VIBRATIONAL ANALYSIS OF DIOPTASE CU6SI6O18-CENTER-DOT-6(H2O) AND ITS PUCKERED 6-MEMBERED RING

The normal modes of vibration and their frequencies are calculated for dioptase, a mineral whose crystal structure (space group R $($) over bar$$ 3 or C-3i(2) consists of puckered six-membered silicate rings (S i6O18) linked by CU2+ ions and H2O groups. The calculation employs a v alence force potential consisting of central interactions between near est neighbors and bond-bending interactions centered at the Si4+ and C U2+ ions. The force constants are determined by fitting the calculated frequencies to values obtained by measuring the single-crystal Raman spectra. The calculated frequencies are in reasonable agreement with e xperiment, permitting assignment of normal modes to the observed spect ral frequencies. Considerable mixing of Cu and H2O motions with those of the ring is found for the Raman-active modes below 430 cm(-1). The normal modes and frequencies of the hypothetical isolated ring with C- 3i symmetry are determined by neglecting all interactions between the rings and the surrounding Cu and H2O. The identification of normal mod es characteristic of the puckered six-membered silicate rings and the effect of the environment on these modes may prove useful in the inter pretation of the Raman spectra of amorphous silicates.