β-Ca11B2Si4O22: six-fold twinning, crystal structure and thermal expansion

Abstract The low-temperature polymorph β -Ca 11 B 2 Si 4 O 22 crystallizes as a monoclinic structure [space group is P 2 1 / c , a =14.059(9), b =6.834(5), c =10.597(7) Å, β =100.735(8)°]. The crystal investigated by single-crystal X-ray diffraction was a twin composed of six individuals. The crystal structure is similar to that of mineral spurrite, Ca 5 (SiO 4 ) 2 CO 3 , and can be described as a framework of [CaO 5 ] and [CaO 6 ] polyhedra, the cavities of which are filled with [SiO 4 ] and [BO 3 ] groups. The orientation relationship of twin domains was investigated by electron backscatter diffraction (EBSD). Thermal expansion was studied by high-temperature X-ray powder diffraction. It is slightly anisotropic: α 11 =10, α 22 =16, α 33 =12×10 −6 °C −1 at 200°C.