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Peraluminous Viscosity Maxima in Na2O-Al2O3-SiO2 Liquids: The Role of Triclusters in Tectosilicate Melts
Abstract
We present enhanced-precision viscosity determinations for Na2OAl22O3SiO2 liquids which challenge the commonly held view that melts of tectosilicate stoichiometry in this system are fully polymerised. The maximum in melt viscosity produced by the substitution of Al for Na does not generally occur at molar Na/Al = 1; rather, it deviates systematically into the peraluminous field for SiO2 > 50 mol%. This deviation is greatest at 67 mol% SiO2 and increases with decreasing temperature. These features may be most simply explained by the presence of “triclusters” consisting of a threefold coordinated oxygen to which one aluminate and two silicate tetrahedra are attached. The presence of such triclusters has important implications for phase equilibria, mechanisms of viscous flow, and the solubilities of elements in natural and synthetic highly polymerised aluminosilicate melts.
... The presence of triclusters results in a more compact packing and, thereby, higher densities are expected (Toplis et al., 1997b). More importantly, triclusters can play a dominant role in transport properties, particularly for self-diffusion of oxygen in melts and viscous flow processes (Toplis et al., 1997b). ...
... The presence of triclusters results in a more compact packing and, thereby, higher densities are expected (Toplis et al., 1997b). More importantly, triclusters can play a dominant role in transport properties, particularly for self-diffusion of oxygen in melts and viscous flow processes (Toplis et al., 1997b). However, their presence is still debated and their influence on properties not ascertained (Tandia et al., 2011). ...
... The extremum in viscosity and its occurrence slightly in the peraluminous region has been strongly debated. This anomaly has not yet been truly understood since it can be attributed to the formation of O tri or Al in high coordination states (Lacy, 1963;Riebling, 1966;Toplis et al., 1997b). ...
... As R approaches the subaluminous join (i.e., R = 1), all modifier cations become associated with Al tetrahedra for charge balancing, which should result in a fully connected network with no expected NBOs. 7,8 When R > 1, i.e., peraluminous region, the alumina content exceeds the amount of modifier in the system and requires a mechanism for charge balancing the Alcentered tetrahedra since not enough modifiers exist. A schematic which also visualizes these regions can be seen in Figure 2. As the glass system changes from metaluminous to peraluminous, anomalous trends in properties occur which have been attributed to structural rearrangements within the glass network. ...
... 24 From the work by Zirl and Garofalini, the extrema observed in the viscosity of the NAS glasses were explained by the change in activation energy for sodium diffusion due to tricluster formation and reduction of NBOs. 24 This theory was further studied and supported by Toplis et al., 7 in which triclusters were thought to explain the anomaly in the viscosity for alkali aluminosilicates. ...
... In addition, the highest percentage of NBOs was hypothesized to occur in the 67 mol% SiO 2 composition (Fig. 1). 7 Toplis et al. also concluded that some portion of sodium ions at R = 1 must not be in a charge-balancing role for alumina tetrahedra which would explain the excess NBOs. Theoretically, the R = 1 composition should be fully charge-balanced with no population of NBOs since all modifiers are "consumed" by the alumina tetrahedra. ...
Aluminosilicate glasses are ubiquitous in high‐performance displays due to their favorable thermal, mechanical, and optical properties. They also exhibit interesting structural features depending on the ratio of alumina to modifiers in the glass system. Excess modifiers exist in the metaluminous region, while the peraluminous region contains more negatively charged alumina structures than modifiers. As the composition switches from metaluminous to peraluminous, anomalous changes in properties such as the glass transition temperature, viscosity, and refractive index occur. This has been explained with two contrasting structural transformations to accommodate the lack of charge‐balancing modifiers: either aluminum increases in coordination (forming five‐coordinated or six‐coordinated Al) and/or oxygens become three‐coordinated (known as triclusters). The precise charge‐balancing mechanism remains a subject of much debate in the community. This review highlights this structural debate by providing a chronological understanding of how these two theories evolved. We also summarize the state‐of‐the‐art understanding of the aluminosilicate glass structure. By gaining a more comprehensive view of the two opposing structural views within the aluminosilicate glass system, we can gain insights on valuable future research from both experimental and modeling perspectives.
... The presence of triclusters results in a more compact packing and, thereby, higher densities are expected (Toplis et al., 1997b). More importantly, triclusters can play a dominant role in transport properties, particularly for self-diffusion of oxygen in melts and viscous flow processes (Toplis et al., 1997b). ...
... The presence of triclusters results in a more compact packing and, thereby, higher densities are expected (Toplis et al., 1997b). More importantly, triclusters can play a dominant role in transport properties, particularly for self-diffusion of oxygen in melts and viscous flow processes (Toplis et al., 1997b). However, their presence is still debated and their influence on properties not ascertained (Tandia et al., 2011). ...
... The extremum in viscosity and its occurrence slightly in the peraluminous region has been strongly debated. This anomaly has not yet been truly understood since it can be attributed to the formation of O tri or Al in high coordination states (Lacy, 1963;Riebling, 1966;Toplis et al., 1997b). ...
Aluminosilicate glasses are of great importance for industrial applications and are good analogs of magmas. In practical applications, almost all silicate glasses incorporate alumina either as impurity or as a large component and Al2O3 usually imparts greater chemical durability, viscosity, glass transformation temperatures, improves mechanical properties, reduces devitrification tendencies and thermal expansion. Recent experimental and modeling techniques have substantially improved our understanding of the atomic-scale structure, providing a strong basis to better control glass properties. In this article, the composition-structure-property relationships in a great variety of glasses containing Al2O3 are reviewed.
... Changes in configurational entropy in the melt are associated with configurational heat capacity, C p conf . 1, 13,14 For silicates, C p conf has been described as the difference in heat capacity between the liquid and the glass at transition temperature. 15 Besides the decrease in viscosity with increasing temperature, the glasses also show a decrease in viscosity with decreasing silica content ( Figure 5c). ...
... The viscosity in the range of 10 4 to 10 8 Pa s could not be tested with the equipment used here, owing to crystallisation of the glasses studied. 13,14 Chemical durability experiments Dissolution rate measurements in dilute conditions using the Stirred Reactor Coupon Analysis (SRCA) method. Here, the glass dissolution rate is calculated from the step height resulting from dissolution of a flat surface (Figure 8a). ...
Silicate glasses are not only used for everyday articles such as windows or drinking glass. By altering their composition, we can adjust their properties to become degradable and help to...
... Such "high-performance" CS glasses often incorporate large amounts of Al, which increase both the melting temperature and the melt viscosity (Silverman, 1939;Cheng et al., 2013;Karlsson, 2021), and thereby making the glasses comparatively costly to produce. The melt viscosity is maximized around a unity molar ratio of the metal oxide to alumina (Toplis et al., 1997;Webb et al., 2007), n(M (2) O)/n(Al 2 O 3 ) = 1 [henceforth referred to as "M (2) O/Al 2 O 3 "]. Conventional soda-lime-silicate (SLS) glasses are in general unsuitable for chemical strengthening because they suffer from higher stress relaxation than high performance AS glasses (Varshneya and Kreski, 2012;Erdem et al., 2017;Güzel et al., 2019;Macrelli et al., 2019;Sun and Dugnani, 2020). ...
... In general, AS glasses frequently exhibit a turning point for properties M (2) O/Al 2 O 3 ≈ 1. Especially transport properties, such as melt viscosity, exhibit clear maxima in both the ternary and the quaternary Na 2 O-(CaO)-Al 2 O 3 -SiO 2 systems (Toplis et al., 1997;Webb et al., 2007;Karlsson, 2021). However, other properties conventionally also give a trend-shift at M (2) O/Al 2 O 3 ≈ 1 (Smedskjaer et al., 2013;Cormier, 2021). ...
