Crystal structure of sibirskite (CaHBO_3) by Monte Carlo simulation and Rietveld refinement

The crystal structure of sibirskite (CaHBO3) was solved by the Monte Carlo simulation using powder X-ray diffraction data and confirmed by the Rietveld refinement. The mineral sibirskite is monoclinic with space group P21/a and cell constants of a = 8.643(6), b = 9.523(2), c = 3.567(3) Å, and β = 119.23(3)°. The unit cell consists of independent atoms such as calcium, boron, hydrogen, and three oxygen atoms. The calcium atom is surrounded by six oxygen atoms in an octahedral coordination, and a symmetrical pair of edge-shared CaO6 octahedra forming a double chain elongates the crystallographic c axis. The CaO6 double chains are not directly connected to each other. The BO3 triangles are linked to one vertical and two shared oxygen atoms in three CaO6 double chains to form the sibirskite structure. Sibirskite is isostructural with nahcolite (NaHCO3), which consists of NaO6 double chains and CO3 triangles.