1ST OCCURRENCE OF A BA-DOMINANT BREWSTERITE - STRUCTURAL FEATURES

The first occurrence of brewsterite in Italy, here reported, is at the Cerchiara mine (La Spezia, Liguria); it is found as prismatic crystal s filling veins in metacherts from Northern Apennine ophiolitic sequen ces. Electron microprobe analysis ((Ba1.80Sr0.02Na0.07Ca0.03K0.04) (Al 4.29Si11.78)O32.nH2O) reveals the dominant presence of barium among th e exchangeable cations. Cell parameters are: a = 6.790(1), b = 17.581( 4), c = 7.735(1) angstrom, beta = 94.50(3)-degrees. Biaxial (+), n(alp ha) = 1.514(2), n(beta) = 1.516(2), n(gamma) = 1.528(2). Single crysta l refinement confirms the main features of the brewsterite structure, apart from the extraframework cations which seem to be spread over thr ee sites, two of which with low occupancy. This occurrence of brewster ite furnishes new information on the crystal chemistry of this zeolite , up to now reported only as a Sr-dominant zeolite.