Raman spectra and lattice-dynamical calculations of natrolite

Polarized single-crystal Raman scattering and powder infrared absorption sp ectra of Fdd2 orthorhombic natural natrolite (Na-1.88 K-0.02 Ca-0.04 )[Al-1 .96 Si-3.03 O-10].2.03 H2O from Khibiny, Kola peninsula, were measured. Usi ng short-range models, lattice-dynamical calculations were performed for an idealized natrolite structure Na-4[Al4Si6O20]4H(2)O containing 46 atoms in the primitive unit cell (Z = 2). By varying the valence force constants, t he calculated frequencies in the Raman and IR spectra were fitted to the ob served frequencies. On considering their calculated intensities as well, ne arly all the observed bands (especially those corresponding to the A(1) mod es) could be unambiguously assigned and interpreted. The external vibration s of H2O could be correctly assigned using deuterated samples. The stronges t Raman band at 534 cm(-1) corresponds to a breathing mode of the four-memb ered aluminosilicate ring. The calculated bulk modulus (52.7 GPa at zero pr essure) is close to the experimental value of 47 +/- 6 GPa. The natrolite structure has some advantages upon other zeolites to understa nd the amorphization mechanism, because samples of this mineral surrounded by a non-penetrating medium show no crystal phase transitions with increasi ng, pressure. Lattice energy minimization calculated with variable unit-cel l dimensions shows the crystal structure to become unstable at about 5.5 GP a, thereby apparently explaining the amorphization process at 4-7 GPa. This instability is connected with shear acoustic modes coupled with soft inter nal framework vibrations.