Jeffrey R. Allwardt

Aluminum coordination and the densification of high-pressure aluminosilicate glasses

Abstract To better understand the relationship between atomic-scale structures and densities of aluminosilicate glasses and liquids, we used 27Al MAS NMR to determine the speciation of aluminum ions in K3AlSi3O9, Na3AlSi3O9, and Ca3Al2Si6O18 glasses quenched from melts at 3 to 10 GPa. These data are a first approximation of high-pressure melt structure and illustrate the effects of the type of modifier cation. High field strength modifier cations (e.g., Ca) clearly induce more high-coordinated Al than lower field strength cations (e.g., Na and K). Measured glass densities show that, especially with rapid decompression, a significant portion of the total densification observed in-situ in melts is retained on return to ambient temperature and pressure. Observed increases in Al coordination are well correlated with decreased volume, which suggests that this structural change is a major part of the mechanism for recovered densification of high-pressure melts. Additionally, 23Na MAS NMR, combined with the 27Al MAS spectra and density determinations, reveal that other changes, such as the compression of modifier cation sites and/or decreased network bond angles, must also be significant, especially at low pressure.

Bonding preferences of non-bridging O atoms: Evidence from 17O MAS and 3QMAS NMR on calcium aluminate and low-silica Ca-aluminosilicate glasses

Abstract The fraction of O atoms as non-bridging O atoms (NBO) can be well approximated based on composition alone in many silicate glasses, but the NBO preference for specific network forming cations is much less well known. Using oxygen-17 (17O) NMR on low-silica calcium aluminosilicate (CAS) glasses, this study shows that Al-NBO (155 ppm) can be readily distinguished from Si-NBO (110-120 ppm), and that there is a strong preference for the latter. This study also presents a consistent equilibrium constant formulation that indicates that for thermodynamic modeling of most CAS melts with Si > Al, Al-NBO are of minor importance, although they could be significant in some models of diffusion and viscosity. Al-27 one pulse NMR and analyses of spinning side bands show that AlO5 and AlO6 species are below detection limits (<0.5%) in the low-silica (SiO2 ≤ 20 mol%) glasses of this study (NBO/T = 0.6 to 0.8). In addition, 17O MAS NMR does not detect any obvious (<2%?) Al3O triclusters; hence calculations of NBO assignments can be assigned unambiguously.

Effect of structural transitions on properties of high-pressure silicate melts: 27Al NMR, glass densities, and melt viscosities

Abstract The densities and viscosities of silicate melts depend strongly on pressure, in part because of potentially measurable structural rearrangements. In an attempt to further understand these changes and how they affect macroscopic properties, we have used 27Al MAS NMR to determine the coordination of the Al cations in a series of aluminosilicate glasses quenched from melts at pressures of 2 to 8 GPa, have measured the glass densities, and have applied an in-situ falling sphere method to measure melt viscosities at high pressure. Spectra from these four- and five-component glasses show increasing Al coordination with increasing pressure and with increasing average field strength of the modifier cation, as was previously reported for simpler compositions. These data also indicate that when multiple modifier cations are present (e.g., Ca and K), the Al coordination is lower than what would be expected from linear combinations of the appropriate aluminosilicate end-members. The viscosity of Ca3Al2Si6O18 melts, measured using a falling sphere method that combines multianvil techniques with synchrotron X-ray radiography, may reach a minimum at a pressure below 6 GPa. A quasi-thermodynamic approach using equilibrium constants for the reactions that generate high-coordinated Al suggests that this pressure may be related to a maximum in the concentration of five-coordinated Al. These results further support the concept that pressure-induced network structural transitions have direct implications for the macroscopic properties of high-pressure melts.

Ca-Mg and K-Mg mixing around non-bridging O atoms in silicate glasses: An investigation using 17O MAS and 3QMAS NMR

Abstract In an effort to improve the physical accuracy of models of the thermodynamics of silicate melts, we describe a systematic study of the extent of modifying cation mixing, using 17O 3QMAS NMR, in a series of Ca-Mg and K-Mg silicate glasses. The spectra for the mixed cation Ca2-2xMg2xSi2O6 glass show that only one large non-bridging O atom (NBO) peak occurs that encompasses the entire range of chemical shifts ranging from Ca-NBO to Mg-NBO. Comparison of the isotropic projections from 3QMAS NMR to spectra predicted by a random model show that mixing in these glasses is highly disordered, but may contain a small amount of ordering at the glass transition temperature. In contrast, cation mixing in K-Mg silicate glasses is very ordered, confirming previous results; however, the results of this study disagree with the interpretation of the previous study and show that the NBO in K-Mg silicate glasses contain mostly Mg-NBO, not a highly ordered K-Mg-NBO species. These order-disorder results have direct implications in constraining entropy models and therefore allowing better predictions of mineral-melt equilibria in silicate melts

