Influence of cation disorder on the mineral physics of ankerite

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Zucchini, Azzurra ; Boffa Ballaran, Tiziana ; Fastelli, Maximiliano ; Comboni, Davide ; Hanfland, Michael ; Frondini, Francesco ; Comodi, Paola:
Influence of cation disorder on the mineral physics of ankerite.
In: American Mineralogist. Bd. 110 (2025) Heft 6 . - S. 908-918.
ISSN 1945-3027
DOI: https://doi.org/10.2138/am-2024-9495

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Abstract

The structural evolution and compressibility of ordered and disordered ankerite were investigated at pressures up to ∼25 GPa using synchrotron single-crystal X-ray diffraction in a diamond-anvil cell. Ordered ankerite (space group R3¯) undergoes a discontinuous phase transition between 12.15 and 13.45 GPa to a high-pressure structure called ankerite-II (space group P1¯) that has Ca in an eightfold coordination. Disordered ankerite (R3¯c space group) does not undergo a phase transition in the investigated pressure range.
A Birch-Murnaghan equation of state was used to fit the volume compressibility. Ordered ankerite K0V = 95(1) GPa, K′ = 3.8(3) appears slightly more compressible than disordered ankerite K0V = 99(1) GPa, K′ = 2.7(1). The phase transition in ordered ankerite has a change in volume of ∼0.6\% and K13.45V = 110(11) GPa, K′ = 7(3) for ankerite-II. The possible significance of this different behavior of ordered and disordered ankerite is discussed.