Peraluminous Glasses Research Articles

Molecular dynamics simulations of magnesium aluminosilicate glass structure: High‐coordinated alumina and oxygen tricluster formation

AbstractPrevious research has shown a consistent discrepancy in the reported structure of alkaline earth aluminosilicate glasses using molecular dynamics (MD) simulations versus nuclear magnetic resonance (NMR) experiments. Past MD results have consistently shown less than 5% five‐coordinated Al units (Al[5]) in peraluminous glass compositions, but with high fractions of triple‐bonded oxygens (TBO, i.e., triclusters). Experimental results have shown a high fraction of Al[5] with no direct evidence for TBO. One of the main criticisms associated with high TBO content found in MD‐generated glass structures is the use of classical interatomic potentials. To investigate this issue, we analyze the formation of both TBO and Al[5] using three independently developed potentials with varying silica content and [Al2O3]/[MgO] ratios for the magnesium aluminosilicate (MAS) system. We specifically choose compositions with high ratios of alumina to magnesium oxide as this region is not as commonly explored. Results indicate that Al[5] charge compensates the Al network in metaluminous compositions (compositions with more Mg than Al) while both TBO and Al[5] are prevalent in peraluminous ranges (high Al content compositions) to charge balance Al units. From the literature, NMR experiments report MAS glasses with varying Al[5] fractions and show significant differences for the same reported compositions. When comparing MD results from this work, the fraction of calculated Al[5] is within the experimental variation found in the literature. This indicates that classical potentials can accurately capture alumina environments and that both Al[5] and TBO can coexist in relatively high fractions. From the consistency in our results, we conclude that TBOs are inherent to the aluminosilicate glass system and are not simulation artifacts.

Chromium enriched peraluminous glasses: Incorporation limit, crystalline phase equilibrium and impact of chromium on the rheological properties of the glass

Chromium is a multivalent element with very low solubility in silica-based glasses. It is incorporated into glass via the solubilisation of Cr2O3 as Cr3+ through an acido-basic reaction, and is in redox equilibrium with Cr6+ and Cr2+ oxidation states depending on the surrounding atmosphere. The least soluble form of chromium is Cr3+, which is also the most common form of chromium in glass elaborated in air. This study investigated the behavior of chromium in peraluminous glasses, which are potential matrices for nuclear waste containment. Chromium is often present in nuclear waste solutions as a corrosion product coming from the waste reprocessing steps. Consequently, it is important to understand the effect of chromium addition to peraluminous glasses, both in terms of the microstructure and the processing ability of the glass melt. The study investigated the effect of chromium added as a Cr2O3 chromium (+III) oxide on the homogeneity, the viscosity, and the glass transition temperature (Tg) of peraluminous glasses. The chromium redox state and its effect on the incorporation limit of chromium was determined. Peraluminous glasses have an incorporation limit of between 0.50 and 0.55 wt% Cr2O3 in air, and between 1.25 and 1.50 wt% Cr2O3 in a reducing environment. Chromium (+III) oxide was found to have a limited effect on the viscosity of peraluminous glasses up to 2 wt% of Cr2O3, despite the presence of crystals above 0.5 wt%. The addition of chromium oxide had no significant effect on the Tg of the glass, except after a slow cooling of the sample containing the highest content of 2.0 wt% Cr2O3. Two forms of chromium were identified under UV-visible spectroscopy for the glasses elaborated in air: Cr3+ (6-fold) and Cr6+ (4-fold).

Compositional dependence of crystallization and chemical durability in alkali aluminoborosilicate glasses

This study aims to understand the impact of composition on crystallization and chemical durability in alkali aluminoborosilicate based model nuclear waste glasses designed in the peralkaline, metaluminous and peraluminous regimes. The glasses have been thermally treated using the canister centerline cooling (CCC) schedule. The chemical durability of both parent and CCC-treated glasses has been assessed by product consistency test (PCT-B) for 120 days. The peraluminous glasses exhibit the highest dissolution rates, followed by peralkaline and metaluminous glasses. In general, increasing B2O3 content in glasses tends to suppress nepheline formation, thus, decreasing the negative impact of nepheline on durability of the final waste form. However, higher B2O3 content itself may result in detrimental impact on the durability of the final waste form. The thermal history has been shown to have a significant impact on the durability of the glasses.

