Evolution of the bikitaite structure at high pressure: A single-crystal X-ray diffraction study

Abstract

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.