CRYSTALLOGRAPHIC ANALYSIS OF PHASE TRANSFORMATIONS IN DANBURITE CaSi2B2O8

The crystallographic analysis of the structures of danburite CaSi2B2O8, which is orthorhombic (Pnma) at normal pressure, triclinic (P $$\bar{1}$$ ) at a pressure of 25.4 GPa, and monoclinic (P21/c) at a pressure of 32.3 GPa, shows that they are based on a joint packing of Ca cations and O anions whose regularity increases with pressure. A common structural fragment is the configuration of atoms related by the glide reflection plane retaining for Ca and a part of O, B positions with a possible Si migration according to the seesaw mechanism (N. V. Belov).