R. Dupree

A multinuclear magnetic resonance study of the structure of hydrous albite glasses

The structures of a series of hydrous albite glasses quenched from melts at high pressures and temperatures have been studied using 29Si, 23Na, 27Al, and 1H nuclear magnetic resonance. Changes in the isotropic chemical shift, the chemical shift dispersion, and the mean nuclear quadrupole coupling constant for 23Na as a function of dissolved water concentration were deduced from spectra obtained at two different magnetic fields. Major changes in the sodium environment occur, but the spectra for 29Si and 27Al, and hence their structural environments, are similar throughout the range of water concentrations studied (0–67 mol%). No previous model is consistent with the results of this study. The data suggest the existence of the following structural features: i) exchange of H+ for Na+ as a charge-balancing cation; ii) formation of Na(OH) complexes; iii) incorporation of molecular water; iv) no octahedrally coordinated aluminium; v) no Al-OH or Si-OH. These features can be summarised in terms of the equilibrium NaAlSi3O8 + H2O ⇋ HAlSi3O8 + Na(OH). n nIn contrast to all previous interpretations, we see no evidence for depolymerisation of the aluminosilicate framework, although an increase in the symmetry of the aluminium environments and decrease in the chemical shift dispersion of the sodium environments suggests a more ‘ordered’ structure than in the dry glass. If the structures of hydrous albite melts are the same as those of the glasses studied here the current understanding of the effect of dissolved water on the physical properties of felsic melts must be reassessed.

NMR investigation of the structure of some bioactive and related glasses

Abstract A magic angle spinning nuclear magnetic resonance (MAS-NMR) investigation of the environments of 29 Si, 31 P and 23 Na, in glasses from the Na 2 Oue5f8CaOue5f8SiO 2 ue5f8(6 wt% P 2 O 5 ) system, has shown that the distribution of non-bridging oxygens can be described by a binary distribution of Q 2 and Q 3 silicon species. The changes in the chemical shifts of these two species with composition are interpreted as resulting from the preferential association of Na + with Q 3 and Ca 2+ with Q 2 . It is suggested that it is this partitioning that determines bioactivity by controlling the dissolution, hydrolysis and condensation reactions which occur at the interface between the glass and the physiological environment.

NMR determinations of SiOSi bond angle distributions in silica

Abstract The 29 Si NMR chemical shift in crystalline silicates is strongly dependant on the Siue5fcOue5fcSi 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 Siue5f8Si 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 Siue5fcOue5fcSi 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.

A study of the structural role of water in hydrous silica glass using cross-polarisation magic angle spinning NMR

Hydrous silica glasses containing 2.5 wt.% and 8.7 wt.% H2O were prepared by quenching melts from high pressures and temperatures, and investigated by 29Si NMR. Cross-polarisation magic angle spinning NMR experiments are described and used to identify silicon in different structural units within the glasses. The relative amounts of each structural unit are obtained by quantitative single pulse MASNMR and used together with the known water content to give the extent of hydroxyl formation. The hydroxyl concentration appears to be lower than that suggested by most previous work on hydrous silicate glasses using other experimental techniques.

A MAS-NMR investigation of lithium silicate glasses and glass ceramics

Abstract MAS-NMR of binary lithia-silica glasses, containing 25–39 mol% Li 2 O, shows that the Q n distribution largely follows that predicted by the binary (constrained) model but with some deviation when the lithia content exceeds 29 mol%. The deviation consists of disproportionation of Q 3 to Q 4 + Q 2 to an extent dependent on the lithia concentration, decreasing from 33 mol% Li 2 O onwards. Heat treatment of the glasses produces two polymorphs of lithium disilicate as the devitrification products. The structural changes during the early stages of crystallisation are reflected in variations in both 29 Si peak halfwidth and chemical shift but with these variations occurring at different temperatures. The static 7 Li spectra for the crystallised samples exhibit a Pake splitting indicative of the presence of 7 Li pair of separation ∼ 2.1 A .