The effect of fictive temperature on Al coordination in high-pressure (10 GPa) sodium aluminosilicate glasses

Abstract Typical liquidus temperatures can be over 1000°C greater than the glass transition temperatures for high-pressure aluminosilicate melts so the effect of temperature must be determined if glass data is to be used to approximate the structural speciation present in geologic melts. This study has investigated the effect of fictive temperature (Tf, taken as the temperature where the melt structure is the same as that of the glass) on the percentage of [5]Al and [6]Al species in two high-pressure (10 GPa) Na-aluminosilicate glasses (Na3AlSi7O17 and NaAlSi3O8) where one glass of each composition was quenched from the high-pressure melt while the other was annealed near the glass transition temperature. The 27Al MAS NMR spectra of the Na3AlSi7O17 samples show that the higher Tf (quenched) glass contains more high-coordinated Al than the lower Tf (annealed, 475°C) glass. However, the 27Al spectra of the NaAlSi3O8 samples show the opposite temperature dependency, which in addition to the lack of NBO in this glass, may suggest differing mechanisms for the generation of high-coordinated Al

Chapter 11 – The effect of composition, compression, and decompression on the structure of high-pressure aluminosilicate glasses: an investigation utilizing 17O and 27Al NMR

This contribution reviews recent work on the determination of the structure of high-pressure aluminosilicate glasses and presents new 17 O and 27 Al magic-angle spinning (MAS) and 17 O triple-quantum (3Q) MAS NMR spectra that investigate the structure of K 3 AlSi 3 0 9 (KAS) and Ca 3 Al 2 Si 6 O 18 (CAS) glasses quenched at 5 GPa. Comparison of the 27 Al MAS spectra for the high-pressure and ambient-pressure CAS and KAS glasses shows that the average Al-coordination increases with increasing pressure and that there is a strong compositional dependence to the amount of high-coordinated Al, as the CAS glass contains significantly more [5] Al and [6] Al than the KAS glasses. The spectra of the 5 GPa and the ambient pressure glasses were compared and show that high-coordinated Al are formed by the mechanisms: [4] Si–NBO+4 [4] Si–0– [4] Al →5 [4] Si–0– [5] Al and 2 [4] Si–NBO+4 [4] Si–0– [4] Al → 6 [4] Si–0– [6] Al, which are similar to the mechanisms for the generation of [5], Si and [6], Si in high-pressure silicate glasses. Decompression times were varied (14 h and 1 s) and longer times are shown to reduce the average Al-coordination. Additionally, the reproducibility of glass making in a multi-anvil apparatus was investigated by comparing the Al MAS spectra of two glasses made in the same high-pressure experiment, but in separate capsules. This suggests that the reproducibility of the average Al-coordination is about 0.1 (e.g. 5.0 ± 0.1) for glasses with an Al/(AAl + Si) ratio of 0.25 .

The Diversity of Nuclear Magnetic Resonance Spectroscopy

The discovery of the physical phenomenon of Nuclear Magnetic Resonance (NMR) in 1946 gave rise to the spectroscopic technique that has become a remarkably versatile research tool. One could oversimplify NMR spectros-copy by categorizing it into the two broad applications of structure eluci-dation of molecules (associated with chemistry and biology) and imaging (associated with medicine). But, this certainly does not do NMR spectros-copy justice in demonstrating its general acceptance and utilization across the sciences. This manuscript is not an effort to present an exhaustive, or even partial review of NMR spectroscopy applications, but rather to pro-vide a glimpse at the wide-ranging uses of NMR spectroscopy found within the confines of a single magnetic resonance research facility, the Stanford Magnetic Resonance Laboratory. Included here are summaries of projects involving protein structure determination, mapping of intermolecular inter-actions, exploring fundamental biological mechanisms, following compound cycling in the environmental, analysis of synthetic solid compounds, and microimaging of a model organism.