Iron enriched peraluminous glasses: Incorporation limit and effect of iron on glass transition temperature and viscosity

This study investigates the effect of iron oxide Fe2O3 on the homogeneity, viscosity, and glass transition temperature (Tg) of peraluminous glasses. As-quenched glass samples elaborated at 1350 °C show an incorporation limit around 17.5 wt% of Fe2O3 (10.4 mol%), while slowly cooled glass samples show a limit of 7.5 wt% of Fe2O3 (4.2 mol%). Iron in the peraluminous glasses investigated was detected under three configurations: 6-fold coordinated 6Fe3+ in majority, 4-fold coordinated 4Fe3+ and 6-fold coordinated 6Fe2+. It was observed that Fe2O3 addition has a strong fluidifying effect on peraluminous glasses and tends to slightly decrease the glass transition temperature Tg, depending on its thermal history.

Structural and rheological analysis of nickel enriched peraluminous glasses

This study describes the role of nickel oxide (NiO) in peraluminous glass. Peraluminous glass is defined by an excess of aluminium ions (Al 3+ ) compared to modifier elements such as Na + , Li + or Ca 2+ ions. The study investigates the effect NiO incorporation on peraluminous glass homogeneity and process ability. Quenched NiO-containing glass showed an incorporation limit greater than 12 wt% NiO (13.8 mol%), while slowly cooled glass showed a limit of 5 wt% NiO (5.8 mol%). Nickel oxide is also found to have a fludifying effect on peraluminous glasses. When the nickel oxide content is above its incorporation limit, the resulting crystals were found to have a significant effect on glass rheological properties. Finally, hypotheses are proposed as to the structural role of Ni in peraluminous glass based on the experimental results obtained. • The effect of NiO incorporation on peraluminous glass homogeneity and process ability is investigated • Quenched glass showed an incorporation limit greater than 12 wt% NiO (13.8 mol%) • Slowly cooled glass showed an incorporation limit of 5 wt% NiO (5.8 mol%) • NiO is found to have a fludifying effect • Ni-containing crystals were found to have a significant effect on the glass rheological properties • Hypotheses on the structural role of Ni in peraluminous glass are proposed

Investigation of alumino-silicate glasses by coupling experiments and simulations: Part I - Structures

A set of alumino-silicate glasses with SiO2-Al2O3-Na2O-CaO compositions was investigated using Raman and NMR spectroscopy techniques and by classical molecular dynamics, in order to study the structural modifications which occur when CaO progressively replaces Na2O and when Al2O3 progressively replaces SiO2. The effect of a gradual increase in the Al2O3 content was also studied. Al was always 4 coordinated for all the compositions studied, including the peraluminous glasses. An increase in the number of Al-O-Al bonds was found when the Al2O3 or CaO content increases. The study confirmed that AlO4 groupings are preferentially compensated by Na rather than by Ca, and that there are non-bridging oxygens surrounded by mixed environments containing both Na and Ca. Combining the information given by the Raman and NMR spectroscopy results enabled the attribution of the Raman band at 570cm-1 to the Si-O-Al bonds. It confirmed that the band at 510cm-1 can be attributed to the Al-O-Al bonds, while that at 985cm-1 can be attributed to the Q4-Al entities. Lastly, it was demonstrated that the classical molecular dynamics potentials of Deng et al., specifically developed for this type of glasses, permit the reproduction of a large number of the experimental results quantitatively or sometimes simply qualitatively.

Impact of pressure on structure and properties of hot‐compressed Na2O–Al2O3–SiO2 glass by molecular dynamics simulations

