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 are very small. 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 involve Si-O-Si bending (near 600 cm-1) or represent mainly 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 stretching modes. For both diopside and pseudowollastonite, premelting thus 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.