| Popis: |
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. |
