In situ structural changes of amorphous diopside (CaMgSi2O6) up to 20 GPa: A Raman and O K-edge X-ray Raman spectroscopic study

Abstract

Diopside, CaMgSi2O6, is an important analogue for depolymerized silicate melts. We have used the complimentary spectroscopies, X-ray Raman Scattering (XRS), X-ray absorption near edge structure (XANES) and Raman, to investigate diopside glass in situ to 20GPa. We observe a stark change in behavior of CaMgSi2O6 near 4GPa that corresponds to a major change in the rate of inter-tetrahedral angle (∠Si–O–Si) closure. Below 4GPa, the ∠Si–O–Si closes rapidly at 2–3°/GPa whereas above 4GPa it decreases by ∼1°/GPa. A distinct shift to higher wavenumbers of the 870 and 905cm−1 Raman bands are observed at 1.3GPa suggesting that Q0 species have been completely converted to Q1 or higher Qn species.XRS measurements at the O K-edge suggest that [5]Si is formed by 3GPa. This formation is accompanied by a rapid decrease in the ∠Si–O–Si and a decrease in Q0 species. A linear increase in the geometric mean of the high frequency envelope, the χb value, from 999 to 1018cm−1 suggests that the conversion of NBO to BO is continuous up to 14GPa. Above 14GPa, the Raman spectra show an obvious negative shift in both, the high frequency peak maximum, and the χb position. Simultaneously, the low frequency envelope looses its asymmetry at 14GPa. This may be explained by either a loss of a vibrational mode in the range 1000–1200cm−1 and/or the formation of [6]Si. The structural evolution of CaMgSi2O6 correlates well with a major change in the compressibility and diffusivity around 5GPa.