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.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.