AbstractWe conduct systematic studies to investigate the impact of pressure on structure and properties of Na2O–Al2O3–SiO2 glass system using molecular dynamics simulations. Glass compositions range from peralkaline, to charge‐balancing, to peraluminous. Hot‐compression operation is applied to as‐prepared glasses with pressure up to 4 GPa. Several key learnings are obtained from the study: (a) Density and elastic moduli increase with increasing Al/Na ratio (RAl/Na) and pressure, which are mainly driven by average internetwork bond angle reduction and void size reduction upon compression. (b) Population of three bonded oxygen (TBO) show some presence in the peralkaline composition space, while gain significantly in the peraluminous regime. Pressure essentially encourages conversion of bridging oxygen to nonbridging and three bonded oxygens. Local nearest neighbor environment of TBO is predominately Al atoms rather than Si atoms regardless of RAl/Na and applied pressure. (c) Analysis of first sharp diffraction peak (FSDP) from neutron scattering suggests that medium‐range structure is compacted and small size rings are noticeably distorted over the hot‐compression process. (d) Ring size distribution analysis shows that pressure has a more pronounced effect on altering medium‐range structure for peralkaline glasses as compared to peraluminous glasses.

Structure and crystallization behavior of phosphorus-containing nepheline (NaAlSiO4) based sodium aluminosilicate glasses

The study aims at understanding the structural role of P2O5 in meta- and per-aluminous sodium aluminosilicate glasses, followed by its correlation with their crystallization behavior. Experimental results suggest an increase in the degree of polymerization of the phosphate network and a change in the aluminum environment in the glasses from Al(OSi)4 to Al(OP)4 with an increasing P2O5/SiO2 ratio. The substitution of SiO2–by–P2O5 in the amount varying between 10 – 20 mol.% suppresses the crystallization of nepheline. This has been attributed to the unavailability of Al3+ (to form Si–O–Al linkages) due to the formation of Al–O–P linkages in the glass structure. An increase in P2O5 content (>20 mol.%) in peraluminous glasses results in the crystallization of AlPO4 as the primary phase. The results have been discussed in the light of various models developed to predict the compositional dependence of nepheline crystallization in Hanford's high-level waste glasses.

Chemical durability of peraluminous glasses for nuclear waste conditioning

For the handling of high level nuclear waste (HLW), new glass formulations with a high waste capacity and an enhanced thermal stability, chemical durability, and processability are under consideration. This study focuses on the durability of peraluminous glasses in the SiO2–Al2O3–B2O3–Na2O–CaO–La2O3 system, defined by an excess of Al3+ ions compared with the network-modifying cations Na+ and Ca2+. To qualify the behavior of such a peraluminous glass in a geological storage situation, its chemical durability was studied in various environments (pure water, groundwater, and alkaline solutions related to a cement environment) and glass alteration regimes (initial rate, residual rate, and resumption of alteration). The alteration solution was characterized by inductively coupled plasma, and the altered glass by scanning electron microscopy, X-ray diffraction and secondary ion mass spectrometry. A comparative study of the chemical durability of these and reference glasses (ISG and SON68) over all timescales highlights the remarkable properties of the former. While their initial dissolution rate is of the same order as the reference glasses, the gel formed under silica saturation conditions is more passivating, making its dissolution rate at least one order of magnitude lower, while its low alkalinity makes it less susceptible to clayey groundwater and highly alkaline solutions.

Fragility and configurational heat capacity of calcium aluminosilicate glass-forming liquids

Enabling accurate prediction of the properties of aluminosilicate glasses is important for the development of new glass compositions for high-tech applications. In this study, we use a combined topological and thermodynamic approach to connect the configurational heat capacity (Cp,conf) with the liquid fragility (m) and glass transition temperature (Tg) of calcium aluminosilicate glass-forming liquids. To study structural and dynamical features of these systems, we choose two representative glass composition series: one at the tectosilicate join with varying SiO2 content and one with constant CaO content but varying Al2O3/SiO2 ratio. Cp,conf is determined using differential scanning calorimetry (DSC), while m and Tg are determined through both DSC and viscosity measurements. The Cp,conf model is found to predict the measured data well for most systems, but deviations between the modeled and measured Cp,conf values appear for the "strongest" glasses in the tectosilicate series and for the most peraluminous glasses in the constant CaO series. We discuss the structural origins of these model-from-data deviations based on Raman spectroscopy measurements.