The interaction between water and aluminosilicate magmas

29Si, 27Al and 23Na NMR data are presented for a variety of dry and dydrous aluminosilicate glasses. For 29Si and 27Al spectra only minor differences were observed between dry and hydrous glasses, but significant changes were observed in 23Na spectra. As found in our earlier study of hydrous albite glasses, the data are consistent with a model for the mechanism of water dissolution which involves no breakage of T-O-T linkages. Viscosity, phase equilibrium and vibrational spectroscopic data from other studies are discussed and shown to be consistent with such a model.

Magic angle spinning NMR of alkali phospho-alumino-silicate glasses

Abstract MAS-NMR has been used to explore the structure of sodium phospho-alumino-silicate glasses. Adding P 2 O 5 to sodium silicates causes repolymerisation of the silicate network and formation of sodium ortho-, pyro- and eventually metaphosphate units. At high P 2 O 5 concentrations the structure is determined by the incorporation of silica into a sodium phosphate glass as either chain end species or as an “SiP 2 O 7 ” phase. Removal of sodium from these glasses or substitution of alumina for P 2 O 5 destroys the “SiP 2 O 7 ” phase.

Different water solubility mechanisms in hydrous glasses along the Qz-Ab join:: Evidence from NMR spectroscopy

Abstract The compositional dependence of water incorporation mechanisms in melts and glasses belonging to the quartz (Qz)-albite (Ab) join was studied with nuclear magnetic resonance (NMR) spectroscopy. Dry and hydrous glasses (containing 3.8 ± 0.1 wt% water) ranging in composition from Qz90Ab10 to Qz28Ab72 (in wt%) were synthesised and studied with 1H, 23Na, 27Al and 29Si magic angle spinning (MAS) NMR and 1H-29Si cross polarisation (CP) MAS NMR at magnetic fields of 8.45 and 14.1 T. The results show that both molecular H2O and OH groups are present in the hydrous glasses, represented by a broad and a narrow component in the static 1H NMR spectra. The changes in the 23Na MAS NMR spectra of the hydrated glasses indicate that sodium associates with water in all compositions studied. In contrast to the Ab-rich glasses, the 29Si MAS spectra of Qz-rich glasses (Qz73Ab27 - Qz90Ab10) change upon water incorporation, indicating the presence of Si-OH groups at least in the Qz-richest sample. The 27Al MAS data demonstrate that Al is only present in tetrahedral coordination for all glasses studied and that unless δiso for Al Q4(4Si) and Al Q3(3Si)-OH are identical, Al-OH groups cannot be present in significant concentrations. Thus, in hydrous Ab-rich glasses there is probably no significant depolymerisation as suggested by Kohn et al. (1989a) . However, more 27Al data for appropriate model compounds or reliable calculations of 27Al shifts and quadrupolar coupling constants are required before this conclusion can be considered to be definitive. For Qz-rich glass compositions a second solubility mechanism involving the formation of Si-OH and depolymerisation of the silicate network is inferred. The data suggest that only in Qz-rich glasses do both mechanisms coexist.

NATURAL ABUNDANCE SOLID STATE 43CA NMR

Abstract 43Ca MAS NMR spectra have been collected for 12 solid phases including silicates, carbonates and sulphates. In all cases 43Ca was present at natural abundance, the first time such experiments have been reported. In general the lines are broad and featureless, though in the case of calcite a second order quadrupolar lineshape was observed. Within each class of compounds (silicates, carbonates etc.), the isotropic chemical shift appears to be linearly correlated with the mean Caue5f8O bond length of the first coordination shell of calcium, with a similar gradient for all of the classes of compounds studied.

13C MAS NMR: A method for studying CO2 speciation in glasses

13C-enriched glasses of albite, nepheline, and sodamelilite compositions were quenched from melts at 1350–1625°C and 10–35 kb, and studied using 13C and 27A1 MAS NMR. Distinct resonances for molecular CO2 and carbonate groups were observed in 13C spectra for a variety of albite glasses, whereas nepheline and sodamelilite glasses contained carbonate groups only. Two distinct carbonate groups were observed in glasses of both albite and nepheline compositions. Preliminary data on the effect of H2O and ƒH2 on carbon speciation in albite glasses suggest that the CO32− :CO2 ratio is increased in the presence of water, and that molecular CO is present under reducing conditions. 27Al spectra for albite and nepheline glasses show that CO2 dissolution is not accompanied by a [4]Al to [6]Al coordination change.