Occurrence and crystal structure of a Ca-pure willhendersonite

The paper reports the occurrence and the crystal-chem. data of willhendersonite from the breccia ridge of a melilitite plug at Colle Fabbri, Terni, Italy. The chem. data suggests extensive chem. variability, ranging from the compn. of the holotype sample (CaKAl3Si3O12: 5 H2O) through intermediate compns., to a Ca-pure term; thus it becomes necessary for a chem. redefinition of the zeolite. The structural refinement of a Ca-pure willhendersonite single crystal exhibits the same space group and Si/Al ordering as the holotype willhendersonite, but it displays significant differences in the distribution of the extra-framework species. These structural features are discussed in the light of a comparison with the structures of the holotype willhendersonite and natural and exchanged chabazites.