High-temperature X-ray diffraction and Raman spectroscopy of diopside and pseudowollastonite |
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| Authors: | Pascal Richet Bjorn O Mysen Jannick Ingrin |
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| Institution: | (1) Laboratoire des Géomatériaux, URA 734, CNRS, Institut de Physique du Globe, 4 place Jussieu, F-75252 Paris cedex 05, France, FR;(2) Geophysical Laboratory and NSF-sponsored Center for High-Pressure Research (CHiPR) Carnegie Institution of Washington, 5251 Broad Branch Rd, Washington, DC, 20015, USA, US;(3) Laboratoire de Géophysique et Géodynamique Interne, URA CNRS 1369, Université Paris-Sud, F-91405 Orsay, France, FR |
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| Abstract: | Diopside (CaMgSi2O6) and pseudowollastonite (CaSiO3) have been studied by X-ray powder diffraction and Raman spectroscopy up to their respective melting points. In agreement
with previous unit-cell parameters determinations below 1100 K, thermal expansion of diopside along the a and c axis is much smaller than along the b axis. For pseudowollastonite, the axis expansivity increases slightly in the order b>a>c. For both minerals, the change in unit-cell angles is very small and there are no anomalous variations of the other unit-cell
parameters near the melting point. With increasing temperatures, the main changes observed in the Raman spectra are strong
increases of the linewidths for those bands which mainly represent Si−O−Si bending (near 600 cm−1) or involve Ca−O or Mg−O stretching, in the range 270–500 cm−1 for diopside, and 240–450 cm−1 for pseudowollastonite. At temperatures near the onset of calorimetric premelting effects, this extensive band widening results
in a broad Raman feature that can no longer be deconvoluted into its individual components. No significant changes affect
the Si−O streching modes. For both diopside and pseudowollastonite, premelting appears to be associated with enhanced dynamics
of the alkaline-earth elements. This conclusion contrasts markedly with that drawn for sodium metasilicate in which weaker
bonding of sodium allows the silicate framework to distort and deform in such a way as to prefigure the silicate entities
present in the melt.
Received 16 July 1997 / Revised, accepted: 6 March 1998 |
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