NATROLITE .2. DETERMINATION OF DEFORMATION ELECTRON-DENSITIES BY THE X-X METHOD

A single crystal of natrolite, Na2Al2Si3O10.2H(2)O (space group Fdd2), was studied by X-ray diffraction methods at room temperature. The int ensities were measured in a complete sphere of reflection up to sin Th eta/ lambda 1=0.903 Angstrom(-1). A refinement of high-order diffracti on data yielded residuals of R/(F)=0.9%, R(w)(F)=0.8%, GoF=1.40 for 18 56 high-angle reflections (0.7 less than or equal to sin Theta/ lambda less than or equal to 0.903 Angstrom(-1)) and R(F)=1.0%, R(W)(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 elect ron densities (DED) in natrolite. Within all tetrahedra, residual elec tron density-was found in the T-O bond directions indicating a conside rable covalent contribution to the chemical bond. The range of the int eratomic peak heights was from 0.19 to 0.34 e/Angstrom(3) in the SiO4 tetrahedra and from 0.11 to 0.23 e/Angstrom(3) in the AlO4 tetrahedron . The ionic contribution to the chemical bond manifests itself in the displacement of the peaks towards the oxygen atoms. Charge displacemen t due to interaction of nonframework cations with framework oxygen ato ms as well as electron densities attributable to the lone pair orbital s in the water molecule have been observed.