Effect of composition on peraluminous glass properties: An application to HLW containment

Part of the Research and Development program concerning high level nuclear waste (HLW) glasses aims to assess new glass formulations able to incorporate a high waste content with enhanced properties in terms of thermal stability, chemical durability, and process ability. This study focuses on peraluminous glasses of the SiO2 – Al2O3 – B2O3 – Na2O – Li2O – CaO – La2O3 system, defined by an excess of aluminum ions Al3+ in comparison with modifier elements such as Na+, Li+ or Ca2+. To understand the effect of composition on physical properties of glasses (viscosity, density, Tg), a Design Of Experiments (DOE) approach was applied to investigate the peraluminous glass domain. The influence of each oxide was quantified to build predictive models for each property. Lanthanum and lithium oxides appear to be the most influential factors on peraluminous glass properties.

Constraints on the incorporation mechanism of chlorine in peralkaline and peraluminous Na2O-CaO-Al2O3-SiO2 glasses

Incorporation mechanisms of Cl in peralkaline and peraluminous Na2O-CaO-Al2O3-SiO2 glasses as a model system for phonolitic melts were investigated using 35Cl, 23Na, 27Al, and 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The size and large distribution of electric field gradients for 35Cl causes loss of signal in the MAS NMR experiment and this, in combination with the low concentration of Cl and the large chemical shift dispersion, means that even at the highest available fields we are at the limits of MAS NMR. Nevertheless clear differences in the Cl environment in peralkaline and peraluminous glasses can readily be seen. In both glass types Cl exists in relatively symmetric Na-Ca-Cl environments. The 35Cl chemical shift indicates that the Cl environment is dominated by the presence of Na cations, consistent with the Na/Ca ratio of 5/1 in the glasses. 35Cl MAS NMR spectra of the peraluminous glasses show a larger chemical shift distribution and a more positive isotropic chemical shift, ~-75 ppm, than the peralkaline glasses, ~-100 ppm. They also have a larger quadrupole coupling constant with a larger distribution, indicating greater disorder in the peraluminous glasses. It is likely that there are more Ca cations present in the Cl environments in the peraluminous glasses than in the peralkaline glasses despite their having the same Na/Ca ratio. In the peralkaline glasses the formation of Na-Ca-Cl environments leads to a decrease in the number of network-modifying cations, which causes a polymerization of the glass network. No effect on the glass polymerization was observed in the peraluminous glasses. Some 35Cl signal is also lost in the static spectra indicating that ~20% of Cl for a peralkaline glass and more than ~70% for a peraluminous glass must be in environments where there is a large enough electric field gradient that the resulting very broad line is unobservable. These environments could be simply Na-Ca-Cl with higher electric field gradients than those producing the observed 35Cl signal or non-bridging Cl environments like for example Al-Cl. The Cl environment in the present mixed Na2O-CaO aluminosilicate glasses appears to be more disordered than was to be expected from previous NMR spectroscopic studies on simpler glass compositions.

Effects of fluorine on the solubilities of Nb, Ta, Zr and Hf minerals in highly fluxed water-saturated haplogranitic melts

The effect of fluorine on the solubilities of Mn-columbite (MnNb2O6), Mn-tantalite (MnTa2O6), zircon (ZrSiO4) and hafnon (HfSiO4) were determined in highly fluxed, water-saturated haplogranitic melts at 800 to 1000°C and 2kbar. The melt composition corresponds to the intersection of the granite minimum with the albite–orthoclase tieline (Ab72Or28) in the quartz–albite–orthoclase system (Q–Ab–Or), which is representative of a highly fluxed melt, from which high field strength element minerals may crystallize. The melt contains 1.7wt.% P2O5, 1.05wt.% Li2O and 1.83wt.% B2O3. The main purpose of this study is to examine the effect of F on columbite, tantalite, zircon and hafnon solubility for a melt with this composition. Up to 6wt.% fluorine was added as AgF in order to keep the aluminum saturation index (ASI, molar Al/[Na+K]) of the melt constant. In an additional experiment F was added as AlF3 to make a glass peraluminous. The nominal ASI of the melts are close to 1 for the minimum composition and approximately 1.32 in peraluminous glasses, but if Li is considered as an alkali, the molar ratio Al/[Na+K+Li] of the melts are alkaline (0.87) and subaluminous (1.09), respectively.The molar solubility products [MnO]∗[Nb2O5] and [MnO]∗[Ta2O5] are nearly independent of the F content of the melt, at approximately 18.19±1.2 and 43.65±2.5×10−4 (mol2/kg2), respectively for the minimum composition. By contrast, there is a positive dependence of zircon and hafnon solubilities on the fluorine content in the minimum composition, which increases from 2.03±0.03×10−4 (mol/kg) ZrO2 and 4.04±0.2×10−4 (mol/kg) HfO2 for melts with 0wt.%F to 3.81±0.3×10−4 (mol/kg) ZrO2 and 6.18±0.04×10−4 (mol/kg) HfO2 for melts with 8wt.%F. Comparison of the data from this work and previous studies indicates that ASI of the melt seems to have a stronger effect than the contents of fluxing elements in the melt and the overall conclusion is that fluorine is less important (relative to melt compositions) than previously thought for the control on the behavior of high field strength elements in highly evolved granitic melts. Moreover, this study confirms that although Nb, Ta, Zr and Hf are all high field strength elements, Nb–Ta and Zr–Hf are complexed differently in the melt.

