ON THE SIGNIFICANCE OF SMALL DEVIATIONS FROM HIGHER SYMMETRY

Abstract

Small deviations from higher symmetry can be established by single-crystal diffraction methods by (1) measuring the metric of the unit cell with high, non-routine precision; (2) measuring a full reflection sphere of diffraction data and testing it for the presence or absence of symmetry elements in the dataset; (3) refining the structure in both the higher and the lower symmetry and comparing the results; (4) checking the refined crystal structures for the presence or absence of hidden symmetry elements. By these criteria it is not established that the v{120} growth sector of yugawaralite really has triclinic symmetry (Tanaka et al., 2002). On the contrary, the refined metrical parameters and the atomic positions conform to monoclinic symmetry within the margins of experimental error.