resulting phase diagrams inform our understanding of carbon storage, seismic velocity anomalies and the global redistribution of carbon into the deep Earth. Advances in computational modelling and high-pressure synthesis continue to refine the stability fields of these phases, elucidating mechanisms of decarbonation, self-oxidation and pressure-induced polymerisation that bear on both planetary interiors and materials science applications.

Research from Nature Portfolio

Recent studies have detailed the stability and transformations of iron-bearing carbonates at pressures exceeding lower-mantle conditions. Investigations of pure FeCO3 under simultaneous high pressure and temperature have uncovered self-oxidation–reduction reactions leading to the formation of novel orthocarbonate and tetracarbonate structures with CO4 units, stable to depths of at least 2,500 km. Complementary work on (Mg,Fe)CO3 demonstrates that above ~80 GPa these minerals adopt a phase with tetrahedrally coordinated carbon, profoundly altering their reactivity and potential role as deep-carbon carriers. Further experiments on ferromagnesite reveal a pressure-driven spin transition of iron that triggers a volume collapse and stabilises an orthorhombic Pmm2 phase at ~50 GPa and 1,400 K, suggesting that low-spin ferromagnesite may dominate carbon transport in the lower mantle.

High-Pressure Phase Behavior of Carbonate Minerals publication trend

The graph below shows the total number of articles in high-pressure phase behavior of carbonate minerals across all publications each year (not limited to Nature Index journals).

Technical terms

Diamond anvil cell: A high-pressure device that compresses a sample between two diamond tips, enabling in situ study of materials under extreme pressure.

Phase transition: A change in crystal structure or coordination environment of a mineral induced by variations in pressure and temperature.

Coordination number: The number of atoms directly bonded to a central atom; e.g., threefold (CO3) or fourfold (CO4) carbon coordination in carbonates.

Tetrahedral coordination: A structural motif in which a central atom (carbon) is bonded to four surrounding atoms (oxygen) at the corners of a tetrahedron.

Spin crossover: A pressure- or temperature-induced change in the electronic spin state of transition-metal ions, affecting volume and stability of the host mineral.

References

1. Synthesis and Characterization of Lithium Pyrocarbonate (Li2[C2O5]) and Lithium Hydrogen Pyrocarbonate (Li[HC2O5]). Angewandte Chemie International Edition (2024).
2. CaCO3 phase diagram studied with Raman spectroscopy at pressures up to 50 GPa and high temperatures and DFT modeling. Physics of The Earth and Planetary Interiors (2018).
3. Stability of iron-bearing carbonates in the deep Earth’s interior. Nature Communications (2017).
4. Tetrahedrally coordinated carbonates in Earth’s lower mantle. Nature Communications (2015).
5. High-Pressure Orthorhombic Ferromagnesite as a Potential Deep-Mantle Carbon Carrier. Scientific Reports (2015).
6. Pressure driven spin transition in siderite and magnesiosiderite single crystals. Scientific Reports (2017).