OCCURRENCE AND CRYSTAL-STRUCTURE OF A CA-PURE WILLHENDERSONITE

The paper reports the occurrence and the crystallochemical data of wil lhendersonite from the breccia ridge of a melilitite plug at Colle Fab bri, Terni, Italy. The chemical data point to great chemical variabili ty, ranging from the composition of the holotype sample (CaKAl3Si3O12: 5 H2O) through intermediate compositions, to a Ca-pure term, and so th ey call for a chemical redefinition of the zeolite. The structural ref inement of a Ca-pure willhendersonite single crystal exibits the same space group and Si/Al ordering as the holotype willhendersonite, but i t displays significant differences in the distribution of the extrafra mework species. These structural features are discussed in the light o f a comparison with the structures of the holotype willhendersonite an d natural and exchanged chabazites. (C) Elsevier Science Inc. 1997.