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Crystal structure of afghanite, the eight-layer member of the cancrinite-group; evidence for long-range Si, Al ordering

Paolo Ballirano, Elena Bonaccorsi, Adriana Maras and Stefano Merlino
Crystal structure of afghanite, the eight-layer member of the cancrinite-group; evidence for long-range Si, Al ordering
European Journal of Mineralogy (February 1997) 9 (1): 21-30

Abstract

Afghanite, ideally [(Na,K) (sub 22) Ca (sub 10) ][Si (sub 24) Al (sub 24) O (sub 96) ](SO (sub 4) ) (sub 6) Cl (sub 6) , is the eight-layer member of the cancrinitegroup (ABABACAC 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) A, c = 21.4119(18) A. The P6 (sub 3) mc 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) A and 1.72(2) A 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 P6 (sub 3) mc. 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.


ISSN: 0935-1221
Serial Title: European Journal of Mineralogy
Serial Volume: 9
Serial Issue: 1
Title: Crystal structure of afghanite, the eight-layer member of the cancrinite-group; evidence for long-range Si, Al ordering
Affiliation: Universita di Pisa, Dipartimento di Scienze della Terra, Pisa, Italy
Pages: 21-30
Published: 199702
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 17
Accession Number: 1997-030088
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: 3 tables
Secondary Affiliation: Universita di Roma "La Sapienza", ITA, Italy
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 199710
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