Ian Farnan

The nature of the glass transition in a silica-rich oxide melt.

The atomic-scale dynamics of the glass-to-liquid transition are, in general, poorly understood in inorganic materials. Here, two-dimensional magic angle spinning nuclear magnetic resonance spectra collected just above the glass transition of K2Si4O9 at temperatures as high as 583�C are presented. Rates of exchange for silicon among silicate species, which involves Si—O bond breaking, have been measured and are shown to be closely related in time scale to those defined by viscosity. Thus, even at viscosities as high as 1010 pascal seconds, local bond breaking (in contrast to the cooperative motion of large clusters) is of major importance in the control of macroscopic flow and diffusion.

NMR determinations of SiOSi bond angle distributions in silica

Abstract The 29 Si NMR chemical shift in crystalline silicates is strongly dependant on the SiOSi bond angle, hence, if a suitable relationship between shift and angle can be obtained then the shape of the NMR line in glasses can be used to give information about the statistical distribution of bond angles and by transformation the SiSi partial pair correlation function in these materials. Several functional relationships between shift and angle have been given in the literature and these result in different SiOSi bond angle distributions deduced. This paper will try to review the current situation and comment on the prospect of an NMR alternative to X-ray diffraction structural determinations.

The structure and dynamics of alkali silicate liquids: A view from NMR spectroscopy

Abstract Understanding and predicting the macroscopic properties of silicate liquids require information on both structure and dynamics. Studies of glass structure provide a convenient starting point, but only represent the liquid at the rather low temperature of transition from glass to liquid. In situ high- T studies, as well as work on glasses with varying fictive temperature, can provide more information on structural changes with T and on dynamical mechanisms. Here we discuss the application of NMR spectroscopy to both kinds of approaches. We present results, primarily from our laboratory, on anionic species distributions, on silicon and oxygen site exchange in liquids and its relationship to viscosity, and on the quantification of the abundances of five- and six-coordinated Si in high-pressure and 1-bar-pressure glass samples.

Effects of High Temperature on Silicate Liquid Structure: A Multinuclear NMR Study

The structure of a silicate liquid changes with temperature, and this substantially affects its thermodynamic and transport properties. Models used by geochemists, geophysicists, and glass scientists need to include such effects. In situ, high-temperature nuclear magnetic resonance (NMR) spectroscopy on 23Na, 27A1, and 29Si was used to help determine the time-averaged structure of a series of alkali aluminosilicate liquids at temperatures to 1320�C. Isotropic chemical shifts for 29Si increase (to higher frequencies) with increasing temperature, probably in response to intermediate-range structural changes such as the expansion of bonds between nonbridging oxygens and alkali cations. In contrast, isotropic chemical shifts for 27Al decrease with increasing temperature, indicating that more significant short-range structural changes take place for aluminum, such as an increase in mean coordination number. The spectrum of a sodium aluminosilicate glass clearly indicates that at least a few percent of six-coordinated aluminum was present in the liquid at high temperature.

Dynamics of the α-β phase transitions in quartz and cristobalite as observed by in-situ high temperature 29Si and 17O NMR

Relaxation times (T1) and lineshapes were examined as a function of temperature through the α-β transition for 29Si in a single crystal of amethyst, and for 29Si and 17O in cristobalite powders. For single crystal quartz, the three 29Si peaks observed at room temperature, representing each of the three differently oriented SiO4 tetrahedra in the unit cell, coalesce with increasing temperature such that at the α-β transition only one peak is observed. 29Si T1s decrease with increasing temperature up to the transition, above which they remain constant. Although these results are not uniquely interpretable, hopping between the Dauphiné twin related configurations, α1 and α2, may be the fluctuations responsible for both effects. This exchange becomes observable up to 150° C below the transition, and persists above the transition, resulting in β-quartz being a time and space average of α1 and α2. 29Si T1s for isotopically enriched powdered cristobalite show much the same behavior as observed for quartz. In addition, 17O T1s decrease slowly up to the α-β transition at which point there is an abrupt 1.5 order of magnitude drop. Fitting of static powder 17O spectra for cristobalite gives an asymmetry parameter (η) of 0.125 at room T, which decreases to <0.040 at the transition temperature. The electric field gradient (EFG) and chemical shift anisotropy (CSA), however, remain the same, suggesting that the decrease in η is caused by a dynamical rotation of the tetrahedra below the transition. Thus, the mechanisms of the α-β phase transitions in quartz and cristobalite are similar: there appears to be some fluctuation of the tetrahedra between twin-related orientations below the transition temperature, and the β-phase is characterized by a dynamical average of the twin domains on a unit cell scale.

