Original paper
Crystal structure of afghanite, the eight-layer member of the cancrinite-group: evidence for long-range Si,Al ordering
Ballirano, Paolo; Bonaccorsi, Elena; Maras, Adriana; Merlino, Stefanο
European Journal of Mineralogy Volume 9 Number 1 (1996), p. 21 - 30
54 references
published: Dec 30, 1996
manuscript accepted: Sep 17, 1996
manuscript received: Mar 10, 1996
DOI: 10.1127/ejm/9/1/0021
Abstract
Abstract Afghanite, ideally [(Na,K)22Ca10][Si24Al24O96](SO4)6Cl6, is the eight-layer member of the cancrinitegroup (AB ABAC AC stacking sequence). Its structure was refined in the P31c space group to R = 4.5 % by means of single-crystal X-ray diffraction data. The cell parameters are a = 12.8013(7) Å, c = 21.4119(18) Å. The P63imc space group proposed in a previous structure refinement is not consistent with the ordered Si,Al pattern suggested by an Si/Al ratio equal to 1 shown by afghanite and other members of the cancrinite-group. The Si-O and Al-O bond distances, 1.61(2) Å and 1.72(2) Å respectively, found in the structure refinement, are in accordance with an ordered Si,Al distribution which is allowed by the P31c space group, a maximal non isomorphic subgroup of P63mc. Afghanite contains six 11-hedra (cancrinite) cages and two 23-hedra (liottite) cages. Four cancrinite cages are stacked along [O O z]. They contain a regular ....Ca-Cl-Ca-Cl.... chain similar to that observed in davyne and related phases: in particular Ca is located near the center of the bases whereas Cl is near the center of the cage. A liottite cage with a base-sharing cancrinite cage is stacked along [2/3 1/3 z] and [1/3 2/3 z]. The liottite cage hosts a maximum of three sulphate groups which alternate regularly with cation-containing planes. The cancrinite cage, that shares the bases with the liottite cages, presents a disordered distribution of Cl and F leading to two possible configurations similar to those observed in liottite
Keywords
afghanite • cancrinite-group • feldspathoids • structure refinement • stacking sequence