P. Richet et al., HIGH-TEMPERATURE X-RAY-DIFFRACTION AND RAMAN-SPECTROSCOPY OF DIOPSIDEAND PSEUDOWOLLASTONITE, Physics and chemistry of minerals, 25(6), 1998, pp. 401-414
Diopside(CaMESi2O6) and pseudowollastonite (CaSiO3) have been studied
by X-ray powder diffraction and Raman spectroscopy up to their respect
ive melting points. In agreement with previous unit-cell parameters de
terminations below 1100 K, thermal expansion of diopside along the a a
nd c axis is much smaller than along the b axis. For pseudowollastonit
e, the axis expansivity increases slightly in the order b>a>c. For bot
h minerals, the change in unit-cell angles is very small and there are
no anomalous variations of the other unit-cell parameters near the me
lting point. With increasing temperatures, the main changes observed i
n the Raman spectra are strong increases of the linewidths for those b
ands which mainly represent Si-O-Si bending (near 600 cm(-1)) or invol
ve 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 on
set of calorimetric premelting effects, this extensive band widening r
esults in a broad Raman feature that can no longer be deconvoluted int
o its individual components. No significant changes affect the Si-O st
reching modes. For both diopside and pseudowollastonite, premelting ap
pears to be associated with enhanced dynamics of the alkaline-earth el
ements. This conclusion contrasts markedly with that drawn for sodium
metasilicate in which weaker bonding of sodium allows the silicate fra
mework to distort and deform in such a way as to prefigure the silicat
e entities present in the melt.