SILICATE SPECIES EXCHANGE, VISCOSITY, AND CRYSTALLIZATION IN A LOW-SILICA MELT: IN SITU HIGH-TEMPERATURE MAS NMR SPECTROSCOPY

Abstract Aluminous armalcolite has been found in two sillimanite-bearing xenoliths that were recently exhumed from the lower crust of central Mexico. The ranges of compositions are (Fe2+0.58-0.66Mg0.18-0.28Al0.15-0.18V3+0.06-0.10Fe3+0.00-0.06Ti4+1.84-1.86)O5 and (Fe2+0.20-0.51Mg0.18-0.29Al0.16-0.19-V3+0.02-0.06Fe3+0.30-0.81Ti4+1.49-1.71)O5. The occurrence of armalcolite is unusual in crustal paragneisses because most terrestrial armalcolite occurs in volcanic rocks that are derived from partial melting in the Earths mantle. Textures suggest that armalcolite is a late product formed by the reaction rutile + ilmenitess = armalcolitess during rapid transport of the xenoliths to the surface. Phase equilibria in the system MgO-FeO-Fe2O3 - TiO2, which indicate that armalcolite is stable in the crust at 900-1200 °C, are consistent with this interpretation. Thermodynamic properties are estimated for oxides in the system MgO-FeO-Fe2O3 - TiO2 to constrain activity-composition relations for armalcolite and conditions of formation. Activity coefficients calculated for armalcolite range from 0.27 to 1.36, depending on the ilmenite model used, at temperatures between 1000 and 1300 °C. Depth of formation of armalcolite in the crust is not well constrained. Thermodynamic calculations at 800-1200°C for the compositions observed indicate that the armalcolite in one xenolith would have been in equilibrium with rutile at values of fO₂ between the hematite + magnetite buffer (HM) and the fayalite + magnetite + quartz (FMQ) buffer, and that armalcolite in the other xenolith would have been in equilibrium with rutile and ilmenite at values of fO₂ between FMQ and two log units below the FMQ buffer.

Observation of slow atomic motions close to the glass transition using 2-D 29Si NMR

Abstract Experiments were performed to investigate atomic re-arrangements just above the glass transition in potassium tetrasilicate (K 2 Si 4 O 9 ). These experiments employed two-dimensional NMR techniques which were developed to study chemical exchange in non-viscous liquids and organic polymers. The major feature of the 29 Si 2-D NMR exchange spectra of potassium tetrasilicate above its glass transition was a slow exchange ( Q 3 and Q 4 environments. A similar experiment performed just below the glass transition showed no such exchange, which suggests that this Q species exchange is the motion that is ‘frozen out’ as a viscous liquid cools through its glass transition. The results are consistent with a very local region of atomic re-arrangement and contradict a polymeric view of silicate liquids.

Nuclear Magnetic Resonance Spectroscopy in the Earth Sciences: Structure and Dynamics

Detailed knowledge of the structure and dynamics of the materials that make up the earth is necessary for fundamental understanding of most geological processes. Nuclear magnetic resonance spectroscopy is beginning to play an important role in investigations of inorganic solid materials, as well as of liquids and organic compounds; it has already contributed substantially to our knowledge of minerals and rocks, compositionally simplified analogs of magmas, and the surfaces of silicate crystals. The technique is particularly useful for determining local structure and ordering state in crystals, glasses, and liquids, and is sensitive to atomic motion at the time scales of diffusion and viscosity in silicates. New techniques offer promise for increased resolution for quadrupolar nuclei and for extension of experiments to high temperature and pressure.

Bonding and dynamical phenomena in MgO: A high temperature 17O and 25Mg NMR study

We have measured the isotropic chemical shifts (δiso) and the spin-lattice relaxation times (T1) for 17O and 25Mg in MgO from room temperature up to 1300° C. The 17O chemical shifts increase linearly from 47 ppm at room temperature to 57 ppm at 1300° C, and over the same temperature range the 25Mg chemical shift increases linearly from 25 to 27 ppm. These changes are not the result of changes in the bulk magnetic susceptibility of the samples, but may be due to increased orbital overlap which is the result of the increase in thermal vibration of the ions with temperature. In the case of 25Mg, the shift to lower shielding with increasing temperature is opposite to that expected from simple bond length versus chemical shift trends established for the oxides at room temperature. If this is a general phenomenon, high-temperature NMR data may be biased to lower shielding.Spin-lattice relaxation times (T1) were measured in order to study the energetics of defect motion. T1s for 17O and 25Mg exhibit similar behavior over the range of temperatures studied. Up to 800° C, T1s decrease gradually, but above 800° C, T1s drop rapidly, with slopes corresponding to apparent activation energies of 192±9 kJ/mol (2.0±0.1 eV) for 17O and 151±6 kJ/mol (1.56±0.06 eV) for 25Mg. While direct comparison of these activation energies to those derived from diffusion or conductivity measurements is complicated, the similar behavior for both nuclei suggests their relaxation phenomena are related.

Magic angle spinning NMR observation of sodium site exchange in nepheline at 500° C

Dynamics in minerals at time scales from seconds to microseconds are important in understanding mechanisms of displacive phase transitions, diffusion, and conductivity. High resolution, magic-angle-spinning (MAS) NMR spectroscopy can directly show the rates of exchange among sites, potentially providing less model-dependent information than more traditional NMR relaxation time measurements. Here we use a newly developed high temperature MAS probe (Doty Scientific, Inc.) to observe the exchange of Na+ among the alkali sites in a high-Na nepheline at temperatures as high as 500° C. Observed exchange rates are consistent with correlation times derived from cation diffusivity.