The structural evolution of natural bikitaite Li2(H2O)2[Al2Si4O12], compressed in penetrating (water-containing) medium up to 4 GPa, was studied using single-crystal X-ray diffraction data from a diamond-anvil cell. A nearly isotropic compression of bikitaite up to 1.2 GPa proceeds through a slight decrease of the framework T-O-T angles. Further pressurizing leads to anisotropic compression: The compressibility of b-axis is half as smaller compared to c-axis, and the a-axis is the least compressible. The structure can be described as hexagonal sheets of six-membered rings parallel to (001), connected by pyroxene-like chains. Upon the compression, the hexagonal sheets approach each other, leading to the shortening of the c-parameter. The deformation of hexagonal sheets, reinforced by O-Li bonds, is defined by the corrugation of 6-membered rings. The deformation of more flexible pyroxene chains, responding to the deformation of hexagonal sheets, consists of axial rotation of tetrahedra with only minor change in T-O-T angles. The arrangement of extraframework species changed slightly. The system of H-bonds between water molecules remains intact upon pressurizing. At the same time, the formation of new H-bonds with framework O-atoms becomes possible above 2 GPa due to the shortening of the distances between Ow positions and framework O-atoms. All pressure-induced structural changes are completely reversible and the recovered crystal structure returns to its ambient structure. The results clearly demonstrate the absence of pressure-induced hydration in the bikitaite structure. The pressure-induced changes in the unit cell metrics are similar for bikitaite compressed in water-containing medium, silicon oil, and glycerol.
- Crystal structure
- High pressure
- Single-crystal X-ray diffraction