Abstract
<title>Abstract</title> <p> The high-pressure behaviour of REE-free calcioburbankite, Na <sub>2</sub> Ca <sub>4</sub> (CO <sub>3</sub> ) <sub>5</sub> , was investigated by <italic>in situ</italic> single-crystal X-ray diffraction at ambient temperature to ~ 20 GPa. The phase remains stable throughout the experimental range and exhibits pronounced compressional anisotropy, with the <italic>c</italic> axis (linear modulus <italic>M</italic> <sub>0</sub> = 158(3) GPa) substantially more compressible than the <italic>a</italic> axis (linear modulus <italic>M</italic> <sub>0</sub> = 239.8(18) GPa). The pressure-volume data are well fitted by a 3 <sup>rd</sup> order Birch-Murnaghan equation of state with <italic>V</italic> <sub>0</sub> = 584.12(16) Å <sup>3</sup> , <italic>K</italic> <sub>0</sub> = 68.1(7) GPa and <italic>K</italic> ' <sub>0</sub> = 5.26(13). Structural analysis shows that compression is accommodated chiefly by contraction of the cation kagome-layer stacking and can be described both from the point of view of ‘perovskite’ tesselation tiles and from the conventional point of view of cation-centred coordination polyhedra. The results define the elastic response of the REE-free end-member of calcioburbankite and provide constraints relevant to modelling burbankite-group minerals under mantle conditions. </p>