Abstract

The crystal structure of semenovite, (Fe2+,Mn,Zn,Ti)RE2Na0_2(Ca,Na)8(Si,Be)20 (O,OH,F)48, Z = 2, orthorhombic, a = 13.879(5), b = 13.835(5), c = 9.942(6)A, space group Pmnn, has been determined by Patterson and Fourier methods. The X-ray diffraction data were collected from a twin crystal by a Philips PW 1100 diffractometer. The refinement was carried out taking into account the average of the chemical analyses made on a number of samples (final conventional R factor = 0.05 for 1366 independent reflections).

The structure consists of tetrahedral layers parallel to (001), superposed at intervals of c/2 and connected by cation polyhedra (RE,Na,Ca-antiprisms and Fe,Mn-octahedra).

The {110} twinning, always present and responsible for the tetragonal pseudosymmetry, is explained on structural grounds.

The structure of semenovite is related to the structures of meliphanite, leucophanite, gadolinite, nordite, and hellandite; aminoffite, which has a marked resemblance both in the stoichiometric unit and lattice parameters, shows little similarity with semenovite in regard to the crystal structure.

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