Fluorine speciation as a function of composition in peralkaline and peraluminous Na2O–CaO–Al2O3–SiO2 glasses: A multinuclear NMR study

The incorporation mechanisms of fluorine (F) into peralkaline and peraluminous Na2O–CaO aluminosilicate glasses with ∼65mol% SiO2 (model system for phonolites) were investigated by magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. In 19F MAS NMR spectra of the fluorine-bearing peralkaline glasses at least five F sites could be distinguished, while only three of these sites could be found in the corresponding peraluminous glasses, which shows that there are more F incorporation mechanisms in peralkaline than in peraluminous glasses. In the peralkaline glasses containing up to 6.2mol% F the following F environments were identified: F–Ca(n) at ∼−113ppm, Si–F–Na(n) or Al–F–Ca(n) at ∼−146ppm, Al–F–Al at ∼−168ppm, Al–F–Na(n) at ∼−188ppm and F–Na(n) at ∼−225ppm (“n” indicates that the number of atoms is variable or uncertain). F–Ca(n) is the most abundant site which is surprising as Ca is the least common cation in the glasses. The fraction of F–Ca(n) sites increases from 42% to 53% as the F content increases from 1.2 to 6.2mol%. The addition of up to 16.5mol% (5.3wt%) water strongly affects F speciation in peralkaline glasses and results in a decrease in the fraction of F–Al sites compared to F–Ca(n) sites. It seems that hydroxyl groups (OH) and F occupy similar Al environments and that F cannot compete with OH.In the peraluminous glasses containing up to 18.3mol% F only three F environments Si–F–Na(n) or Al–F–Ca(n) at ∼−149ppm, Al–F–Al at ∼−170ppm and Al–F–Na(n) at ∼−190ppm are observed. Al–F–Na(n) is the most abundant site with a fraction of 54–61%. The F speciation also changes with the F concentration, with a minimum in Al–F–Na(n) sites between 3.5 and 9.7mol% F.Fluorine has only a small effect on the 23Na and 29Si MAS NMR spectra. 27Al MAS NMR spectra of the peralkaline glasses show only four-coordinated Al while in the peraluminous glasses ∼5% of the Al was found to be five-coordinated. The amount of five-coordinate Al does not change with increasing F content, but the environment of the five-coordinate Al becomes more symmetric with increasing F.

The role of Al3+ on rheology and structural changes in sodium silicate and aluminosilicate glasses and melts

