Bull. Soc. fr. Minéral. Cristallogr.

(1975), 98, 3&-540.

The crystal structure of barrerite

Structure cristalline de la barrérite.

Introduction

The new mineral name, Barrerite, in honour of R. M. Barrer, who has largely contributed to zeo¬ lite studies, was approved by the Commission for New Minerals and Mineral Names, of the I. M. A., for a mineral first described by Passaglia and Pon-giluppi (1974 and 1975) as sodian stellerite. It was found at Capo Pula, Sardinia, Italy, and it is simi¬ lar to stellerite in both its powder pattern and its Si/Al ratio, but it differs from stellerite in its sym¬ metry and in its exchangeable cations, which are mainly alkalies. The aim of this research is to verify whether barrerite has the same framework of stilbite (Galli, 1971) and stellerite (Galli and Alberti, 1975) and to investigate the reasons for its different symmetry.

Experimental

Barrerite from Capo Pula (Passaglia and Pongi-luppi, 1974) used in the present work has space group Amma, with a = 13.643 (2), b — 18.200 (3), c = 17.842 (3) (all A, errors in parentheses). The unit cell content is :

(Cali68Mg0.34Na10-90K2.12)l5.04

[ (All6.38-F �0.02) 16. 40�55. 41�144 ] • 5I.56H2O

A single crystal (~ 0.4 x 0.35 x 0.25 mm) was X-rayed with Zr-filtered Mo-radiation in the 0-2 0

Refinement

scan mode on a Philips PW 1100 four-circle com¬ puter-controlled diffractometer of the «Centro di Cristallografia Strutturale del C. N. R. di Pavia, Italy ». 3,543 diffraction intensities possible in space group Ammm were inspected. The systema¬ tically absent reflections of the type hko with h = 2 n + confirmed the space group to be Amma. Of the remaining 3,444 diffractions compa¬ tible with the correct space group, corresponding to 13.3 per cent of the possible diffractions in the molybdenum sphere, 2,483 diffractions (72 % of the measured reflections) were considered as ob¬ served having 1 > 2 a (I). No absorption correction was applied

Atomic scattering factors given by Hanson et al. (1964) for neutral atoms were used throughout ; for the exchangeable cations an average scattering curve was obtained considering all the elements detected by the chemical analysis, each with its own weight, as follows : Na : 72,4 %, K : 14. 1 %, Ca : 11. 1 %, Mg : 0.2 % ; for the Si atoms an ave¬ rage curve was obtained with 77.2 % of Si and 22.8 of Al.

The refinement was carried out on the CDC 6600 computer of the CINECA as an alternation of least square cycles and electron density maps, the last maps allowed the contènts of the channels in the tetrahedral framework to be located. The atomic parameters of the framework atoms given by Galli and Alberti (1975) for stellerite were used to start