Crystal structure and microtwinning of a calcium-rich analogue of labuntsovite

The structure of a new representative of the labuntsovite family, which was found in the Khibiny alkaline massif (the Kola Peninsula), was established by the method of X-ray diffraction analysis. The unit-cell parameters are a = 14.365(7) Å, b = 13.887(7) Å, c = 7.814(2) Å, β = 117.36(5)°, sp. gr. Cm, Raniso = 0.058, 1385 F > 4σ(F). The mineral differs from other members of the family by the absence of Na-atoms and the high Ca-content. The Ca-polyhedron is reduced to a seven-vertex polyhedron (the average Ca-O distance is 2.48 Å) compared to the eight-vertex Na-polyhedron (the average Na-O distance is 2.57 Å). The structure channels are orderly occupied, thus lowering the symmetry to Cm.