Structural modifications induced by dehydration in the zeolite gismondine

Gismondine from Ontalto di Castro, Italy [Ca3.91Al7.77Si8.22O32.17.57 H2O] with a 10.0199(4), b 10.6773(5), c 9.8316(1) Å, β 92.561(6)°, space group P21/c, dehydrated in vacuum for 1 and 24 h transformed into 2 phases, here called gismondine (1 h) and gismondine (24 h), resp. Gismondine (1 h) is characterized by 9.5% H2O loss and by a small decrease in the cell vol. (ΔV = 0.6%) with a 9.989(3), b 10.616(3), c 9.820(3) Å, and β 92.57(2)°, Rw = 7.6%. The framework is almost undistorted, but a rearrangement of H2O mols. causes a change in space group to P21, with formation of a more regular 6-coordinated Ca polyhedron. Gismondine (24 h) is characterized by the orthorhombic space group P212121 and a unit cell doubled with respect to the nondehydrated sample; a 13.902(9), b 8.892(4), and c 13.952(5) Å, Rw = 7.6%. More than 50% of H2O is lost, the framework is highly distorted, and the channels are strongly squashed. Residual H2O sites are fully occupied. Ca polyhedra are 7-fold coordinated and are linked by vertexes to form infinite chains.