For a series of conventional soda-lime-silicate glasses with increasing Al2O3 content, we investigated the thermal, mechanical, and structural properties before and after K+-for-Na+ ion-exchange strengthening by exposure to molten KNO3. The Al-for-Si replacement resulted in increased glass network polymerization and lowered compactness. The glass transition temperature (Tg), hardness (H) and reduced elastic modulus (Er), of the pristine glasses enhanced monotonically for increasing Al2O3 content. H and Er increased linearly up to a glass composition with roughly equal stoichiometric amounts of Na2O and Al2O3 where a nonlinear dependence on Al2O3 was observed, whereas H and Er of the chemically strengthened (CS) glasses revealed a strictly linear dependence. Tg, on the other hand, showed linear increase with Al-for-Si for pristine glasses while for the CS glasses a linear to nonlinear trend was observed. Solid-state 27Al nuclear magnetic resonance (NMR) revealed the sole presence of AlO4 groups in both the pristine and CS glasses. 23Na NMR and wet-chemical analysis manifested that all Al-bearing glasses had a lower and near-constant K+-for-Na+ ion exchange ratio than the soda-lime-silicate glass. Differential thermal analysis of CS glasses revealed a “blurred” glass transition temperature (Tg) and an exothermic step below Tg; the latter stems from the relaxation of residual compressive stresses. The nanoindentation-derived hardness at low loads and <5 mol% Al2O3 showed evidence of stress relaxation for prolonged ion exchange treatment. The crack resistance is maximized for molar ratios n(M(2)O)/n(Al2O3)≈1 for the CS glasses, which is attributed to an increased elastic energy recovery that is linked to the glass compactness.
... VII D) between three network-forming (Al-or Si-centered) tetrahedral units. 43,44 Notwithstanding, the majority of the negative values are zero or positive within the error bounds. Figure 7 shows the dependence of f NBO on R along constant mol% SiO 2 tie-lines for several different aluminosilicate glasses. ...
... 58 In practice, there is often a displacement of this extremum from R = 1, and it is speculated that oxygen triclusters may play a crucial structural role. 44 The presence of triclusters in the peraluminous regime is indicated in the present work by some negative values of f NBO for the KAS, BaAS, and CaAS systems, as calculated from Eq. (12). The present model shows, however, that along a constant mol % SiO 2 tie-line, the minimum in f NBO is asymmetric about R = 1 with a shape and depth that are dependent on F M . ...
An analytical model is developed for the composition-dependent structure of the amorphous aluminosilicate materials
(M2O)x(Al2O3)y(SiO2)1−x−y and (MO)x(Al2O3)y(SiO2)1−x−y, where 0 ≤ x ≤ 1 and 0 ≤ y ≤ 1. The model is based on a simple set of
reactions and contains a single adjustable parameter p (0 ≤ p ≤ 1). The latter is found from 27Al solid-state nuclear magnetic resonance
(NMR) experiments in the regime where R = x/y ≥ 1, aided by new experiments on the magnesium and zinc aluminosilicate systems.
The parameter p decreases linearly as the cation field strength of M+ or M2+ increases, as per the observation previously made for the
degree of aluminum avoidance [Lee et al., J. Phys. Chem. C 120, 737 (2016)]. The results indicate that as the cation field strength increases,
there are less fourfold coordinated aluminum atoms to contribute toward the glass network, and Al–O–Al bonds become more prevalent
in a progressive breakdown of Loewenstein’s aluminum avoidance rule. The model gives a good account of the composition-dependent
fraction of non-bridging oxygen (NBO) atoms for R ≥ 1, as assessed from the results obtained from solid-state NMR experiments. An
extension of the model to (M2O3)x(Al2O3)y(SiO2)1−x−y glasses leads, however, to an excess of NBO atoms, the proportion of which
can be reduced by invoking network-forming fivefold coordinated Al atoms and/or oxygen triclusters. The model provides a benchmark
for predicting the structure-related properties of aluminosilicate materials and a starting point for predicting the evolution in
the structure of these materials under the extreme conditions encountered in the Earth’s interior or in processes such as sharp-contact
loading.
... This makes aluminosilicate glass costly and rather difficult to produce. Moreover, at the metaluminous composition, i.e., when the non-bridging oxygens are minimized, the melt viscosity is maximized [9]. This has similarly been found to exist in the quaternary soda lime aluminosilicate system when the ratio R 2 O+RO/Al 2 O 3 is equal to 1 [10]. ...
... The viscosity results show, as expected, that the viscosity increases with increasing alumina content, see Fig. 2 and the supplementary material (Table S1). This was also noted when melting and quenching the glass samples and corresponds well to previous studies [9,10,29]. As shown in Fig. 2, both viscosity models fit the data very well in the liquid range. ...
Adding alumina to the conventional soda lime silicate glass composition improves many properties, however, also increases the melting temperature. In the current paper, alumina doping of soda lime silicate glasses and its implications to high temperature viscosity are investigated in order to verify the linearity when replacing SiO2 for Al2O3. An anomaly in the linearly increasing viscosity was found when the ratio CaO/Al2O3 is <1, which is explained by Ca²⁺ acting as an inhibitor for viscous flow. The Angell and Rao and the Waterton-MYEGA models show very similar results, even for the extrapolated Tg. Thermodynamic data extracted using the Ojovan method generally increases linearly with the Al2O3 content. The Doremus fragility index shows a deviation in the linearity while the Angell fragility index on the other hand shows a linear increase with the Al2O3 content.
... However, the actual equilibrium position shifts because some Al 2 O 3 forms higher-coordination AlO 5 and AlO 6 structures that do not require charge compensation. For oxides with high basicity, such as BaO, CaO and Na 2 O, the compensating equilibrium may occur at an M x O/Al 2 O 3 molar ratio of less than 1, [23,24,27,28] while for oxides with lower basicity, such as MgO, the equilibrium may occur at a molar ratio greater than 1. [27] In the current slag system, the relatively high optical basicity of CaO compared to MgO gives CaO priority in charge compensation for Al 3+ ions. [29] By converting the slag compositions into molar fractions, it can be observed that the CaO/Al 2 O 3 molar ratios in all four slags are greater than 1, indicating that charge compensation equilibrium for Al 3+ ions has been achieved in all cases. ...
This study explored the effect of CaO/Al2O3 ratio on the fluidity and structure of CaO–Al2O3–MgO–V2O3 slag used for smelting Al–V alloys. With increasing CaO/Al2O3 ratio, viscosity progressively decreased, while the free running temperature initially decreased and then increased, with a turning point at a ratio of 0.8. Introducing CaO disrupted the bridging oxygen by releasing free oxygen, leading to the formation of non-bridging oxygen. Consequently, bridging oxygen decreased while non-bridging oxygen and free oxygen increased with higher CaO/Al2O3 ratio, as demonstrated by XPS analysis. 27Al MAS-NMR spectra revealed that AlO4 tetrahedra consisting of Q2(Al), Q3(Al), and Q4(Al) units dominated the slag, accounting for more than 90 mol pct of the Al3+ species, with smaller amounts of AlO5 and AlO6 units. As the CaO/Al2O3 ratio increased, Q4(Al) and Q3(Al) units with highly coordinated bridging oxygens were continuously converted to Q2(Al) units, resulting in a lower degree of polymerization within the aluminate network.
... In this sense, many studies have focused on the ratios of Na 2 O/K 2 O, MgO/CaO, and SiO 2 /Al 2 O 3 . As a result of these studies, it has been determined that the vitrification rate strongly depends on the amounts and/or ratios of these factors in the starting composition [44][45][46][47][48][49][50][51]. ...
This study deals with the intricate relationship between composition, microstructure and the final properties of porcelain tiles namely bulk density, water absorption, thermal expansion coefficient, strength, and Young’s modulus. In order to achieve this, a comprehensive set of 16 experimental trials were carried out, involving the preparation of eight distinct formulations characterized by varying Seger values and ratios which were subsequently subjected to sintering at two distinct firing temperatures. The analysis was conducted through a comprehensive correlation matrix, revealing significant relationships between composition, phase content and the final properties. The glassy phase emerges as a key contributor, positively influencing bulk density, reducing water absorption and linear shrinkage, and enhancing Young’s modulus. The coefficient of thermal expansion correlates positively with residual quartz and inversely with mullite content, highlighting their impact on this property. Strength is positively correlated with mullite content, while quartz, albite, and anorthite phases exhibit negative correlations due to their intricate influence on the overall microstructure. These findings provide essential insights into the complex interplay between composition, microstructure and properties, contributing to the advancement of materials engineering for tailored porcelain tiles.
... [22] However, to the authors' knowledge, it seems that research on this assumption is limited [25][26][27] and mainly stems from the presence of a clear effect of charge compensation on slag viscosity and the assumed reciprocity between electrical conductivity and viscosity of slags. [28][29][30] Thus, experimental evidence for the assumption that the charge compensating cations are immobile seems to be lacking for the moment. It might be possible though that they require a higher activation energy to move from their position and thus cause a decrease in electrical conductivity. ...
