Contribution
First occurrence of a Ba-dominant brewsterite: structural features
Cabella, Roberto; Lucchetti, Gabriella; Palenzona, Andrea; Quartieri, Simona; Vezzalini, Glovanna
European Journal of Mineralogy Volume 5 Number 2 (1993), p. 353 - 360
25 références bibliographiques
publié: Apr 19, 1993
manuscrit accepté: Nov 30, 1992
manuscrit reçu: Jul 22, 1992
DOI: 10.1127/ejm/5/2/0353
Abstract
Abstract The first occurrence of brewsterite in Italy, here reported, is at the Cerchiara mine (La Spezia, Liguria); it is found as prismatic crystals filling veins in metacherts from Northern Apennine ophiolitic sequences. Electron microprobe analysis ((Ba1.80Sr0.02Na0.07Ca0.03K0.04) (Al4.29Sin11.78)O32·nH2O) reveals the dominant presence of barium among the exchangeable cations. Cell parameters are: a = 6.790(1), b = 17.581(4), c = 7.735(1) Å, ß = 94.50(3)°. Biaxial (+), nα = 1.514(2), nß = 1.516(2), nγ = 1.528(2). Single crystal refinement confirms the main features of the brewsterite structure, apart from the extraframework cations which seem to be spread over three sites, two of which with low occupancy. This occurrence of brewsterite furnishes new information on the crystal chemistry of this zeolite, up to now reported only as a Sr-dominant zeolite
Mots-clefs
natural zeolite • Ba-dominant brewsterite • Cerchiara mine (La Spezia • Italy) • chemical analysis • optical properties • crystal structure refinement