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

Abstract

Abstract The low-temperature polymorph β-Ca11B2Si4O22 crystallizes as a monoclinic structure [space group is P21/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, Ca5(SiO4)2CO3, and can be described as a framework of [CaO5] and [CaO6] polyhedra, the cavities of which are filled with [SiO4] and [BO3] 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.