47-11 Thermal Transformation of Vaterite to Calcite: A High-Temperature Synchrotron XRD Study

Session: Minerals in Motion: Tracking Mineral Reactions Using In Situ and Synchrotron Techniques, A Celebration of the Career of Peter Heaney

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Abstract:

Calcium carbonate (CaCO₃) has three crystalline polymorphs, calcite, aragonite and vaterite, along with amorphous CaCO₃. Calcite is the thermodynamically stable phase of CaCO₃ at standard conditions, while aragonite is its high-pressure polymorph. Vaterite occurs in a variety of biogenic and abiotic systems, such as pearls, fish otoliths and cement. One of the most enigmatic aspects of vaterite is its complex crystal structure, which has been debated for decades. Although its basic structure (hexagonal sublattice composed of alternating layers of Ca²⁺ cations and CO₃^2- anions) was resolved more than 60 years ago, the electrostatic interactions between Ca²⁺ and neighboring CO₃^2- ions promote ordering of the triangular [CO₃] groups, resulting in a superstructure. The details of this superstructure have not been fully resolved due to the lack of single crystals that are sufficiently large for high-resolution structural characterization. Thermodynamically, vaterite is metastable at ambient conditions, and it transforms to the stable calcite upon heating. In this study, a vaterite sample was synthesized via the reaction of CaCl₂∙2H₂O and Na₂CO₃ solutions with glycine as the directing agent. In-situ high-temperature synchrotron X-ray diffraction (XRD) was conducted from room temperature to 973 °C at a heating rate of 1 °C/min. Simultaneous thermal analysis using differential scanning calorimetry and thermogravimetry was also done using the same heating rate. Vaterite was stable up to ~460 °C, above which it started to convert to calcite. At ~550 °C, the vaterite-to-calcite transition was complete. Further heating led to decomposition of calcite into CaO and CO₂, which started at ~700 °C and completed at ~815 °C. Rietveld analyses of the XRD data allowed determination of structural parameters of vaterite and its transformed calcite and decomposed CaO as a function of temperature. Although the focus of this work is not to determine the detailed superstructure of vaterite, structural analysis indicates that the vaterite superstructure persisted at high temperatures until its transformation to calcite. In addition, from the high-temperature XRD results, the thermal expansion coefficients of vaterite and calcite have been derived, and the structural mechanisms of their transformation discussed.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6866

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