Vibrational analysis of wadeite K2ZrSi3O9 and comparisons with benitoite BaTiSi3O9.

Abstract

Raman scattering measurements and lattice dynamics calculations are reported for the cyclosilicate mineral wadeite K2ZrSi3O9 . The structure of this crystal ~space group P6 or C 3h 1 ! consists of planar three-membered silicate rings ~Si3O9! linked by K 1 and Zr 41 ions. The rings in wadeite are nearly identical to those in the mineral benitoite BaTiSi3O9 . The wadeite structure differs from that of benitoite, however, in that the rings are in a staggered configuration, rather than arranged one directly above the other in columns. The calculation reported here employs a valence force potential consisting of central interactions between nearest neighbors and bond-bending interactions centered at the K 1 ,Z r 4 1 ,S i 4 1 , and O 22 ions, with force constants determined by fitting the frequencies observed in the Raman spectra. The calculation is in reasonable agreement with experiment, permitting assignment of normal modes to the observed spectral frequencies. Considerable mixing of K 1 and Zr 41 motions with those of the rings is found for Raman-active modes below 250 cm 21 . The highly localized normal modes of the planar three-membered silicate rings in wadeite are nearly identical to those determined for the rings in benitoite, at frequencies above 800 cm 21 . At lower frequencies, however, the similarities become less obvious as internal ring modes can be mixtures of idealized three-membered ring modes that are unique to each mineral. A striking result in the case of wadeite is the absence of the bridging oxygen breathing mode that was found for benitoite and two other cyclosilicates studied previously. This finding may have consequences for the interpretation of the Raman spectra of silicate glasses, since a breathing mode of this type has been postulated to be a universal feature of networks containing three-membered rings. @S0163-1829~96!07621-7#