Cation exchange of Nl4 for Na in natrolite, Na2A12ShOlO.2H20, yields an anhydrous ammonium natrolite, (Nl4)zA12SiJOlO. The symmetry is reduced from space group Fddl in Na-natrolite to space group Pl121 in Nl4-natrolite, but the description of the structure has been given in space group Cl121 for better comparison with the Na-form. The volume of the unit cell of NH4-natrolite (a = 17.899(2) A, b = 18.390(2) A, c = 6.529(1) A, y = 90°, V = 2149.1(8) A3) is reduced compared with the original Na-natrolite (a = 18.293(1) A, b = 18.641(1) A, c = 6.587(1) A, V = 2246.2(2) A3) by more than 4 %. The agreement factors are R = 4.67 % and Rw = 2.98 % for 9269 Fobs. The crystal is twinned on a plane normal to [100] or [010]. The ammonium groups in the Nl4form occupy similar positions to those of the water molecules in Na-natrolite. The distances Si-O and Al-O within the framework are strongly influenced by the hydrogen bonds from the NH4-groUPS. Hydrogen bonded oxygen atoms have, on average, larger TO distances (0.01 A) and smaller TO -T angles (6.8°) than non-hydrogen bonded oxygen atoms of the framework. The effect of the hydrogen bonds on the T-0 bond lengths is stronger than the influence exerted on them by the change in the T-0-T angles. This is the first instance in which changes in the geometry of the framework can be attributed solely to hydrogen bonding effects. The contraction of the framework due to the exchange of Nl4 for Na is caused by tilting and twisting of the chains within themselves and not by a rotation of the chains as a whole around their hinges between the flbers. The NAT framework of natrolite is collapsible, which means that all angles TO -T (the hinges between the essentially rigid T04 tetrahedra) change upon contraction or expansion in the same sense.

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Abstract.A single crystal of natrolite, Na2A12Si3010 9 2H20 (space group Fdd2), was studied by X-ray diffraction methods at room temperature. The intensities were measured in a complete sphere of reflection up to sinO/ )~=0.903 ~-1. A refinement of high-order diffraction data yielded residuals of R/(F)=0.9%, Rw(F)=0.8%, GoF=I.40 for 1856 high-angle reflections (0.7 <_ sinO/ 2_<0.903 ,~-1) and R(F)= 1.0%, Rw(F) = 1.2%, GoF=3.07 for all 3471 independent reflections in the complete sphere of reflection. The X-X method was used to calculate deformation electron densities (DED) in natrolite. Within all tetrahedra, residual electron densitywas found in the T-O bond directions indicating a considerable covalent contribution to the chemical bond. The range of the interatomic peak heights was from 0.19 to 0.3@/~k 3 in the SiO 4 tetrahedra and from 0.11 to 0.23 e/A 3 in the A10 4 tetrahedron. The ionic contribution to the chemical bond manifests itself in the displacement of the peaks towards the oxygen atoms. Charge displacement due to interaction of nonframework cations with framework oxygen atoms as well as electron densities attributable to the lone pair orbitals in the water molecule have been observed.

Diffusion / Neutron Scattering / Transport PropertiesResults on the motion of water molecules in six natural zeolites (natrolite, phillipsite, harmotome, heulandite, stilbite and chabazite) from incoherent inelastic neutron scattering are presented. The spectra for five zeolite samples exhibit broad maxima and few resolved peaks. Only for natrolite with water molecules tightly bound to the alumosilicate framework a spectrum with well-resolved peaks is observed. A detailed interpretation is attempted for the Ba-zeolite harmotome where data were recorded at various temperatures and different amount of water. -Differential scanning calorimetry gave a broad dehydration peak for chabazite (T = 443 K) and a sharp single peak for natrolite ( T = 633 K), whereas three maxima (T = 433, 523 and 673 K) were observed for harmotome.

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