Crystal structure of a K-rich natural gmelinite and comparison with the other refined gmelinite samples.

A complete crystallochem. characterization of a gmelinite sample from Fara Vicentina (Vicenza, Italy) is given. The mineral is anomalous for the prevalence of K (2.72 atoms per unit cell) among the exchangeable cations. The lattice consts. are a = 13.621(3) and c = 10.254(1) Å. Its crystal structure was refined down to Rw = 5.92% and compared with the structures reported for Na-, Ca- and K-rich gmelinites in the literature; the sample studied is characterized by a very low occupancy (3%) of 1 (C2) of the 2 exchangeable cation sites and by water mols. distributed among 8 partially occupied sites. A correlation involving the C2 site and the geometry of the tetrahedral framework was found between the bond strengths: with decrease in the bond strength, the shape of the 8-ring of the gmelinite cage becomes more circular, the double 6-ring becomes more hexagonal and, consequently, the c parameter increases and the a parameter decreases. Post-crystn. cation exchanges explain the present chem. compn. of the mineral.