Raman spectra of chondrodite at various temperatures

The effect of temperature on the Raman spectra of natural and synthetic cho ndrodite was investigated over the range 81 to 873 K at atmospheric pressur e. All samples remained intact up to 873 K, This high-temperature stability is unusual, and contrasts with the behaviour of all other synthetic dense hydrous magnesium silicates which have, so far, been found to dehydrate at temperatures below 873 K at atmospheric pressure, Also, unlike the other de nse hydrous magnesium silicates, the Raman-active OH-stretching bands of bo th natural and synthetic chondrodite were observed to decrease in wavenumbe r with increasing temperature, The negative temperature dependence of these vibrations suggests that there is a rather open environment around the O-H bonds. All the Raman-active modes below 1000 cm(-1) (except the 571 cm(-1) band in synthetic, F-free, chondrodite) have a negative temperature depend ence in accord with most other silicates. The temperature data, combined with published high-pressure data, were also used to determine the intrinsic anharmonicity of the Raman-active modes. S ynthetic chondrodite was found to have smaller intrinsic anharmonic paramet ers (a(i)) than natural chondrodite, Comparison of the OH-stretching modes with the internal modes of synthetic chondrodite showed that they were simi lar but had opposite behaviour, while the internal modes of natural chondro dite were found to have much larger a(i) parameters. The modes at 931 and 9 66 cm(-1) in natural chondrodite had the highest levels of intrinsic anharm onicity indicating that they may arise from Si-OH vibrations rather than Si O4 vibrations. The larger temperature dependence of the Raman-active modes and the higher intrinsic anharmonicity of natural chondrodite indicate that the presence of fluorine atoms has a strong influence on the vibrational p roperties of chondrodite and the OH vibrations in particular, Copyright (C) 1999 John Wiley & Sons, Ltd.