Unusual thermal expansion of a B-O bond in the structure of danburite CaB2Si2O8

The crystal structure of danburite has been studied at high temperatures up to 900 °C based on single-crystal diffraction intensities collected on a four-circle diffractometer. The thermal response of the structure is basically characterized by an unusual expansion of a specific bond, B –O(2b), of the borate tetrahedron, the thermal expansion coefficient of the bond being 1.6xl0-5C-1 in the temperature range from about 400 °C to 900° C. In accordance with such an expansion, the Ca –O(2b) bond shows a relative contraction with an increase of temperature. The result supports a nine-coordination, rather than seven, for the Ca atom as geometrically defined based on the consideration of Voronoi polyhedron around the atom.