Currently, the valorization of slags from the lead production/recycling industry is difficult due to their high lead content. Moreover, lead is still mainly produced via blast furnaces which use cokes for heating and thus emitting a large quantity of greenhouse gasses. A potential solution for both issues is to change the production to a submerged electric arc furnace which relies on electricity for heat generation. The operation of this furnace is highly dependent on the slag’s electrical conductivity and thus, knowledge of the dependency of this property as a function of the slag’s composition is essential, but mostly lacking in literature. To obtain fundamental insights on this property as a function of slag composition, a simple ternary PbO–CaO–SiO2 system was studied wherein PbO was gradually replaced by CaO at a constant SiO2 content. The presented experimental results show a significant non-linear decrease in the electrical conductivity as a function of PbO by CaO substitution. This decrease cannot be predicted by the currently available electrical conductivity model based solely on the slag’s degree of polymerization. Similarities between the observed trends in electrical conductivity due to this substitution and mixed-(earth) alkali slags/glasses may further prove that Pb2+ and Ca2+ exhibit a mixed modifier effect in the molten state.
... Understanding the structure of aluminosilicate glasses is of great importance both in geological sciences and materials science. Magmas are melts of aluminosilicate composition, [1][2][3][4][5][6] and aluminosilicate glasses are used in a variety of applications such as reinforcement fibres in composites, 7-10 the entrapment of radioactive waste, 11,12 as optical components such as photonic waveguides and optical amplifiers 13,14 and as host media for high power lasers. 15,16 A detailed knowledge of the structure-property relationships in such glasses is key to tailoring mechanical and optical properties. ...
The average shift of high-frequency IR and Raman spectra, calculated as their centre of gravity (COG), correlates linearly with optical basicity and allows for estimating the average degree of covalent/ionic bonding by vibrational spectroscopy.
... In this sense, many studies have focused on the ratios of Na 2 O/K 2 O, MgO/CaO, and SiO 2 /Al 2 O 3 . As a result of these studies, it has been determined that the vitrification rate strongly depends on the amounts and/or ratios of these factors in the starting composition [44][45][46][47][48][49][50][51]. ...
This study focuses on optimizing the composition and firing temperature of porcelain tiles using statistical analysis techniques. A full factorial design, including model adequacy checking, analysis of variance, Pareto charts, interaction plots, regression model, and response optimizer is employed. The key factors were the Seger ratios of SiO2/Al2O3, Na2O/K2O, MgO/CaO, and firing temperature. The response variables investigated were bulk density, water absorption, linear shrinkage, coefficient of thermal expansion (at 500°C), and strength. The statistical analysis revealed highly significant results, which were further validated, confirming their reliability for practical use in the production of porcelain tiles. The study demonstrated the effectiveness of utilizing Seger formulas and properties of typical raw materials to accurately predict the final properties of ceramic tiles. By employing SiO2/Al2O3=5.2, Na2O/K2O=1.50, MgO/CaO=3.0, and firing temperature of 1180°C, optimized properties, such as maximum strength, maximum bulk density, and minimum water absorption, was achieved with a composite desirability of 0.9821.
... 8 Further molecular dynamics simulations of other modified aluminosilicate glasses have supported Zirl and Garofalini. 4,9 Later works by Neuville et al. studied calcium aluminosilicates (CAS) using nuclear magnetic resonance (NMR) and found significant fractions of five coordinated aluminum in the peraluminous regime. It was concluded that TBO were most likely trivial in these aluminosilicate glass systems and were not the preferred charge-balancing mechanism. ...
Previous 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.
... Al 2 O 3 ), the Al can sit in the tetrahedral sites, but because Al carries a 3+ charge, the mean coordination for O must increase in order to maintain charge balance. This is achieved by the formation of oxygen triclusters, 19,20 O atoms that link three tetrahedra. From a topological analysis, these triclusters "harden" the network, 21 leading to lower glass forming ability. ...
Understanding and controlling liquid–liquid phase separation in aluminosilicates is crucial for optimizing glass properties. However, the metastable nature of aluminosilicates’ phase separation has made it difficult to study experimentally, and uncertainty persists regarding the compositional and temperature extents of the miscibility gap. Here, we present new experimental evidence that suggests a consolute temperature between 1440 and 1590°C and endmember compositions of 7 and 62 mol.% Al2O3 for the phase‐separated glasses. Using containerless melt processing, deeply supercooled liquids over the 0–60 mol.% Al2O3 range are probed with in situ small‐ and wide‐angle X‐ray scattering, which simultaneously reveals changes in nanoscale density heterogeneity and atomic structure. Correlations between phase separation and atomic coordination environments are compared for liquids and glasses. Pair distribution function analysis shows mean O–(Si + Al) coordination increases with Al2O3 content and decreases with temperature.
... Above 6 GPa, the formation of the [4] Si-O- [5,6] Al bond and [5,6] Al in boron-bearing aluminosilicate glasses indicates that the oxygen tricluster contributes to the formation of [5,6] Al as NBO species are not available in both glasses. Particularly, two [4] Si-O- [4] Al bonds may participate to form [3] O with highly coordinated Al in Scheme (1.5), as proposed by earlier studies (Lacy, 1963;Toplis et al., 1997). This indicates that [3] O clusters easily form in glasses with a larger fraction of [4] Si-O- [4] Al species in NaAl 0.5 B 0.5 SiO 4 glasses. ...
The pressure-induced structural evolutions of boron-bearing model rhyolitic melts under high pressures enable to infer the detailed geochemical processes (melting and fluid-rock-melt interactions) occurring in Earth interiors and to control the melt properties (viscosity and the boron isotope composition, δ¹¹B) of complex magmatic melts, providing insights into the boron cycle toward the deeper part of the upper mantle (∼10 GPa). Despite the importance, the structures of multicomponent boron-bearing silicate melts above 3 GPa are currently unavailable. Here, we explore the structures, particularly, coordination transformation of constituent elements in boron-bearing nepheline and albite glasses – a model rhyolitic melts - upon compression to a depth of ∼270 km (∼9.2 GPa) in the mantle using multi-nuclear solid-state nuclear magnetic resonance (NMR) spectroscopy. The results showed that the conversion of [3]B into [4]B is prominent upon compression up to 6 GPa. In contrast, the formation of [5,6]Al is accompanied by the formation of oxygen tricluster above 6 GPa, where all the nonbridging oxygens are consumed. We quantify how the melt composition affects tendency to form highly coordinated B, Al, and Si upon compression. Particularly, the increase in the [4]B population tends to be larger for the glasses with low Si content as pressure increases to 9.2 GPa. We reveal the relationship between such structural adaptations of the compressed melts at high pressure and the melt properties, including viscosity and element partition coefficient in boron-bearing melts. The current NMR results also unravel the structural origins of ¹¹B/¹⁰B ratios in rhyolitic melts at high pressure. Considering a preferential partitioning of ¹⁰B to [4]B, an increase in [4]B population in the melts leads to an pressure-induced enrichment of ¹⁰B. As the increase in Si/B ratio in the melts tends to decrease the pressure-induced increase in [4]B fraction, the contribution of boron coordination transformation on the ¹¹B/¹⁰B ratios in silicate melt would be somewhat minor in deep mantle melts with increasing Si content. The detailed boron environments in rhyolitic melts at high pressure yield useful constraints for the isotope composition (¹¹B/¹⁰B) of dense mantle melts, thereby enabling quantification of deep boron cycle.
... The glass formation regions were defined for alkali and alkaline earth aluminosilicates and demonstrated that the alumina saturation limit extended up to 1.2 mol of alumina per mole of flux. 1 The hardness of these glasses appears to be strongly linked to the network connectivity through the formation of fivefold coordinated aluminum ([AlO 5 ] − ) species, the formation of Si 2 AlO x and SiAl 2 O x triclusters, and the elimination of nonbridging oxygen (NBO). [2][3][4] It was demonstrated that hardness increased with the extension of the alumina saturation level from 1.0 to 1.2 in aluminosilicate glasses. The highest hardness glasses possessed chemistries similar to stable crystalline phases, specifically anorthite in the CaO-Al 2 O 3 -SiO 2 (CAS) system. ...
Vickers hardnesses of RO–Al2O3–SiO2 glasses were measured over a broad range of compositions, ranging from ternary endpoints to mixed ratios of RO (CaO with MgO, SrO, or BaO), with the systematic variation of Al2O3 and SiO2 levels. The hardnesses of CAS and MAS glasses are similar, ranging from 6.7 to 7.2 GPa, with the replacement of CaO with MgO producing a marginal increase in hardness. The substitution of SrO or BaO for CaO generally produced in a decrease in hardness down to 4.5 GPa with BaO. The sensitivity to alumina and silica levels, however, was much greater ranging from a minimum of 4.5 GPa to a maximum of ∼8.2 GPa. The correlation of the Vickers hardness with melting temperature was observed in the CAS system but generally not in the RO‐blended glasses. Overall, a combined cation field strength of modifier cations determined the hardness above the critical RO blending ratio.
