Dachiardite from Hokiya-Dake: Evidence of a new topology

A refinement of the crystal structure of dachiardite from Hokiya-dak, Japan, shows the presence (16% modal abundance) of domains of the hypothetical structure called "modified" dachiardite by S. Merlino (1975, 1976) assocd. with domains of "normal" dachiardite. As in the Elba dachiardite, 2 acentric framework configurations are present with the same frequency in the Hokiya-dake dachiardite, so that the statistical symmetry C2/m is maintained. This feature is interpreted as a tendency (common to all zeolites) to avoid T-O-T angles of 180° in the topol. C2/m symmetry of dachiardite. As with the related structures of mordenite and epistilbite, a remarkable Al-enrichment is found in the tetrahydrons of the 4-membered ring.