Crystal-structure determination of twinned kettnerite

The crystal structure of kettnerite, CaBi(CO3)OF, a 3.7976(5), b, 3.7976(5) , c 13.569(4) Angstrom, V 195.71(8) Angstrom(3), space group Pmmm, Z = 2, h as been refined to an R index of 0.016 on the basis of 265 unique, observed reflections. Although the cell and the distribution of diffraction intensi ties obey tetragonal symmetry, kettnerite is optically biaxial, and attempt s to solve the structure with tetragonal symmetry led to stereochemical non sense. Introduction of a twin plane via reflection on {110} for orthorhombi c symmetry simulates tetragonal diffraction-intensity distribution for the merohedral twin, and gives a structure with sensible stereochemistry. Both large cations, Ca and Bi, have [8]-coordination, and Bi3+ shows stereoactiv e lone-pair behavior. The kettnerite structure is layered with a Ca-F layer , a Bi-O layer, and a CO3 layer with the plane of the (CO3) group orthogona l to the plane of the layer. Oxy-carbonate minerals are rare relative to hy droxy- and hydrated-carbonate minerals. The only cations that can satisfy t he bond-valence requirements for a non-carbonate oxygen atom are those for which there is an electronic driving mechanism to produce a marked asymmetr ical distribution of bond valences, such as for the uranyl group, (UO2)(2+) , and for large cations with stereoactive lone-pairs of electrons, e.g., Bi 3+, Pb2+. In this regard, all oxy-carbonate minerals contain such cations.