... Rare-earth titanate glasses generally exhibit 1.9 ≤ n OTi ≤ 2.1 4 , so the EPSR-predicted n OTi would suggest more O-Ti 3 than previously reported. A significant fraction of these triclusters, a term first used for O linking three Al-O 4 tetrahedra in aluminosilicate networks32,33 , leads to topological hardening and lower glass-, the large fraction of triclusters prevents glass formation via melt-quenching, though amorphous forms can be prepared by other means ...
Rare-earth titanates form very fragile liquids that can be made into glasses with useful optical properties. We investigate the atomic structure of 83TiO2-17Nd2O3 glass using pair distribution function (PDF) analysis of X-ray and neutron diffraction with double isotope substitutions for both Ti and Nd. Six total structure factors are analyzed (5 neutron + 1 X-ray) to obtain complementary sensitivities to O and Ti/Nd scattering, and an empirical potential structure refinement (EPSR) provides a structural model consistent with the experimental measurements. Glass density is estimated as 4.72(13) g cm−3, consistent with direct measurements. The EPSR model indicates nearest neighbor interactions for Ti-O at r¯TiO = 1.984(11) Å with coordination of nTiO = 5.72(6) and for Nd-O at r¯NdO = 2.598(22) Å with coordination of nNdO = 7.70(26), in reasonable agreement with neutron first order difference functions for Ti and Nd. The titanate glass network comprises a mixture of distorted Ti-O5 and Ti-O6 polyhedra connected via 71% corner-sharing and 23% edge-sharing. The O-Ti coordination environments include 15% nonbridging O-Ti1, 51% bridging O-Ti2, and 32% tricluster O-Ti3. This structure is highly unusual for oxide glasses melt-quenched at ambient pressure, as it consists of Ti-Ox predominantly in octahedral (with nearly no tetrahedral) coordination.
... It is also noted that Al [6] is negligible ( < 0.2%) in MD simulation, however a new species, Al [3] , is traced. Toplis et al. [62] considered Al [3] species as a metastable intermediate during tricluster formation through interactions between BOs and cations. But it is more likely to be a coordination defect created by specific potential functions in simulation process [63] , which has not been evidenced experimentally. ...
Blast furnace slag stores a massive amount of thermal energy which commonly dissipates during cooling. To lay the foundation for the development of heat retrieval technique, it is vital to realize the thermophysical properties of the slags. In this work, thermal conductivity of CaO-SiO2-Al2O3-MgO melts with 0–15 mol% Al2O3 was investigated using the hot-wire method. The compositional dependence of thermal conductivity was discussed with respect to the structure characterized by Raman spectroscopy, MAS-NMR (magic angle spinning-nuclear magnetic resonance) and MD (molecular dynamics) simulation in short- and intermediate-range order. The additions of Al2O3 up to 9 mol% notably promote the thermal conductivity. The formation of Al-O-Si linkages is greatly enhanced, establishing a polymerized aluminosilicate network. The significant reduction of NBO/Treal contributes to a higher thermal conductivity. Further additions of Al2O3 up to 15 mol% do not significantly increase or reduce the thermal conductivity. Although NBO/Treal still decreases, the decremental rate becomes less rapid. The topological ring size of aluminosilicate network becomes smaller since the formation of Al-O-Al linkages is greatly facilitated at higher Al2O3 content, which reduces the mean free path of phonon vibrations. It may counteract the enhancement of thermal conductivity by the reduction of network modifying oxides.
... There is, however, increasing evidence for deviations from this simple model, with significant proportions AlO5 and even 5-fold coordinated SiO5 units observed in glasses that are sufficiently charge compensated by metal cations Toplis et al. 2000;Stebbins et al. 1991). Significant fractions of AlO4 have also been observed in the alumina-rich glasses (Mysen and Richet 2005) in which charge neutrality may be accomplished by the formation of O:(Al/Si)3 triclusters in which one O atom is shared by three Al/Si tetrahedral units (Lacy 1963;Toplis et al. 1997;Toplis et al. 2000;Stebbins et al. 2001;Mysen and Toplis 2007). ...
... For the synthesis of the glasses, the addition of a network intermediate i.e. Al 2 O 3 can minimize the NBOs and further enhance the viscosity of the melt [19,20]. Moreover, the addition of Al 2 O 3 in the silicate glasses results in lowering the refractive index of the studied glassy matrix [21]. ...
Attempts have been made to synthesize alumina doped calcium silicate glasses in the system 40SiO 2-35K 2 O-(25-x)CaO (x = 0, 2, 4, and 6) via melt-quenching method. Density of the glasses were estimated by employing Archimedes' principle and obtained within the range of 2.421±0.012 to 2.176±0.011 gm/cm 3 , the oxygen packing density (OPD) and ionic concentrations were estimated and found to be in the range of 58.818±0.294-54.161±0.271 × 10 −3 gm-atom/cm 3 and 0-195.725±0.979 ion/cm 3 respectively. FTIR and Raman investigations showed the existence of numerous bonds of Si-O-Si, β-CaSiO 3 , Si-O − , C = O, CO 3 2− and O H, which are due to diverse modes of vibrations. As doping concentration of Al 2 O 3 increases optical band-gap increase from 1.5 to 2.5 eV. Refractive index (n), and optical dielectric constant (ε) were determined and lies within the range of 1.210±0.024-0.827±0.017 and 1.464±0.029-0.684±0.014. CO 2 gas sensing measurements were done at room temperature, 32°C and maximum sensor response (9.93) was recorded for the glass SKCA6 at 1000 ppm.
... Finally, i-Melt predicts small S conf (T g ) values for K-rich and Al-rich melts (Fig. 9b,f), in agreement with experimental findings (Richet and Bottinga, 1984;Le Losq and Neuville, 2013;Le Losq et al., 2017;Robert et al., 2019). This is explained by Al and K respectively promoting the polymerization of the melt network (decrease in NBO/T) and the formation of larger cooperative molecular domains involved in the melt viscous flow (e.g., Riebling, 1966;Taylor and Rindone, 1970;Rammensee and Fraser, 1982;Mysen, 1988;Toplis et al., 1997b;Mysen and Toplis, 2007;Xiang et al., 2013;Le Losq et al., 2017). The variations in S conf (T g ) with the composition of aluminosilicate melts, thus, depend mostly on (i) how metal cations interact together, and (ii) on how those interactions are affected by the presence of Al, and by Si-Al interactions. ...
Aluminosilicate glasses and melts are of paramount importance for geo- and materials sciences. They include most magmas, and are used to produce a wide variety of everyday materials, from windows to smartphone displays. Despite this importance, no general model exists with which to predict the atomic structure, thermodynamic and viscous properties of aluminosilicate melts. To address this, we introduce a deep learning framework, ‘i-Melt’, which combines a deep artificial neural network with thermodynamic equations. It is trained to predict 18 different latent and observed properties of melts and glasses in the K2O-Na2O-Al2O3-SiO2 system, including configurational entropy, viscosity, optical refractive index, density, and Raman signals. Viscosity can be predicted in the 10⁰-10¹⁵ log10 Pa·s range using five different theoretical frameworks (Adam-Gibbs, Free Volume, MYEGA, VFT, Avramov-Milchev), with a precision equal to, or better than, 0.4 log10 Pa·s on unseen data. Density and optical refractive index (through the Sellmeier equation) can be predicted with errors equal or lower than 0.02 and 0.006, respectively. Raman spectra for K2O-Na2O-Al2O3-SiO2 glasses are also predicted, with a relatively high mean error of ∼25 % due to the limited data set available for training. Latent variables can also be predicted with good precisions. For example, the glass transition temperature, Tg, can be predicted to within 19 K, while the melt configurational entropy at the glass transition, Sconf(Tg), can be predicted to within 0.8 J mol⁻¹ K⁻¹.
