Rehydration mechanisms in zeolites: reversibility of T-O-T breaking and oftetrahedral cation migration in brewsterite

The rehydration process of the zeolite brewsterite (space group P2(1)/m, id eal Formula (Sr,Ba)(2)Al4Si12O32. 10H(2)O) was studied by single crystal X- ray diffraction on a sample completely dehydrated (heated at 280 degreesC i n vacuum for 24 h) and subsequently kept at room conditions for four years. The following structural modifications were observed: (1) the mineral almo st regains the unit cell parameters of the untreated phase; (2) a near-comp lete rehydration occurs, with the water molecules reassuming the four origi nal positions; (3) all the extra-framework cations, spread over a number of sites in the dehydrated phase, migrate to the only site originally present in the untreated phase; (4) the framework cations which migrated during de hydration to new four- and Eve-coordinated sites owing to the breaking of a T-O-T bridge, but which do not form new bridges, regain their original pos ition; (5) ill the case of formation of new T-O-T bridges, the return of(Si ,Al) cations to the original sites occurs every time they are five-coordina ted, whereas it is only partial when they are four-coordinated. (C) 2001 El sevier Science B.V. All rights reserved.