Because of their importance in both the geosciences and the glass-making industry, alkali aluminosilicate melts have been the focal point of many past studies, but despite progress many problems remain unresolved, such as the complex behaviour of the thermodynamic properties of aluminium-rich alkali silicate melts. This paper presents a study of Na2O–Al2O3–SiO2 glasses and melts, containing 75mol% SiO2 and different Al/(Al+Na) ratios. Their structure has been investigated by using Raman spectroscopy, as well as, 23Na, 27Al and 29Si 1D MAS NMR spectroscopy. Results confirm the role change of Na+ cations from network modifier to charge compensator in the presence of Al3+ ions. In addition, polymerization increases with increase of the Al/(Al+Na) ratio. These structural changes explain the observed variations in the viscosity of these melts. The viscosity data in turn allow us to calculate the configurational entropy of melts at the glass transition temperature [the Sconf(Tg)]. The variations of the Sconf(Tg) are strongly nonlinear, with sharp increases and decreases depending on the Al/(Al+Na) ratio. More importantly, a strong increase of the Sconf(Tg) is observed when a few Al2O3 is added to sodium silicate melt. A strong decrease is observed after crossing the tectosilicate join, when Al/(Al+Na)>0.5 and when Al3+ ions are present in fivefold coordination, Al[5], in the glass. Furthermore, in situ27Al NMR spectra of the peraluminous melt show a clear increase of the Al[5] concentration with increasing temperature. When considered in combination with melt fragility and heat capacity, our data demonstrate that Al[5] is clearly a transient unit at high temperature in highly polymerized tectosilicate and peraluminous melts. However, when present in glasses, Al[5] increases the stability of the aluminosilicate network, hence the Tg of glasses. This could be explained by the ability of Al[5] to carry threefold coordinated oxygen atoms in its first coordination shell, as observed in minerals. Localisation of threefold coordinated oxygen atoms on Al[5] implies an increase of the medium-range order in the glass, an hypothesis that is in agreement with the low Sconf(Tg) of peraluminous glasses.

Amorphous Materials: Properties, Structure, and Durability: Arsenic enrichment in hydrous peraluminous melts: Insights from femtosecond laser ablation-inductively coupled plasma-quadrupole mass spectrometry, and in situ X-ray absorption fine structure spectroscopy

Despite the ubiquity of arsenic in hydrothermal-magmatic environments, its abundance, distribution, and chemical and structural status in natural silicate melts and glasses remain poorly known. Here we report the first in situ measurements of the redox state and molecular structure of As, using X-ray absorption fine structure (XAFS) spectroscopy, in a rhyolitic peraluminous glass from Macusani (SE Peru) that is representative of anatectic melts derived from metasedimentary crustal protoliths. Arsenic abundances as well as the concentrations of other trace elements were measured in the glass using a femtosecond laser ablation-inductively coupled plasma-quadrupole mass spectrometry (LA-ICP-QMS). The glass shows enrichments, by factors of 10 to 100, in comparison with the mean continental crust values, for As and other incompatible trace elements (e.g., Be, B, Rb, Sn, Sb, and Ta), and by factors of 100 to 200 for Li, Cd, and Cs. Arsenic is present in the peraluminous glass in trivalent state in the form of oxy-hydroxide complexes like AsO(OH) 2 - and/or As(OH) 3 , similar to those dominant in the aqueous fluid vapor phases at hydrothermal-magmatic conditions. The similar As chemical speciation between the fluid and the melt is consistent with As fluid/melt partitioning coefficients close to one, as observed in experiments on rhyolite-water systems. The depolymerized melt structure caused by elevated H 2 O, F, and P contents is likely to allow accomodation of high concentrations of metalloid hydroxide/hydrated complexes. Consequently, hydrous silicate melts may be important transporting media in shallow magmatic-hydrothermal settings for As and similar elements like B and Sb due to their high affinity to water and hydroxide ligands

Disequilibrium thermal breakdown of staurolite: a natural example

A partially melted pelitic xenolith, Wehr volcano, East Eifel, Germany, contains staurolite porphyroblasts pseudomorphed by a hercynite-rich assemblage that includes ferrogedrite, sillimanite-mullite ss , quartz and siliceous, peraluminous glass, which can be schematically represented by the reaction, staurolite → hercynite + gedrite + Al-silicate + quartz + melt. Minerals and glass of the pseudomorph are a disequilibrium breakdown assemblage that represents a time-temperature-transformation of staurolite during short-term heating and cooling. Microstructural evidence indicates the hercynite formed first together with melt, followed by ferrogedrite, and crystallization of Al-silicate and quartz from the melt. The absence of cordierite, the presence of ferrogedrite (a possible metastable intermediate phase to formation of almandine) in the staurolite breakdown assemblage and evidence for melting of muscovite + quartz and plagioclase (An 20 ) + quartz in the xenolith indicates minimum pressure conditions of 0.35 GPa at ca . 665–700 °C with probable overstepping of staurolite stability by 40–95 °C. Such conditions were kinetically favourable for the formation of the metastable breakdown assemblage that was preserved by quenching on eruption.

