Superlattice ordering transitions driven by short-range structure in barium calcium carbonates

Area of Science:

• Mineralogy
• Materials Science
• Geochemistry

Background:

• Balcite (BaCa_1-CO₃) is a synthetic carbonate mineral.
• It exhibits disorder in cation and anion sublattices.
• It serves as a model for rhombohedral carbonates like calcite and dolomite.

Purpose of the Study:

• Investigate high-temperature transformations of balcite.
• Characterize exotic superlattice structures formed from balcite.
• Compare synthetic pathways with naturally occurring polymorphs.

Main Methods:

• Heating balcite under controlled temperatures (150-600 °C).
• Applying elevated pressure to induce structural changes.
• X-ray diffraction and structural analysis to identify new phases.

Main Results:

• Balcite transforms into balcomite, a dolomite analog, via a second-order phase transition.
• Elevated pressure creates a larger supercell structure with 3D cation segregation.
• Naturally occurring polymorphs (alstonite, paralstonite, barytocalcite) did not form directly from balcite.
• Alstonite showed complex transformations involving balcomite-like and paralstonite-like structures.

Conclusions:

• High-temperature transformations in the Ba-Ca carbonate system are driven by coarsening and short-range order similarity.
• Ephemeral, exotic carbonate structures can play a role in mineral phase transformations.
• These pathways influence the distribution of natural mineral phases.