The aim of this study is to understand the stability and compressional behaviour of ankerite as a function of Fe content and cation distribution under typical mantle conditions. Depending on the subduction regime, these conditions may be suitable (warm subduction) or not (cold subduction) for the order-disorder phase transition. High-pressure single-crystal X-ray diffraction experiments were performed at the ID15B beamline (ESRF, Grenoble, France), up to ~50 GPa, on fully ordered and fully disordered crystals belonging to two compositions: Ca1.0(2)[Mg0.3(1)Fe0.7(1)](CO3)2 (ank7) Ca1.0(2)[Mg0.6(1)Fe0.3(1)Mn0.1(1)](CO3)2 (ank3). Preliminary results show that ordered ankerite undergoes a phase transition from R-3 (ankerite-I) to a P-1 (ankerite-II) structure, with the transition pressure decreasing with increasing Fe content (~13 GPa in ank7 and ~15 GPa in ank3). At higher pressures, ank7 transforms to a rhombohedral ankerite-IIIb [1] between 37 and 40 GPa, while ank3 undergoes a phase transition to a triclinic structure for which structure refinements are in progress. During decompression, both ank7 and ank3 recover their original R-3 symmetry and high crystallinity.

As for the disordered structures, the onset of the ankerite-I to ankerite-II phase transition was observed at pressures higher than those observed for the ordered crystals (more than 10 GPa), with transition pressure that seems to increase with increasing Fe content.

Both order-disorder and high-pressure phase transitions strongly influence the compressibility behavior of ankerite, with ordered ankerite and ankerite-II being the most and least compressible structures, respectively, while disordered ankerite lies in between [2]. Ongoing research dedicated to study their elasticity under non-ambient conditions, will allow also to take into account the strong anisotropic behavior expected for both (dis)ordered ankerite and ankerite-II, which is likely to be an additional factor that could condition the observed seismic wave anomalies in the mantle wedge.

[1]doi:10.2138/am-2017-6161;[2]doi:10.2138/am-2024-9495