T18 A Session in Honor of Georges Calas, Professor Emeritus, Sorbonne Université, Paris, France, and 2023 Roebling Medalist of the Mineralogical Society of America I

Sunday, 15 October 2023
8:00 AM - 12:00 PM
315 / 316

POLYMERIZATION AND DISPROPORTIONATION REACTIONS IN SILICATE GLASSES: NEW EVIDENCE FOR "FREE" OXIDE

Paper number: T18-2
Program: Technical Sessions
Time: 8:05 AM – 8:20 AM

Authors

Abstract

The number and type of Qn species (n=0-4) strongly affects the chemical and physical properties of silicate glasses and melts. 29Si MAS NMR and Raman spectra of alkali and alkaline earth silicate glasses enable the identification and quantification of Qn; but even the assignment of the resolved spectral bands remains controversial. Nearly all publications since 1980 have related the Q species in these glasses1,2 using the disproportionation reactions:

2Q3 → Q2 + Q4 (1); 2Q2 → Q1 + Q3 (2); 2Q1 → Q0 + Q2 (3)

Comparison of the best resolved 29Si NMR2 and Raman3 spectra for ~40 mol% Na2O glass show that both spectra are dominated by two resolved bands: one band from Q2 and one larger, broader band from Q3. Estimation of the small amount of Q4 (~2%) from static 29Si NMR enabled Stebbins2 to calculate a disproportionation constant of 0.011 for Eq. (1). The recently published 40% BaO and K2O Raman spectra4,5 are remarkably similar to the 40% Na2O spectrum, implying the same assignments for all three glasses- one band from each of Q2 and Q3. The Raman spectra from 50 mol% Li2O, Na2O and K2O glasses and melts4,5 are again all qualitatively similar, displaying one band for each of Q1 (weak), Q2 (strong) and Q3 (medium) species. A requirement of Eq.2 is that at 50 mole% alkali oxide, Q1 = Q3 from mass balance considerations, NBO/Si = 2, and “free“oxide (FO) = 0. These spectra all demonstrate that Q3>Q1, so Eq.2 does not hold and it cannot be the only reaction relating the Q species. Polymerization reactions such as:

2Q2 = 2Q3 + FO (4)

must be considered. Indeed, for these 50 mol% glasses and melts, NBO/Si = ≤1.8, and mol% FO is present. Rxn. 4 dominates for the K metasilicate glass, yielding NBO/Si of 1.5 and a large FO of 7-8%, in agreement with O 1s XPS and the 29Si NMR spectra.

1) Virgo et al. Science 208, 1371 (1980). 2) Stebbins, Nature 330, 465 (1987). 3) Nesbitt et al. Chem. Geol. 562 , 129940 (2022). 4) Moulton et al. Chem. Geol.586, 120611 (2021). 5) Koroleva et al. Minerals 13, 94 (2023).

Geological Society of America Abstracts with Programs. Vol. 55, No. 6

doi: 10.1130/abs/2023AM-391046

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