Jonathan F. Stebbins

NMR evidence for excess non-bridging oxygen in an aluminosilicate glass

The most common of man-made glasses have aluminosilicate compositions, and such glasses also form from rapidly cooling magmas. Oxygen is the most abundant element in these materials, where it occupies either ‘bridging’ (BO) or ‘non-bridging’ (NBO) sites. BOs link two AlO4 or SiO4 tetrahedra, thereby providing strong, long-lived bonds between the smallest structural units of the aluminosilicate network. NBOs provide a relatively weak connection between one tetrahedral cation (Al or Si) and one or more network modifier cations — such as Ca2+or Na+ — that are not an integral part of the tetrahedral network. The relative abundance of these weakly bonded NBOs is critical in determining the thermodynamic and dynamical properties of aluminosilicate glasses and melts. For glasses of ‘tectosilicate’ composition, where the charge of the modifier cation equals the number of aluminium atoms (as in NaAlSi3O8 or CaAl2Si2O8), the conventional view of glass structure is that only BOs are present,. Here we present experimental observations that contradict this view. Our NMR measurements of CaAl2Si2O8, which determine directly the relative abundances of BO and NBO, indicate that a considerable amount of NBO can be present in a tectosilicate glass. These excess NBOs will increase the entropy and heat capacity of the corresponding liquid and decrease its viscosity, as well as modifying flow and diffusion mechanisms,. As the most common rhyolitic magmas and the molten precursors of glass ceramics have near-tectosilicate compositions,, our results require a reassessment of the high-temperature liquid properties that control many processes in the Earth and in industry.

Heat capacities and entropies of silicate liquids and glasses

The heat capacities of several dozen silicate glasses and liquids composed of SiO2, TiO2, Al2O3, Fe2O3, FeO, MgO, CaO, BaO, Li2O, Na2O, K2O, and Rb2O have been measured by differential scanning and drop calorimetry. These results have been combined with data from the literature to fit Cpas a function of composition. A model assuming ideal mixing (linear combination) of partial molar heat capacities of oxide components (each of which is independent of composition), reproduces the glass data within error. The assumption of constancy of ¯Cp,iis less accurate for the liquids, but data are not sufficient to adequately constrain a more complex model. For liquids containing alkali metal and alkaline earth oxides, heat capacities are systematically greater in liquids with high “field strength” network modifying cations. Entropies of fusion (per g-atom) and changes of configurational entropy with temperature, are similarly affected by composition. Both smaller cation size and greater charge are therefore inferred to lead to greater development of new structural configurations with increasing temperature in silicate liquids.

Effects of temperature and composition on silicate glass structure and dynamics: SI-29 NMR results

Abstract The distribution of bridging and non-bridging oxygens is a fundamental aspect of the intermediate range order of silicate glasses and liquids. Disorder in this distribution can be characterized by the relative abundances of Qn species, which can be quantified by 29Si NMR on both spinning and static samples. We emphasize the latter here, and show that increasing the field strength of the network modifying cation and increasing temperature have similar randomizing effects on this distribution. These changes have a major influence on thermodynamic activities. In the liquid, exchange between sites is rapid, but the exchange rate may become slow enough with decreasing temperature to actually define Tg. In silicate liquids, some kind of high energy, low abundance, defects probably are present to allow this structural change to occur, to account for the bulk of the configurational heat capacity, and to explain the observed spin-lattice relaxation times.

Quantification of five- and six-coordinated aluminum ions in aluminosilicate and fluoride-containing glasses by high-field, high-resolution 27Al NMR

Aluminum cation sites with five ([5]Al) or six ([6]Al) anion neighbors are minor species in aluminosilicate glasses that are interesting in models of structure, diffusion, and viscous flow. Using 27Al NMR, we present the first direct evidence for [5]Al in a calcium-aluminosilicate glass without excess aluminum over charge-balancing cations, and quantify small concentrations of both [5]Al and [6]Al in several fluoride-containing aluminosilicate glasses. NMR techniques that enhance resolution by decreasing the effects of second-order quadrupolar broadening, including triple-quantum magic-angle spinning (3QMAS), analysis of spinning sidebands, and data collection at very high external magnetic fields (14.1 and 18.8 T), are particularly effective in accurately observing such species.

