Raman spectroscopy of silicate melts at magmatic temperatures: Na2O-SiO2, K2O-SiO2 and Li2O-SiO2 binary compositions in the temperature range 25–1475°C

Abstract

High-quality Raman spectra of silicate glasses, supercooled liquids and liquids have been obtained in situ to temperatures of 1475°C. The success of the spectroscopic technique is fundamentally dependent on the ability to avoid spectra degradation caused by black-body radiation from the furnace. This can be accomplished by focussing the diameter of the exciting laser beam to ∼ 1 mm, and to control the depth of focus in the sample to 6–40-μm depth. The samples can be viewed visually through a microscope throughout the process, which ensures that the distance between the sources of blackbody radiation (furnace wall and bubbles in glass and liquid) and focus is optimized.With this technique, more than 100 spectra of glasses, supercooled melts and melts in the systems Li2O-SiO2, Na2-SiO2 and K2O-SiO2 have been recorded at 25–1475°C in the frequency range most sensitive to the overall anionic structure of amorphous silicates (800–1300 cm−1). The coexisting structural units generally are SiO32− (Q2), Si2O52− (O3) and SiO2 (Q4) in all glasses and melts in the temperature range investigated. The abundance of the structural units appear sensitive to temperature. The temperature dependence is qualitatively consistent with a shift to the right of the reaction: Si2O52−(2Q3)⇌SiO32−(Q2)+SiO2(Q4 with increasing temperature. From intensity variations with temperature of relevant Si-O stretch bands, the free energy for this reaction probably is sensitive to both bulk melt NBO/Si and to the electronic properties of the alkali metal.