Probing the structure and the disorder of multi-component silicate glasses and melts: Insights from high-resolution solid-state NMR

Abstract

The structures and the extent of chemical and topological disorder in multi-component silicate glasses and melts provide insight into the macroscopic properties of natural silicate melts. Despite the importance and implications, the detailed structures and disorder in multi-component silicate glasses and melts including basaltic and andesitic glasses and melts are not fully understood. This dissertation is for a systematic exploration of the atomic structure in multi-component silicate glasses and melts with varying composition–a model system for natural melts- using high-resolution multi-nuclear (29Si, 27Al, 17O and 7Li) solid-state nuclear magnetic resonance (NMR) that can provide detailed coordination environments of framework cations as well as the degree of network polymerization in silicate glasses. The main objective of this thesis is probing the effect of composition on the atomic structures and their effect on macroscopic properties in multi-component silicate glasses including basaltic and andesitic glasses and melts. The effects of composition on the structure of quaternary CaO-MgO-Al2O3-SiO2 (CMAS) glasses in diopside (CaMgSi2O6) and Ca-tschermakite (CaAl2SiO6) join and glass in the diopside-anorthite (CaAl2Si2O8) eutectic composition (Di64An36)-model systems for basaltic melts- was explored using multi-nuclear solid-state NMR. The predominance of [4]Al and its extensive mixing with [4]Si is consistent with the negative enthalpy of mixing for CMAS glasses obtained by solution calorimetry. The observed increase in non-bridging oxygen (NBO) fraction with increasing mole fraction of diopside (XDiopside) indicates an obvious decrease in melt viscosity toward a diopside endmember. The partitioning of Ca2+ and Mg2+ and/or unmixing of these cations between NBOs and bridging oxygens (BOs) may result in variations in the activity coefficients of CaO and MgO, thus the compositions of melts. The structural details of model andesitic glasses [CaO-MgO-Na2O-Al2O3-SiO2 (CMNAS)] in the diopside and jadeite (NaAlSi2O6) join were investigated using high-resolution solid-state 27Al, 17O MAS and triple-quantum (3Q) MAS NMR and 29Si MAS NMR spectroscopies. The presence of Al-O-Al in jadeite glass implies a violation of the Al-avoidance rule in the glasses and the decrease in the fraction of NBOs with increasing XDiopside is consistent with a decrease in their viscosity. Considering all the experimental Al coordination environments available in the literature, together with the current experimental studies, we attempt to establish the relationship between the fractions of highly coordinated Al and composition, particularly average cationic potential of non-network forming cations (<c/r>ave, defined as cationic potential normalized by the mole fraction of each non-network cation). The fraction of highly coordinated Al increases nonlinearly with increasing <c/r>ave. The detailed atomic environments and the extent of cation mixing in Li-Ba silicate glasses with varying XBaO [BaO/(Li2O+BaO)] using high-resolution solid-state NMR spectroscopy were explored. Considering the previously reported experimental results on chemical ordering in mixed-cation silicate glasses, the current results reveal the effect of difference in ionic radius of the cation on a hierarchy in the degree of chemical order for various network modifying cations in the glasses. The extent of chemical and topological disorder in multi-component Na2O-MgO-Al2O3-SiO2 (NMAS) glasses in nepheline-forsterite-quartz eutectic composition and KLB-1 basaltic glasses were investigated using high-resolution solid-state 27Al and 17O NMR. The fraction of [5]Al in KLB-1 basaltic glasses increases upto ~2.6% with increasing XMgO [MgO/(MgO+Al2O3)]. The 17O 3QMAS NMR spectra for the KLB-1 basaltic glasses studied here confirm that the degree of polymerization (BO content) decreases with increasing XMgO. Based on the analysis of the peak position of {Ca, Mg}-mixed NBOs, non-random distributions of Na+, Ca2+, and Mg2+ around both NBOs and BOs are manifested by a moderate degree of partitioning of Ca2+ and Mg2+ into NBOs and by the spatial proximity between Na+ and BOs (Al-O-Al and Al-O-Si) in the KLB-1 basaltic glasses studied here.