Applied to rhyolite compositions, i-Melt shows that the rheological threshold separating explosive and effusive eruptions correlates with an increase in the fraction of non-bridging oxygens in rhyolite melts as their alkali/Al ratio becomes larger than 1. Exploring further the effect of the K/(K+Na) ratio on the properties of alkali aluminosilicate melts with compositions varying along a simplified alkali magmatic series trend, we observe that K-rich melts have systematically different structures and higher viscosities compared to Na-rich melts. Combined with the effects of the K/(K+Na) ratio on other parameters, such as the solubility, solution mechanisms and speciation of volatile elements, this could ultimately influence the eruptive dynamics of volcanic systems emitting Na-rich or K-rich alkali magmas.
... Le NBO/T permet aussi d'avoir une idée de la viscosité des liquides, qui elle-même est corrélée à la température du système (Mysen, 1995;Giordano et Dingwell, 2003) permettant de contraindre la migration des liquides silicatés dans différents types de contextes magmatiques. La composition globale du silicate influence par conséquent les propriétés physico-chimiques du liquide, comme par exemple la viscosité, l'entropie de configuration et l'enthalpie de mélange (Richet, 1984;Toplis et al., 1997). Jambon et Richet, 1993 (Jambon et Richet, 1993). ...
L'azote (N) fait parti des éléments volatils (comme l'hydrogène et le carbone) essentiels à la vie sur Terre. Le comportement de cet élément lors des processus géologiques à haute température comme par exemple durant les processus magmatiques reste aujourd'hui très parcellaire. Cette thèse est centrée sur la solubilité, la spéciation et la diffusion de N dans les verres et silicates fondus dans le but de mieux comprendre comment cet élément se comporte lors de ces processus. La solubilité de N dans les silicates fondus a été étudiée pour différentes compositions et pour une large gamme de fO₂ (IW –8 à IW +4.1) à 1425°C et 1 atm. Les résultats montrent un effet majeur de la fO₂ et du degré de polymérisation des silicates fondus sur la solubilité de N. Ces nouvelles données suggèrent qu'un océan magmatique mafique à ultra-mafique aurait pu incorporer autant voir plus de N que ce qui est présent dans la Terre silicatée actuelle, supposant qu'une partie de N se soit échappée vers l'atmosphère ou que celle-ci est stockée dans la Terre profonde (i.e., manteau profond, noyau). Pour l'étude de la diffusion de N dans les verres et liquides silicatés, un développement expérimental et analytique a été nécessaire. Le premier coefficient de diffusion de N (sous la forme N³⁻) dans les silicates fondus, à partir de diffusion uni-axiale, a été mis en évidence pour un liquide de type basalte à andésite (i.e., 4.2 × 10⁻⁸ cm².s⁻¹). Cette étude a aussi permis de mettre en évidence l'effet de la composition du liquide sur la vitesse de diffusion de N³⁻ dans les silicates fondus. Cette dépendance à la composition des silicates fondus est plus importante pour N³⁻ que pour l'argon (Ar). Cela implique que le rapport N/Ar peut alors être fractionné lors de processus magmatiques en conditions réductrices (e.g., durant l'océan magmatique terrestre).
... In turn, replacing a BO by a TO in the Al polytopes increases the local charge by +1/3 since a BO contributes a charge of −1 to the central Al (i.e., −2/2), whereas a TO only contributes a charge of −2/3 since it is shared by three Si or Al polytopes. 55 Altogether, since a TO is connected to three distinct polytopes, the overall increase in charge resulting from the transformation of a BO into a TO is equal to +1 (i.e., 3 × 1∕3). Based on this, the number of TO atoms that is needed to counterbalance the excess negative charge of the Al atoms is then given by: ...
Topological constraint theory (TCT) has enabled the prediction of various properties of oxide glasses as a function of their composition and structure. However, the robust application of TCT relies on accurate knowledge of the network structure and topology. Here, based on classical molecular dynamics simulations, we derive a fully analytical model describing the topology of the calcium aluminosilicate [(CaO)x(Al2O3)y(SiO2)1−x−y, CAS] ternary system. This model yields the state of rigidity (flexible, isostatic, or stressed‐rigid) of CAS systems as a function of composition and temperature. These results reveal the existence of correlations between network topology and glass‐forming ability. This study suggests that glass‐forming ability is encoded in the network topology of the liquid state rather than that of the glassy state.
... 58 In analogy, the positive hole created by the reduction of ytterbium when bound to an aluminum tetrahedron could take the role of a monovalent ion in the dynamic formation of the oxygen tricluster. 69,70 Such process could be responsible for the electrical conductivity results ( Figure S3), where the conductivity of our samples is two orders of magnitude higher than that of fused silica, 71,72 and the estimated activation energies for DC conductivity are much lower than the reported values for diffusion of Al 3+ , Yb 3+ , B 3+ , Si 4+ and O 2− in silicate glasses. 28,73 A combination of polaron hopping 74,75 between the ytterbium ions of different valence and the tricluster diffusion by bond switching 69 is hypothesized to be responsible for these results, as the impurity concentrations measured in our samples are too low to have such a strong effect. ...
We use low‐temperature heat capacity, low‐frequency Raman scattering, and THz time domain spectroscopy in order to scale the vibrational density of states and the Boson peak in SiO2‐Al2O3‐B2O3 Yb‐laser host glasses. When substituting B2O3 for SiO2 at constant Al2O3 dopant level, we find an optimal value for the ratio of B/Al in terms of mixture stability, at which the excess in the electron donor capability of Al2O3 (relative to the SiO2 backbone) is compensated by the more acidic B2O3. At this composition, Al2O3 plays a mediating role in the structure of aluminoborosilicate glasses, facilitating dissolution of Yb2O3 and admixture of B2O3 into the SiO2 network.
... Plus de 90 ans après Zachariasen 1932, ces énoncés sont toujours vrais, à quelques exceptions près, et ont même pu être observés directement (Huang et al. 2012). On sait maintenant que les oxygènes peuvent se lier à trois cations formateurs de réseau dans des conditions particulières (Toplis et al. 1997). Deux polyèdres BO4 peuvent se lier par une arrête (« edge-sharing », Jen et al. 2019). ...
L’129I est un produit de fission de l’uranium dans les centrales nucléaires. Il est radiotoxique, très mobile dans l’environnement, et a une demi-vie longue (15.6 Ma). Le conditionnement de l’129I dans les verres nucléaires pour un stockage géologique est compliqué par sa forte volatilité à haute température. La vitrification des verres nucléaires sous pression est une solution qui peut pallier ce problème, car la solubilité des éléments volatils dans les liquides augmente avec la pression.
Des analogues de verres nucléaires ont été vitrifiés sous haute pression (0.5-2 GPa) pour
déterminer la solubilité de l’iode dans les verres, en fonction de paramètres thermodynamiques et compositionnels.
La solubilité de l’iode dans les verres dépend de la pression, de la teneur en bore, mais
également de la teneur en cations non formateurs de réseau. Les verres borosilicatés
polymérisés comme le simulant de verre nucléaire ISG incorporent ~1 mol.% d’iode,
tandis que les verres dépolymérisés comme les “Low Activity Waste Glass” en incorporent
~2 mol.%.
L’iode s’incorpore dans les verres au voisinage des cations non formateurs de réseau. Ce
faisant, il modifie l’état de polymérisation du réseau. L’iode a un effet dépolymérisant sur
les verres polymérisés, et inversement pour les verres dépolymérisés. La solubilité de l’iode
est également fortement influencée par son état d’oxydation, l’espèce I5+ étant bien plus
soluble dans les verres que l’espèce I-.
Relaxation plays a critical role in all glasses, especially those that undergo heat treatments for industrial applications. However, the fundamental mechanisms of relaxation are still poorly understood, especially in industrially relevant glasses such as aluminosilicate systems. Additionally, there exists a gap between glass chemistry and theories of relaxation in glass physics, where the underlying descriptions of relaxation are not always tied to specific features of glass structure. Here, we present a comprehensive review of experimental and computational models used to study the relaxation behavior of high‐temperature oxide glasses, with an emphasis on aluminosilicate glass compositions relevant to the high‐tech glass industry. At the end of this review, we provide a perspective on bridging the gap between glass physics and chemistry through joint experimental and modeling approaches, as well as potential future experiments for measuring relaxation behavior below the glass transition temperature.
