STRUCTURE OF SILICATE MELTS AT HIGH-TEMPERATURE - IN-SITU MEASUREMENTS IN THE SYSTEM BAO-SIO2 TO 1669-PERCENT-C

The structure of glasses, melts, and supercooled melts in the system B aO-SiO2 with BaO/SiO2 = 0.367 has been investigated in-situ with Raman spectroscopy from 25 to 1669-degrees-C. For this purpose a technique has been developed that integrates confocal micro-Raman spectroscopy w ith a microheating stage that fits on the sample stage of a petrograph ic microscope. The temperatures are precise to +/- 4-degrees-C and acc urate to 5-10-degrees-C. The excited volume is approximately 1 mum in diameter by 20-50 mum in depth. The Raman spectra of room-temperature glasses and of melts and,supercooled melts at higher temperatures are consistent with the presence of SiO32- (or Q2), Si2O52- (or Q3), and S iO2 (or Q4) units coexisting in the samples at all temperatures. No ev idence for changes in types of structural units was found. The Raman s pectra indicate that the abundance at SiO32- and SiO2 units increases, whereas that of Si2O52- decreases with temperature. Under the assumpt ion of ideal mixing, the high-temperature data are fitted in terms of In K vs. 1/T (K), which yields a DELTAH for the reaction Si2O52- <---- > SiO32- + SiO2 of 34 +/- 4 kJ/mol.