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

In celebration of the career of Dr. Peter Heaney, we invite submissions using in situ, synchrotron, X-ray, neutron, or electron techniques to study rates/mechanisms of mineral reactions, alongside contributions exploring new approaches to mineralogy education.

Sunday, 19 October 2025
1:30 PM - 5:30 PM
HBGCC, 217A

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

Paper number: 47-12
Program: Technical Sessions
Time: 4:25 PM – 4:40 PM

Authors

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 CO32- anions) was resolved more than 60 years ago, the electrostatic interactions between Ca2+ and neighboring CO32- 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

doi: 10.1130/abs/2025AM-412468

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