Crystal structure of parasibirskite (CaHBO₃) and polymorphism in sibirskite and parasibirskite

Abstract

The crystal structure of parasibirskite (CaHBO₃) obtained at Fuka, Okayama Prefecture, Japan, was determined by Monte Carlo simulation using powder X-ray diffraction data and confirmed by Rietveld refinement. Parasibirskite is monoclinic with space group P2₁/m and cell constants a = 6.722(4), b = 5.437(2), c = 3.555(2) Å, and β = 93.00(5)°. The calcium atom is surrounded by seven oxygen atoms with a monocapped distorted trigonal prism arrangement. The edge-shared CaO₇ polyhedra form a layer structure elongated along the crystallographic b- and c-axes. These CaO₇ layers are connected to each other by BO₃ triangles, thus forming the parasibirskite structure. The hydrogen atom was presumed to connect to the O1 atom from the bond-valence calculation. Parasibirskite is a dimorph of sibirskite (CaHBO₃), which consists of CaO₆ double chains connected by BO₃ triangles.