Abstract
An intermediate mineral between original type dachiardite and sodium-rich dachiardite has been found from Hatsuneura, the Ogasawara Islands. New data for dachiardite from the Onoyama mine, Kagoshima Pref., are also given. Based on the data for the above two dachiardites and literature data, mode of occurrence, chemical composition, optical, X-ray and thermal properties of dachiardite group zeolites were examined. The wide variation of chemical composition in this group seems to be due to the substitution of NaNa for Ca among the exchangeable cations with apparently independent substitution of Al for Si in the framework. The X-ray powder diffraction patterns are not distinguishable among the dachiardite group zeolites, and structural disorder is commonly observed in them. Correlation between Si/(Al+Fe3+) ratio and β angle is recognized. Refractive indices, 2V and c∧Z are closely related to R2+⁄(R++R2+) ratio, in which R2+ and R+ are numbers of divalent and monovalent exchangeable cations, respectively. Some differences are recognized in TG-DTA curves and thermal stability between the original type dachiardite and the sodium-rich dachiardite. The results suggest that the isomorphous substitution may be expected in the dachiardite group zeolites.