The sodium aluminosilicate (NAS) glass family is important for many different industrial applications, but glass relaxation has not yet been thoroughly studied in this system. Thermal analysis techniques such as differential scanning calorimetry (DSC) and modulated differential scanning calorimetry (MDSC) can provide insight into the enthalpy relaxation of glass by measuring the glass transition temperature (Tg), activation energy, and enthalpy of relaxation. MDSC is mostly used to study nonoxide and low Tg glasses, and there is much debate about whether the nonreversing heat flow analysis method is accurate. To the authors’ knowledge, this is the first paper using MDSC to study these NAS compositions, and one of few papers to report MDSC on high Tg oxide glasses. We report on one set of modulation conditions that obtain a linear response using Lissajous curves, as well as comparing the activation energy calculated from DSC with the enthalpy of relaxation obtained from MDSC. Our results show that the activation energy and enthalpy of relaxation do not give the same compositional minimum in relaxation, and therefore more work is needed to investigate the validity of the nonreversing heat flow approach for high Tg oxide glasses.
The structural and chemical modifications on the surface of pure and alkali-doped aluminosilicate glasses due to hydrolysis are investigated using ab initio molecular dynamics. The effects of water on the glass network are fully elucidated by analyzing the short- and intermediate-range structural orders embedded in the pair distribution function, bond length and angle distribution, coordination number, and interatomic bonding. A novel concept of total bond order is used to quantify and compare the strength of bonds in hydrated and unhydrated glasses. We show that aluminosilicate glass is hydrolyzed by water diffusion near the surface, and by proton (H+) transfers into the bulk, which increases with time. Hence, a dissolved glass-water interface becomes rich in Si-OH and Al-OH. The alkali ions associated with the non-bridging oxygen accelerate the hydrolysis by facilitating water and H+ diffusion. Al is more impacted by hydrolysis than Si, resulting in greater variation in the Al-O bond order than Si-O. Doping of NaCl and KCl enhances the ionization of water and the hydrolysis of aluminosilicates with increased salt concentration. The KCl doping ionizes more water molecules and causes more degradation of the glass network than NaCl. Co-doping of Na and K results in a mixed alkali effect due to complex interatomic bonding from different-sized ions. These exceptionally detailed findings in highly complex glasses with varying salt compositions provide new and unprecedented atomistic insights that can help to understand the hydrolysis and dissolution mechanism of aluminosilicates and other silicate glasses.
The transport properties of high-temperature silicate melts control magma flow and are crucial for a wide variety of industrial processes involving minerals. However, anomalous melt properties have been observed that...
Melt-derived metaluminous (Al/Na = 1) aluminosilicate glasses in the system SiO2-Al2O3-Na2O-P2O5 were prepared with P2O5 and SiO2 contents varying from 0 to 7.5 and 50 to 70 mol %, respectively. The glass structure was investigated by X-ray absorption near edge structure, far- and medium-infrared, and polarized Raman spectroscopic techniques. The results indicate the incorporation of phosphate into the aluminosilicate network not only as partially depolymerized groups but also as fully polymerized groups charge-balanced by aluminate units in Al-O-P bonds. A new analysis method based on polarized Raman spectra in the bending frequency range indicates a preference of phosphate to reorganize the smallest ring structures. Changes in the glass transition temperature with the increase in phosphate content were found to be consistent with the depolymerization of the network structure shown by spectroscopy. By contrast, increasing the silica content by substituting SiO4 for AlO4 tetrahedra, while keeping the phosphate content constant, was found to have a negligible effect on network polymerization. Still, the glass transition temperature decreased and correlated with a far-infrared sodium band shift to higher frequency. This was interpreted as local changes in bond strength caused by complex interactions between the different network formers and sodium ions.
The main transport properties governing materials transfer in the Earth are viscosity, diffusion, and electrical conductivity. These properties can be described in terms of the same melt compositional and structural variables. For example, magma viscosity decreases systematically as magma composition becomes more mafic (from rhyolitic to basaltic and komatiitic). Pressure, temperature, and redox conditions are the other relevant variables. In addition, effects of volatiles such as H2O, halogens, and CO2 on transport properties can be large, up to several orders of magnitude.
In contrast to network-forming chemical components, the transport of network-modifiers such as alkali metals and alkaline earths are decoupled from transport of network-formers except when charge-balancing trivalent cations (Al³⁺ and Fe³⁺) in tetrahedral coordination. Their transfer behavior also is important for our understanding of electrical conductivity because electrical charge transfer usually is accomplished via motion of network-modifying cations.
At low pressure, the activation energy of network-modifying transport agents differs by several orders of magnitude from that of network-formers. However, at pressure corresponding to depths exceeding 300 km in the Earth, oxygen coordination numbers in the structure of magmatic liquids increase, which cause bond energy decrease and the transport energy of network-formers and modifiers to resemble one another.
The solubility of nitrogen in magmatic melts is key for understanding the degassing of Earth’s interior and formation of Earth’s N-rich atmosphere. We performed high-pressure experiments to determine the solubility of nitrogen at N2-saturation in basaltic to granitic melts at pressures of 0.3–8 GPa, temperatures of 1200–1600 °C, and oxygen fugacities between the Fe–FeO and Ni–NiO oxygen buffers. The solubility and speciation of nitrogen in the quench silicate melts were quantified by Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), electron probe microanalyzer (EPMA), and Nano-scale secondary ion mass spectrometry (NanoSIMS). The results show that the predominant N-species in our silicate melts is N2, which dissolves physically by filling the interstitial sites of the silicate melt network. The measured N-solubilities in silicate melts (SN2melt) by using different approaches agree with each other, and they vary from 200 to 17000 ppm by weight. The SN2melt increases with increasing pressure and/or decreasing silicate melt NBO/T (the ratio of non-bridging oxygens to tetrahedrally coordinated cations), but it is temperature-independent. Using our newly obtained SN2melt and previous SN2melt and SArmelt data, we develop an empirical model that describes the physical dissolution of N2 and Ar in silicate melts as a function of gas partial pressure and silicate melt composition, and this model can be used to predict SN2melt and SArmelt in magmatic melts of Earth’s mantle and crust. The comparison of our SN2melt data with previous SArmelt and SCO2melt data shows that the solubility in granitic melts is in the order of SArmelt > SCO2melt > SN2melt, but in basaltic melts it is in the order of SCO2melt > SArmelt > SN2melt. The lower SN2melt than SArmelt at Earth’s mantle and crustal conditions suggests N2–Ar fractionation during degassing of mid-ocean ridge basalts and arc magmas. Therefore, the N2/Ar ratios in degassed magmas may not fully represent those in their mantle source regions. The solubility-controlled, preferential degassing of N2 relative to CO2 in Earth’s oxidized shallow magma ocean, in conjunction with collision-induced atmospheric loss, can explain the superchondritic C/N ratio in the bulk silicate Earth. However, the two orders of magnitude higher N2/³⁶Ar ratio in Earth’s mantle than in atmosphere could have resulted from preferential retention of reduced N-species in Earth’s reduced silicate magma ocean or preferential deep subduction of nitrogen.
High resolution O 1s X-ray Photoelectron Spectroscopy (XPS) was employed to determine oxygen speciation in 31 mol% K2O silicate glass containing 0.0, 1.0 and 3.0 mol% dissolved Al2O3. Highly resolved bridging oxygen (BO) and non-bridging oxygen (NBO) peaks were observed. The NBO peak intensity decreased and the BO increased with dissolution of Al2O3. The enhanced BO intensity is localized at two binding energies (BE), one characteristic of a Si-O-Si moiety (e.g., quartz, or v-SiO2), the other of a Si-O-Al moiety (e.g., kyanite, Al2SiO5). The dissolution reaction proceeds in two steps; NBO⁻ of a Q³ species attacks and bonds to an under-coordinated surface Al atom of crystalline Al2O3. The reaction produces a Si-O-Al surface species thus converting the attacking NBO to a BO and altering the reactant Q³ species to a Q⁴ product species:
[Q³]⁻melt + [AlO1.5]surface → [Q⁴]⁰melt + [AlO2]⁻melt
The second step involves reaction of [AlO2]⁻melt with K⁺converting it from a modifier cation to a charge compensator, resulting in the polymerization reaction:
[K-Q³]⁰melt + [AlO2]⁻melt → [Q⁴]⁰ + [KAlO2]⁰melt + 1/2O²⁻
The stoichiometric reaction is the sum of the two:
[Q³]⁻melt + [K-Q³]⁰melt + [AlO1.5]surface → 2[Q⁴]⁰ + [KAlO2]⁰melt + 1/2O²⁻
Al species detached from the solid surface initially must be bonded to some NBO. Within the melt, nucleophilic substitution attack of the Al-bearing species by Q³ species converts NBO to BO, resulting in [KAlO2]melt being bonded four BOs. This reaction may not go to completion in all melts, depending on dissolved Al content and melt composition.
