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 carr ied out in order to elucidate the relationship between the chemical composi tion of natrolite and the arrangement of extraframework atoms in the zeolit ic cavities. The results of the refinement clearly show the presence of a n ew extraframework site, occupied by potassium, with a slight occupancy (abo ut 3%). The short distance between the new K site and O-W (1.68 Angstrom) p revents the simultaneous presence of K and O-W, so that the occupancy of th e O-W site is accordingly lowered. On the basis of structural refinements o f K-exchanged natrolite and gonnardite, a mechanism of Na-K substitution in natrolite is suggested. This substitution produces remarkable changes in t he natrolite structure through rearrangement of the ions in the zeolitic ch annels. (C) 1999 Elsevier Science B.V. All rights reserved.