Modular aspects of the crystal structures of kurchatovite and clinokurchatovite

The crystal structures of the two polytypes of the compound CaMgB₂O₅, kurchatovite and clinokurchatovite, were determined in the space groups Pc2₁b and P2₁/c respectively by Yakubovich et al. (1976) and Simonov et al. (1980). The crystal structures of both minerals have been re-examined and refined until R_obs = 3.00 and 2.82%. The lattice parameters are for kurchatovite a = 36.34,b= 11.135,c = 5.499 Å, Z = 24, space group Pbca, and for clinokurchatovite a = 12.329,b = 11.146,c = 5.519 Å, ß = 101.62°, Z = 8, space group P2₁/c. Common features of both structures are the B₂O₅ clusters formed by two B-triangles with a common vertex, the Mg-octahedra and Ca-polyhedra with seven corners. The two crystal structures show very close similiarities, both being based on the repeat of a monoclinic module, whose volume is one fourth of that of the unit cell of clinokurchatovite (a₀ = a/4). The possibility of further structures based on the same module is discussed.