The effect of MgO on the viscosity and structure of CaO-Al2O3-B2O3-based non-reactive mold flux was studied by rotational viscometer, molecular dynamics (MD) simulations, and Raman spectroscopy. The results show that with the increase of MgO content from 3 °C to 9 pct, the viscosity (the testing temperature is 1300 °C) of the sample decreases from 0.39 to 0.26 Pa seconds, the activation energy decreases from 158.7 to 119.7 kJ/mol, and the break temperature (Tbr) decreases from 1212 °C to 1157 °C. Triple-coordinated oxygen and highly coordinated Al appear in the mold flux to compensate for the excess negative charge of [AlO4]5- tetrahedron. With the increase of MgO content, the stability of the Al-O network structure is enhanced, but the degree of polymerization of melt and the complexity of network structure decrease. With the increase of MgO content, the amount of bridging oxygen in the system decreases, and the polymerization degree of the aluminate network in mold flux decreases. The results of Raman spectra are consistent with those of MD simulation. Therefore, MgO can simplify the melt structure and reduce the viscosity of mold flux.
TiO2 is an important oxide for property modifications in the conventional soda lime silicate glass family. It offers interesting optical and mechanical properties, for instance, by substituting heavy metals such as lead in consumer glasses. The compositional effects on the hardness, reduced elastic modulus and crack resistance as determined by indentation of chemically strengthened (CS) TiO2-doped soda lime silicate glass was studied in the current paper. The CS, which was performed by a K+ for Na+ ion exchange in a molten KNO3 salt bath at 450 °C for 15 h, yielded significant changes in the indentation mechanical properties. The hardness of the glass samples increased, and this was notably dependent on the SiO2, CaO and TiO2 content. The reduced elastic modulus was less affected by the CS but showed decrease for most samples. The crack resistance, an important property in many applications where glasses are subjected to contact damage, showed very different behaviors among the series. Only one of the series did significantly improve the crack resistance where low CaO content, high TiO2 content, high molar volume and increased elastic deformation favored an increased crack resistance.
In peralkaline and meta-aluminous melts, essentially all Al³⁺ (>95%) occupy tetrahedral coordination, whereas for peraluminous melts, complex mixtures of aluminum triclusters with 4-fold coordinated Al³⁺ and Al³⁺ in 5- and 6-fold coordination with oxygen describe the structure. Aluminum in tetrahedral coordination requires electrical charge-balance. With alkali metals (M⁺) in this role, the proportions are M⁺=Al³⁺. The overall structure is dominated by three-dimensionally interconnected tetrahedra to form 6-membered rings of tetrahedra. The Al/(Al+Si) of these tetrahedra are simple positive functions of the bulk melt Al/(Al+Si). When tetrahedrally-coordinated Al³⁺ is charge-balanced by divalent cations, the M²⁺ cation charge-balances 2Al³⁺ tetrahedrally coordinated cations. This structure is dominated by SiO4, (Si,Al)O4, and AlO4 entities.
In peraluminous melts, where there is insufficient proportion of M²⁺ and M²⁺ cations for charge-balance, aluminum exists in triclusters with Al³⁺ in tetrahedral coordination. In peralkaline aluminosilicate melts, there coexist discrete structural units with different degree of silicate polymerization. These units are termed Qⁿ-species where the superscript, n, is the number of bridging oxygen in individual units. Equilibria among these units are of the type, 2Qⁿ = Qⁿ⁺¹ + Qⁿ⁻¹. In these melts, Al³⁺ is distributed among these units. The Al³⁺ in peralkaline aluminosilicate melts strong preference Q⁴ units. This preference is, however, temperature-dependent as reflected in changes in the ΔH of the Qⁿ-species reaction.
Amorphous materials such as glasses and polyacrylonitrile (PAN) found their application in many modern industry products – lenses, touchscreens, high-strength aerospace structures to name a few. Still, structural investigations of amorphous materials are challenging. Solid state nuclear magnetic resonance (NMR) allows to characterize structural units on short and intermediate length scales and therefore was applied in this work for investigation of phosphorus-silicate glasses and PAN-based carbon fibers. In the first part of this work two glass systems (phosphorus-rich and silicon rich) and the changes in the glass structure under addition of aluminum/phosphorus were investigated with means of solid-state NMR including dipolar-based experiments and high-temperature in-situ NMR. Research on silicon-rich system with two base composition 60P2O5-30Na2O-10SiO2 and 50P2O5-16.6Na2O-33.3SiO2 showed that aluminum and silicon atoms are distributed in the phosphorus network and are connected through phosphorus chains. Six-coordinated aluminum substitutes six-coordinated silicon upon addition. Number of high-coordinated cations depends on P/(Al+Si) ratio and corresponds with trends in Tg and Young’s modulus. In sodium aluminosilicate system with 50 to 70 m. p. of silicon dioxide with small amount of phosphorus (up to 7.5%) the network consists of silicon and aluminum tetrahedra, and phosphorus is getting incorporated in form of Q2/3 phosphorus units. Charge-compensation is realized through Q4 phosphorus units and Al-Si triclusters. PAN- based fibers were investigated by hydrogen, nitrogen and carbon ex-situ NMR which allowed to support partial tautomerization and cyclization+dehydrogenation mechanisms. Role of methacrylate (MA) as copolymer was investigated by following signals from selectively enriched nitrogen and carbon atoms. The results of investigation showed that MA copolymers are not chemically inert and participate actively in the network formation. They fully react with nitrile group in PAN which leads to formation of at least four different moieties with at least one C-N connection.
Understanding the atomic structure of glasses is critical for developing new generations of materials with important technical applications. In particular, the local environment of rare-earth ions and their distribution and clustering is of great relevance for applications of rare earth-containing glasses in photonic devices. In this work, the structure of Gd2O3 doped lithium and potassium aluminosilicate glasses is investigated as a function of their network modifier oxide (NMO–Li2O, K2O) to aluminum oxide ratio using molecular dynamics simulations. The applied simulation procedure yields a set of configurations, the so-called inherent structures, of the liquid state slightly above the glass transition temperature. The generation of a large set of inherent structures allows a statistical sampling of the medium-range order of the Gd3+ ions with less computational effort compared to other simulation methods. The resulting medium-range atomic structures of network former and modifier ions are in good agreement with experimental results and simulations of similar glasses.
Molecular dynamics (MD) simulations have been used to study the effect of Na ions on the structure properties of CaO–Al2O3–Na2O slag. The short- and medium-range structures of CaO–Al2O3–Na2O in this study are consistent with existing data. Through the replacement of Ca2+ ions with Na+ ions in CaO–Al2O3–Na2O slag, the structure of the AlO4 tetrahedron is stabilized as the proportion of AlO4 tetrahedron in the melt increases and the average values of the O–Al–O bond angle are closer to those of an ideal tetrahedron. The changes in the melt structure show that Ca2+ ions mainly play a role in modifying the network, while Na+ ions mainly play a role in the charge compensation of the AlO4 tetrahedron; thus, as more Na+ ions replace Ca2+ ions added to the melt, the charge compensation ability in the melt is enhanced, and the network modification ability is weakened. Part of the weak non-bridge oxygen (NBO) structures in the form of Al–NBO–Ca are transformed to strong bridge oxygen (BO) structures in the form of Al–BO–Al, and the microstructure of the melt gradually becomes complicated, which provides a reasonable explanation for the mechanism for the increase of macroscopic viscosity in CaO–Al2O3–Na2O slag with high Al content.
The rotating cylinder method was used to understand the effects of La2O3 and the La2O3/Al2O3 ratios on the melt viscosity of CaO-SiO2(-Al2O3)-La2O3 at high temperatures. The structural characterization of these quenched glass samples were investigated using Raman spectroscopy. The results show that the viscosity of the CaO-SiO2(-Al2O3)-La2O3 melt is significantly reduced due to additions of La2O3. The La2O3 additions simplified the structure of the silicon-oxygen ion groups in the melt, which created a smaller melt viscosity. The critical transition temperature is proportional to the La2O3 content and inversely proportional to the La2O3/Al2O3 ratio. La³⁺ and Al³⁺ make the viscosity-temperature curve of CaO-SiO2-Al2O3-La2O3 melt show different characteristics. There is no critical transition temperature when the melt without La2O3, and the critical transition temperature appears after adding La2O3. The La2O3 plays a role like alkaline oxide in the CaO-SiO2-Al2O3-La2O3 melt. In addition, the degree of polymerization of the melt decreased relative to the La2O3 content and La2O3/Al2O3 ratio. Consequently, the La2O3 behaves as a network modifier for the complex silicate anions of CaO-SiO2-La2O3 and CaO-SiO2-Al2O3-La2O3 melts.
