The crystal structure of podlesnoite, BaCa2(CO3)2F2

Abstract

Podlesnoite, BaCa2(CO3)2F2, a fluorocarbonate mineral recently discovered at the Kirovskii mine, Khibiny alkaline massif, Kola Peninsula, Russia, is orthorhombic, Cmcm, a = 12.501(8) Å, b = 5.846(3) Å, c = 9.443(5) Å, V = 690.1(7) Å3, Z = 4, D calc = 3.614 g/cm3. The crystal structure was solved from 561 unique reflections with I > 2σ(I) by direct methods and refined to R(F) = 0.0278, Rw(F 2) = 0.0577. Podlesnoite is a representative of a new structure type slightly related to aragonite. The most specific feature of the podlesnoite structure is a honeycomb-like framework built by the columns of edge-sharing CaO6F2 polyhedra; they are stretched parallel to the c axis. Interatomic bond-distances within these polyhedra vary from 2.2912(16) to 2.4883(15) Å. The CO3 groups are parallel to the ac plane, the C—O bond distances range from 1.276(2) to 1.296(4) Å. The bond distances within BaO6F4 polyhedra are 2.771(2) to 2.915(2) Å.