SCOLECITE .1. REFINEMENT OF HIGH-ORDER DATA, SEPARATION OF INTERNAL AND EXTERNAL VIBRATIONAL AMPLITUDES FROM DISPLACEMENT PARAMETERS

A single crystal of scolecite, CaAl2Si3O10. 3H(2)O, was studied by X-r ay diffraction methods at room temperature. The intensities were measu red with MoK alpha radiation (lambda=0.71069 Angstrom) in a complete s phere of reflection up to sin theta/lambda=0.9 Angstrom(-1). The struc ture was refined in the pseudo-orthorhombic setting of space group F1d 1 instead of the conventional setting Cc for better comparison with na trolite (Fdd2). The cell parameters are: a=18.502(1) Angstrom, b=18.97 4(2) Angstrom, c=6.525(1) Angstrom, beta=90.615(7)degrees, V=2290.6(3) Angstrom(3), Z=8. A refinement of high-order diffraction data yielded residuals of R(F)=0.9%, R-w(F)=0.9%, GoF=1.73 for 1831 high-angle ref lections (0.7 less than or equal to sin theta/lambda less than or equa l to 0.9 Angstrom(-1)) and R(F)=1.2%, R-w(F)=1.4%, GoF=3.22 for all 34 78 independent reflections. In comparison with natrolite, the replacem ent of 2 Na+ by 1 Ca2+ and 1 H2O leads to a reduction of symmetry from Fdd2 to F1d1. Each general atomic position in natrolite (except of Na ) splits into two crystallographically independent positions in scolec ite. The T-O distances and T-O-T angles of these two sites differ dist inctly from each other due to the influence of the calcium ions on the framework. An unexpected result of our detailed analysis of the data is that the additional water molecule (O7) disturbs the symmetry of th e framework to a greater extent than the replacement of Na+ by Ca2+. A s a comparison of the displacement parameters indicates, the bonds wit hin the tetrahedral framework and to the extraframework cations are st ronger in scolecite than in natrolite. The isotropic U(equ) values of the framework atoms and extraframework cations are about 10% smaller i n scolecite compared to natrolite. The same tendency is shown by the a nalysis of the internal vibrational amplitudes Delta U. The correspond ing force constants are in the range of F=358 to 3367 Nm(-1) for the T -O bonds in scolecite (in natrolite: F=354 to 824 Nm(-1)). The values of the force constants which determine the vibrations of the Ca ions a nd water molecules against the framework oxygen atoms lie in the range of F=33 to 1757 Nm(-1) (in natrolite: F=57 to 293 Nm(-1)).