The thermal behavior of heulandites: a structural study of the dehydration of Nadap [Hungary] heulandite

Heulandite [1318-63-4] from Nadap (Hungary), characterized by an unusually high heat-induced contraction of the cell, was refined in its A and B phases. Phase A does not differ significantly from other refined heulandites. In phase B, the shift of the CS2 cation site and of the O1-O bonded to it, produces a strong rotation of the SBU around its baricentre and a pronounced zig-zag of the SBU chain. The O1 shift is strong enough to cause the statistical breaking of a T-O-T O bridge with the migration of 1 of the 2 T atoms to a new tetrahedron, which shares 3 vertices with the previously occupied one. The fourth vertex is on the mirror plane and joins 2 SBUs of adjacent layers. The data of the refined heulandite-type minerals show that the O1 shift increases with the cation content of the CS2 site. In heulandites, where the CS2 site has a high occupancy of cations with high ionic potential O breaking occurs; in clinoptilolites, where the CS2 site has a low occupancy, the O1 shift is low and breaking of the O bridge does not occur. As a consequence only heulandite shows a B phase which is stable over a long period of time.