X-ray powder diffraction properties of pavonite homologues

Abstract

Theoretical X-ray powder diffraction patterns of all pavonite homologues with known crystal structures were calculated in order to: (1) determine the powder diffraction effects to be anticipated from homologues with scarce or no powder data available; (2) derive the criteria for discriminating the minerals of this group. The calculated powder patterns show very similar distribution of strong and medium intensity reflections. Most of these effects are produced by a quasi-orthorhombic I-centered submotif located in the galena-like slabs of these crystal structures. In the weighted reciprocal space, the I-centered portions correspond to a pronounced F-centered reciprocal sublattice which is sampled by the reciprocal lattice nodes of each homologue in question. The resulting sampling sublattices differ only slightly from one homologue to another, and thus, the majority of the peaks belonging to these sublattices are of low diagnostic value. The only obvious differences between the powder patterns of different pavonite homologues were found in the regions of high d-spacings (in general >4Aa). Based on these, a series of diagnostic reflections were derived. These criteria were applied to powder patterns published in the literature. A good agreement was found between the diagnostic reflections derived for 5 P and 7 P homologues and the published low-angle powder data of various well-documented synthetic and natural pavonites and benjaminites. Other less reliably diagnosed homologues or unnamed phases that might represent members of the pavonite series were also treated. The distinction between pavonite and lillianite homologues on the basis of (unindexed) X-ray powder patterns is also discussed. Due to insufficient structural data for several lillianite homologues, this problem could not however be completely clarified.