Optical vibrations in sheet silicates

Raman scattering up to 1000 cm -1 Stokes shift, and infrared absorption spectra, from 10 cm -1 to 4000 cm -1 , of several sheet silicates have been measured at room temperature. The samples are natural crystals of two muscovites, two phlogopites, two talcs, one margarite, one lepidolite, one chlorite and one biotite. The optical vibrational spectra of sheet silicates can be interpreted as molecular vibrations of: (1) the distorted octahedron MO 6 with S 6 symmetry below 210 cm -1 , (2) an isosceles triangle O-H-O of C 2 nu symmetry in sheet silicates containing octahedral Al 3+ in the range 190 cm -1 to 270 cm -1 , (3) a distorted tetrahedron SiO 4 of C 3 nu symmetry between 300 cm -1 and 1200 cm -1 , (4) a combination band between the nu 3 (a 1 ) mode of SiO 4 at 450 cm -1 with other bands at lower frequencies and (5) the stretching and librations of OH between 400 cm -1 and 3700 cm -1 .