Structural refinement of a K-rich natrolite: evidence of a new extraframework cation site

The structure refinement of a natural K-rich natrolite from the Kirov mine, Khibina massif (Kola Peninsula), closely intergrown with amicite, was carried out in order to elucidate the relationship between the chemical composition of natrolite and the arrangement of extraframework atoms in the zeolitic cavities. The results of the refinement clearly show the presence of a new extraframework site, occupied by potassium, with a slight occupancy (about 3%). The short distance between the new K site and O-W (1.68 Angstrom) prevents the simultaneous presence of K and O-W, so that the occupancy of the O-W site is accordingly lowered. On the basis of structural refinements of K-exchanged natrolite and gonnardite, a mechanism of Na-K substitution in natrolite is suggested. This substitution produces remarkable changes in the natrolite structure through rearrangement of the ions in the zeolitic channels. (C) 1999 Elsevier Science B.V. All rights reserved.