First occurrence of a barium-dominant brewsterite: Structural features

At the Cerchiara mine (La Spezia, Liguria, Italy), Ba-dominant brewsterite (BB) occurs as prismatic crystals filling veins in metacherts from the northern Apennine ophiolitic sequences. Electron microprobe anal. of BB suggested ((Ba1.80Sr0.02Na0.07Ca0.03K0.04) (Al4.29Si11.78)O32.nH2O); the dominant presence of Ba among the exchangeable cations is obsd.. The unit-cell parameters of BB are: a 6.790(1), b 17.581(4), and c 7.735(1) Å, β = 94.50(3)°. BB is optically biaxial (+), with ns α 1.514(2), β 1.516(2), and γ 1.528(2). Single-crystal refinement confirms the main features of the BB structure, apart from the extra-framework cations which spread over 3 sites, two of which with low occupancy. This occurrence of BB furnishes new information on the crystal chem. of this zeolite, up to now reported only as a Sr-dominant zeolite.