Structural behavior of Al3+ in peralkaline, metaluminous, and peraluminous silicate melts and glasses at ambient pressure

Peralkaline and peraluminous glasses close to the metaluminous join in the systems Na2O-Al2O3-SiO2 (NAS), CaO-Al2O3-SiO2 (CAS), and MgO-Al2O3-SiO2 (MAS) have been examined with Raman spectroscopy. At least three different SiO2 contents in each of the systems (NAS, CAS, and MAS) have been studied. Each series of glasses spans the metaluminous join at constant silica content. The spectra of glasses in all three systems show changes consistent with a continuous decrease in abundance of depolymerized species and an increase in fully polymerized species as compositions change from peralkaline to peraluminous. There is no evidence for maxima or minima of these abundances across the metaluminous joins for any of the studied series. These observations confirm previous suggestions that non-bridging O atoms are a general feature of .fully polymerized. glasses, and that a population of Al exists in the melt structure that is not associated with a charge-balancing cation, even in peralkaline compositions.

STRUCTURAL ENVIRONMENT OF Nb5+ IN DRY AND FLUID-RICH (H2O, F) SILICATE GLASSES: A COMBINED XANES AND EXAFS STUDY

We performed Nb K -edge XAFS spectroscopy experiments on a series of model compounds, natural glasses and anhydrous and fluid-bearing (H 2 O, F) synthetic glasses doped with 0.1 to 5 wt.% Nb 2 O 5 . In addition, we decribe a new ICP–AES method to provide chemical information on micro quantities of glass samples. In dry, H 2 O- and F-bearing peraluminous and peralkaline glasses, Nb 5+ is present as NbO 6 moeties. These share corners with SiO 4 and AlO 4 tetrahedra through non-bridging oxygen (NBO) atoms. No evidence for 4-, 5- or 7-coordinated Nb was found. Slight changes in site centrosymmetry were detected among the compositions (ordered in NBO-rich compositions, more disordered and distorted in peraluminous ones). In agreement with Nb 2 O 5 solubility experiments, bond-valence theory suggests that network-modifying cations such as Na are present in the vicinity of Nb. This environment most closely resembles those observed in alkali niobosilicates such as vuonnemite and labuntsovite-group minerals. In dry, peraluminous glasses, the sites containing Nb are disordered, as in columbite. Similar information on speciation was obtained for a set of natural glasses. The local environment around the Nb atoms is therefore highly sensitive to melt depolymerization, alkalinity and H 2 O, F contents, all of which will increase the solubility of Nb in the melt and explains a number of its geochemical properties in evolved magmatic systems.

Al coordination and speciation in calcium aluminosilicate glasses: Effects of composition determined by 27Al MQ-MAS NMR and Raman spectroscopy

The structure and properties of glasses and melts in the CaO–Al 2O 3–SiO 2 (CAS) system play an important role in earth and material sciences. Aluminum has a crucial role in this ternary system, and its environment is still questioned. In this paper, we present new results using Raman spectroscopy and 27Al Nuclear Magnetic Resonance on CAS glasses obtained by classic and rapid quenching methods. We propose an Al/Si tetrahedral distribution in the glass network in different Q n species. In this system, we show that Al and Si are mainly in Q 4 species along the join R = CaO/Al 2O 3 = 1, and in depolymerized Q 2 and Q 3 units at high CaO content for other joins ( R = 1.57 and 3). Five- ( [5]Al) and six-fold ( [6]Al) coordinated aluminum can be detected in the peraluminous glasses ( R < 1) in agreement with the deficit of charge compensators, Ca 2+ions, near Al atoms. Unexpectedly, between 5% and 8% of [5]Al is also observed for percalcic glasses ( R > 1), except for glasses with low silica and high CaO content. The presence of [5]Al is related to viscous flow mechanisms while, in highly depolymerized glasses, the absence of [5]Al may indicate different mechanisms for melts to flow. This systematic study on the CAS system modifies the simple picture of aluminosilicate glasses, and the existence of [5]Al should be included by geochemists, geophysicists and glass scientists to model appropriately the physical properties of aluminosilicate glasses and melts.