Doctor of Philosophy
Planetary Science and Exploration
Nesbitt, H. Wayne
Secco, Richard A.
In a geological context, glasses are useful analogues for silicate melts as they are more readily studied in the laboratory using a wide range of techniques that are impractical for molten liquids. Understanding the structure of binary silicate glasses can help us understand more about the magmatic processes that affect terrestrial planetary bodies.
Potassium silicate glasses ranging in composition from 10 mol% to 35 mol% K2O were studied using X-ray Photoelectron Spectroscopy (XPS). From high resolution O 1s XPS spectra, Bridging Oxygen (BO) mole fractions were calculated and compared with those of previous 29Si MAS NMR studies. From 25-35 mol% K2O, XPS BO mole fractions were higher resulting in a discrepancy. Thermodynamic analysis indicated the presence of a few mol% of Free Oxygen (O2-) in these glasses, which challenges the longstanding assumption that the reaction
BO + O2- → 2NBO
goes to completion where NBO is Non-Bridging Oxygen. The uncertainties involving the synthesis of glass samples and the fitting of the XPS spectra were addressed. An attempt was made to resolve the discrepancy with the NMR results by examining the NMR data and reconsidering the assignment of Q-species. The amount of O2- in potassium silicate glasses was found to sufficient to affect reactions with CO2 and other volatile species which suggests that it is more reactive than Non-Bridging Oxygen.
XPS was then used to study oxygen speciation in glasses with 31 mol% K2O and 0, 1 and 3 mol% Al2O3. NBO decreased while BO increased in two contributions representing Q4 and KAlO2. Aluminum was found to dissolve according to the reaction:
K-Q3 + AlO1.5 → Q4 + KAlO2
At high temperatures the dissociation of Si-O-K moieties results in a process whereby NBO are converted to BO through nucleophilic attack of Si-NBO- melt species on surface Al2O3 sites. The conversion of NBO to BO results in a more stable melt. Without the stability that aluminum provides in melts, the composition of the upper mantle and crust and the subsequent evolution of the crust would have been different from what we observe today.
Summary for Lay Audience
Glass is a material that has been used for millennia for a variety of artistic, industrial and scientific purposes. The structure of a glass is essentially a snapshot of the melt structure at the point when it becomes a glass. In the Earth and Planetary Sciences, glasses provide useful analogues for natural melts. Glasses can be studied at room temperature using a wide variety of techniques unavailable to melts which require high temperatures. Glasses with simple compositions provide a basis for modeling the properties and behaviour of melts with more complex compositions.
In this work, X-ray Photoelectron Spectroscopy is used to quantify abundances of BO (e.g., Si-O-Si) and NBO (e.g., Si-O-K) atoms in potassium silicate glasses containing 10-35 mol% K2O. XPS results indicate an overabundance of BO atoms if the glasses are assumed to only contain BO and NBO and no O2-. Thermodynamic modeling reveals indirect evidence for O2- (e.g., K-O-K) being present at levels of several mol%. A discrepancy is found between the XPS results and results from previous Nuclear Magnetic Resonance (NMR) studies. An attempt is made to resolve this discrepancy by carefully considering uncertainties related to XPS analysis and sample preparation and offering a more complete interpretation of the NMR results. It is then shown that there is sufficient O2- in these glasses to react with CO2 and other volatile species, suggesting that it is the reactive species in these glasses and not NBO as is commonly assumed.
Small amounts of Al2O3 (1 and 3 mol%) are added to a glass containing 31 mol% K2O to study the dissolution of Al2O3. NBO is found to decrease, and BO is found to increase in two contributions with an increase in Al. From these results, the chemical reactions that are dominant at the start of dissolution are proposed where Si-NBO- melt species attack solid Al2O3 species to produce more BO atoms. The overall result is an increase in melt stability without which the evolution of the crust and mantle may have been much different.
Sawyer, Ryan, "Oxygen Speciation in Potassium Silicate and Potassium Aluminosilicate Glasses: Insights From X-Ray Photoelectron Spectroscopy" (2022). Electronic Thesis and Dissertation Repository. 8821.
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