DIRECT PHASING IN ELECTRON CRYSTALLOGRAPHY - DETERMINATION OF LAYER SILICATE STRUCTURES

Direct phase analysis, based on the probabilistic estimate of triple and quartet structure invariant sums, was used to determine the crystal structures of three layer silicates, viz. celadonite, muscovite and phlogopite-biotite, from oblique texture electron diffraction intensity data published earlier by B.B. Zvyagin and his co-workers. In all cases, the resulting potential maps are sufficient to identify most of the atomic positions (including oxygen) found in the original determinations. In addition, most of the crystallographic phase values are identical to the ones found earlier. The major drawback for this technique is due to the paucity of measured intensities, requiring a multisolution approach to this analysis. However, atomic-resolution data enable the choice of the correct map to be made from chemical knowledge of a reasonable layer structure.