Influence of cation disorder on the mineral physics of ankerite

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 R¯3) undergoes a discontinuous phase transition between 12.15 and 13.45 GPa to a high-pressure structure called ankerite-II (space group P¯1) that has Ca in an eightfold coordination. Disordered ankerite (R¯3c 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, K0 = 3.8(3)] appears slightly more compressible than disordered ankerite [K0V = 99(1) GPa, K0 = 2.7(1)]. The phase transition in ordered ankerite has a change in volume of ∼0.6% and K13.45V = 110(11) GPa, K0 = 7(3) for ankerite-II. The possible significance of this different behavior of ordered and disordered ankerite is discussed.