Crystal structure of sibirskite (CaHBO₃) by Monte Carlo simulation and Rietveld refinement

Abstract

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.