STRUCTURAL REFINEMENTS OF NA-, K-, AND CA-EXCHANGED GMELINITES

Monocationic Ca, Na, and K gmelinite crystals were obtained by cation exchange from three natural samples. Their crystal structures were ref ined from single crystal XRD data in the P6(3)/mmc space group down to R values <8%. The structures of the isocationic forms derived from th e different natural samples agree within the experimental error, where as marked differences were found in the different monocationic forms ( Na, Ca, K) of each sample. Cation site (C1) in the gmelinite cage is f ully occupied in the Na- and K-forms and statistically (approximate to 50%) in the Ca-form. Consequently, the tetrahedral framework undergoe s deformations involving marked c lengthening and weak a shortening in the Ca-Na-K series that are closely correlated to both the occupancy factor and the ionic radius of the cation in the C1 site. The cell par ameters of the Ba-form reported in the literature fit this scheme very well.