Twinning in chalcostibite has been reported to occur on the (104) plane with respect to the standard setting Pnma of the space group. However, different axial settings have been used for this mineral, and in the literature an axial ratio which corresponds to the Pnam setting is often reported. Unfortunately, the same indexing of the twin plane, (104), is incorrectly used for both settings; this has led to an incorrect reticular analysis of the twin, which could have been an example of diperiodic twin, an alleged category which has no known examples to date. In this article, we trace back the origin of the inconsistent indexing, we present a revised reticular analysis which puts back this twin in the category of triperiodic twins, and we discuss the atomic structure at the composition plane, showing that a small adjustment of the metal-sulphur coordination is required to accommodate the change in orientation occurring when the twin forms.
Research Article|May 31, 2018