The degree of aluminum avoidance in aluminosilicate glasses

Epidotes are common rock-forming minerals with the ideal composition Ca2Al3-XFeX[SiO4][Si2O7]O(OH). Their structure consists of edge-sharing octahedral chains parallel to the b axis, connected by SiO4 and Si2O7 groups. Large cavities (A1 and A2) exist in which Ca cations are located. Three nonequivalent octahedral sites are present in the structure: M1 octahedra form branched chains with M3 octahedra alternately attached on opposite sides, whereas M2 octahedra form single chains. M3 is the largest and most distorted site, M2 the smallest and most regular. Fe is preferentially located in the M3 site and enters the M1 site to a lesser extent; the M2 site is usually occupied solely by Al. Bird and Helgeson (1980) developed a theoretical model to relate temperature and Fe intracrystalline distribution on the basis of experimental phase-equilibrium data for epidotes (Holdaway 1972; Liou 1973), Fe occupancies determined by Mössbauer spectroscopy on synthetic samples (Dollase 1973), and the thermodynamic properties of CO2-H2O fluids and coexisting phases. According to this model, Fe located at M1 increases with temperature and total Fe content. Application of this model to the study of hydrothermal epidotes in natural systems yielded substantial disagreement with both the actual temperatures at the sampling sites (Bird et al. 1988) and with estimated paleotemperatures (Patrier et al. 1991). Both papers claimed a metastably higher disorder for some of the samples examined. In particular, Bird et al. (1988) found that an epidote sample from the biotite zone displayed a Fe distribution corresponding to 390 °C that was in agreement with the probable downhole temperature of 340 °C, whereas three other samples from the chlorite+calcite zone (probable ABSTRACT

23Na NMR chemical shifts and local Na coordination environments in silicate crystals, melts and glasses

In order to decipher information about the local coordination environments of Na in anhydrous silicates from 23Na nuclear magnetic resonance spectroscopy (NMR), we have collected 23Na magic angle spinning (MAS) NMR spectra on several sodium-bearing silicate and aluminosilicate crystals with known structures. These data, together with those from the literature, suggest that the 23Na isotropic chemical shift correlates well with both the Na coordination and the degree of polymerization (characterized by NBO/T) of the material. The presence of a dissimilar network modifier also affects the 23Na isotropic chemical shift. From these relations, we found that the average Na coordinations in sodium silicate and aluminosilicate liquids of a range of compositions at 1 bar are nearly constant at around 6–7. The average Na coordinations in glasses of similar compositions also vary little with Na content (degree of polymerization). However, limited data on ternary alkali silicate and aluminosilicate glasses seem to suggest that the introduction of another network-modifier, such as K or Cs, does cause variations in the average local Na coordination. Thus it appears that the average Na coordination environments in silicate glasses are more sensitive to the presence of other network-modifiers than to the variations in the topology of the silicate tetrahedral network. Further studies on silicate glasses containing mixed cations are necessary to confirm this conclusion.

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.

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.

Solid-state NMR study of metastable immiscibility in alkali borosilicate glasses

In several series of lithium, sodium, and potassium borosilicate glasses whose compositions traverse known regions of liquid–liquid phase separation, we have applied triple-quantum magic-angle spinning (3QMAS) 11B and 17O NMR to obtain high-resolution information about short-range structure and connections among various network structural units, and their variation with composition and thermal history. Oxygen-17 3QMAS spectra reveal changes in connectivities between silicate and BO3 ([3]B) and BO4 ([4]B) units, by quantifying populations of bridging oxygens such as B–O–B, Si–O–B and Si–O–Si, and of non-bridging oxygens. [3]B–O–Si and [4]B–O–Si as well as [3]B–O–[3]B and [4]B–O–[3]B linkages can be distinguished. 11B MAS and 3QMAS at a magnetic field of 14.1 T allow proportions of several borate units to be determined, including [3]B in boroxol ring and non-ring sites and [4]B with 3 versus 4 Si neighbors. By combining the 11B and 17O NMR results, detailed information on Si/B mixing in sodium borosilicates can be derived, showing, for example, that [4]B and non-ring [3]B tend to mix with silicate units, while ring [3]B is mainly connected to borate groups. In a preliminary study of the effects of varying alkali cation, potassium-containing glasses are similar to those in the sodium borosilicate system, but a lithium borosilicate seems to exhibit considerably greater chemical heterogeneity. In annealing experiments that converted an optically clear to obviously phase-separated glasses, the ratio of [3]B to [4]B does not change significantly, but part of the non-ring [3]B converts to ring [3]B as the degree of unmixing increases.

Al–O–Al and Si–O–Si sites in framework aluminosilicate glasses with Si/Al=1: quantification of framework disorder

Abstract We present for the first time, direct and clear experimental evidence of Al–O–Al and Si–O–Si linkages in charge-balanced aluminosilicate glasses with Si/Al=1, such as NaAlSiO 4 (nepheline) and LiAlSiO 4 (β-eucryptite) compositions using 17 O triple quantum MAS (3QMAS) NMR and quantify the extent of disorder in framework cations (Si/Al). The degree of Al avoidance in NaAlSiO 4 glass is 0.942 at 1050 K, and in LiAlSiO 4 glass is 0.928 at 930 K (0.902 at 1050 K), which demonstrates the effect of cation field strength on the extent of disorder. In addition, we find a remarkable similarity between the ordering state of the liquid and that of the first, disequilibrium phase to crystallize.