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 Pc2(1)b and P2(1)/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 Angstrom, Z = 24, space group Pbca, and for clinokurchatovite a = 12.329, b = 11.146, c = 5.519 Angstrom, beta = 101.62degrees, Z = 8, space group P2(1)/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 (a(0) = a/4). The possibility of further structures based on the same module is discussed.