This chapter shows the fundamental importance of viscosity to describe the fluid character of the synthetic and natural glass‐forming melts relevant to industry and geosciences. The melt response to shear and normal stresses is in most cases defined by a constant ratio between stress and deformation rate. Only under extreme forming conditions and for heterogenic melts deviations from such a Newtonian viscosity are observed. On the atomistic scale, viscous flow is linked to the complexity of cooperative rearrangements of structural entities leading to a non‐Arrhenian temperature dependence above T g. In the glass transition range, viscosity becomes time‐dependent as the timescale of structural relaxation exceeds those of the deformation process. The increasing inequality leads to the isostructural viscosity of the unrelaxed melt state at temperatures far below T g. The various methods used to measure it over 14 orders of magnitude are described along with three‐parameter equations used in practice to predict the viscosity–temperature dependence with high accuracy. Owing to their technical importance, the effect of oxide components on viscosity is discussed especially in relation to changes in the connectivity of rearranging structural units, which are most prominently seen by the viscosity hump of the boron anomaly. Whereas viscous flow is generally governed by the fractions of major components, trace amounts of water have an exclusive role in accelerating the network dynamics. In addition, viscosity is strongly effected by microstructure, when inclusions are present in the melt. In the case of crystal‐bearing melts, a strong increase in viscosity with increasing crystal fraction becomes evident, while bubbles tend to decrease viscosity if sheared at high capillary numbers.
This is the first of a two-part molecular dynamics (MD) study that examines the effects of temperature, pressure, and composition on the structure and properties of ten compositions in the system NaAlSiO4-SiO2. Results were obtained for collections of at least 1300 atoms at temperatures between 2500 and 4500 K, pressures of 2-5 GPa, and simulation durations on the order of 0.1 ns. Durations and numbers of particles are both about three times larger than for previous MD simulations on molten aluminosilicates. This study addresses experimental matters, including aspects of simulation methodology that affect the accuracy of atom trajectories (hence computed properties). These include Nt (total number of atoms in the MD box) and spatial resolution achieved in the interparticle force calculation. Static melt structures and their systematic variation with temperature and composition are also explored. Results indicate that transport properties computed from time-correlation functions apparently become asymptotic for system sizes (Nt) greater than about 1000 particles. -from Authors
Viscometric data for seven compositions in the system NaAlSiO4-SiO2 have been collected by the technique of concentric cylinder rheometry in the temperature and shear rate ranges 1000-1300°C and 10-2-8/s, respectively, for single-phase melts or supercooled liquids. Measured viscosities vary from near 108 to 104 Pa.s for the range of compositions and temperatures investigated. Temperature-viscosity data are extremely well fitted by both Arrhenian (two-parameter) and Fulcher (three-parameter) expressions. The increase in Vogel temperature and decrease in activation energy for viscous flow and isothermal viscosity as NaAlSiO4 is added to molten silica may be rationalized microscopically on the basis of the increased Al-O bond length relative to Si-O, the reduction in bond energy for Al-O compared with Si-O, and the smaller mean TOT intertetrahedral bridging angle for Al-O-Si relative to Si-O-Si. -from Authors
Viscosities of fourteen melts close to the join SiO2-NaAlO2 were measured in the range 1-10(12) Pa.s (700-1650 degrees C) using a combination of concentric cylinder and micropenetration techniques. These compositions cover five isopleths in silica content from 50 to 82 mol% and vary from mildly peralkaline to mildly peraluminous. Greatly improved constraints on the temperature dependence of viscosity in the system SiO2-NaAlO2 result because exactly the same compositions were used for both high-and low-temperature measurements, viscosities over an extended range of silica contents were measured at temperatures close the glass transition, and several compositions at constant silica content and variable alkali/Al ratio were measured, allowing interpolation of data to compositions exactly along the join SiO2-NaAlO2. At high temperature (1600 degrees C) viscosity and activation energy are shown to be approximately a linear function of silica content, but large nonlinearities occur at temperatures close to the glass transition range. Defining fragility as the gradient of the viscosity curve at the glass transition temperature (T-g taken to be the 10(12) Pa.s isokom) on a reduced temperature scale (T-g/T), it is found that the fragility increases in a nonlinear fashion as NaAlO2 is substituted for SiO,, with fragility increasing more rapidly at lower SiO, contents. The viscosity data are combined with heat capacity data available in the literature to estimate configurational entropies of albite, jadeite, and nepheline glasses using the Adam-Gibbs theory. Fragility, when defined in terms of the Adam-Gibbs parameters, is shown to increase with configurational heat capacity (difference in heat capacity between the liquid and the glassy states) but to decrease with increasing configurational entropy at the glass transition. In the light of independent phase equilibria and spectroscopic and calorimetric evidence that suggests the Al-Si ordering increases as silica content decreases from SiO2 to nepheline, the modeling of configurational entropy in terms of Al-Si mixing suggests the following: (1) Melt configurational entropy has contributions from both cation mixing (chemical contribution), as well as variations in the topology of the O network (topological contribution), of which the latter dominates. (2) The chemical contribution is due to mixing of tetrahedral rather than O sites. (3) At the glass transition (10(12) Pas isokom) the topological contribution shows little, if any, variation.
The effect of the addition of 5, 10, and 20 wt% of the alkali oxides on the viscosity of a haplogranitic melt composition has been investigated at I atm and in the temperature range of 400-1650 degrees C. The high-temperature viscosity data were obtained with concentric cylinder viscometry and the low-temperature viscosity data using micropenetration viscometry. The combined data sets for low- and high-temperature viscosities have been fitted for each composition using the Tamann-Vogel-Fulcher (TVF) equation. The effect of alkali oxide on the viscosity of a haplogranite melt is extreme. The viscosity decreases with added alkali oxide content in a nonlinear fashion. The first few mole percent of alkali oxide added decreases viscosity several orders of magnitude, whereas subsequent addition of alkali oxide has a much smaller effect. The effects of each of the alkalis are broadly similar, implying that the structural role of the alkalis is common to all. In detail however, the viscosity of the strongly peralkaline melts investigated here increases with the size of the added alkali cation in the order Li < Na < K,Rb,Cs. This trend probably reflects a minor influence of the alkal-O bond strengths on the melt viscosity. This distinction of a dominant depolymerizing influence and a minor alkali specific bond-strength influence has important implications for the comparison of these data with those for the addition of other depolymerizing agents on the viscosity of haplogranitic melt (e.g., H2O, F2O-1).
A creep apparatus has been built to measure, with inaccuracies of less than 0.04 log poise, viscosities of supercooled silicate melts in the range 109–1014 poises. Measurements on seven pyroxene and five garnet supercooled liquid compositions along the joins MgSiO3-CaSiO3 and Mg3Al2Si3O12Ca3 Al2Si3O12 made between 1000 and 1150 K show deep minima in the viscosity-composition relationship for both joins. These minima reduce when the temperature increases and disappear eventually. Within the framework of the configurational entropy theory of relaxation processes, these observations can be accounted for quantitatively in terms of the contribution of ideal (Ca, Mg) mixing to the total configurational entropy of the melts. The configurational entropies determined from the viscosity measurements agree with the values determined by calorimetry for liquid CaSiO3, CaMgSi2O6, MgSiO3, and Mg3Al2Si3O12. The heat capacities of Ca3Al2Si3O12 glass and liquid have also been obtained from dropcalorimetry measurements.
The experimental work and published data were used to calculate the anionic constitution of common igneous melts. Apparently all magmas have an (NBO/T) < 1. Basalts have (NBO/T) = 0.9 to 0.6. Andesites are about 0.3 and granite melts are between 0.2 and 0.05. The anionic structure is actually a mixture of 2, 1, and 0. -K.A.R.
The viscosity-temperature relationships of five melts on the join Na
2Si 2O 2-Na 4Al
2O 5 (5, 10, 20, 30 and 40 mole percent Na
4Al 2O 5) have been measured in air, at
1 atm and 1000-1350°C with a concentric cylinder viscometer. All the
melts on this join of constant bulk polymerization behave as Newtonian
fluids, in the range of shear rates investigated, and the melts exhibit
Arrhenian viscosity-temperature relationships. Isothermal viscosities on
this join initially decrease and then increase with increasing mole
percent Na 4Al 2O 5. The minimum
viscosity occurs near 20 mole percent Na 4Al 2O
5 at 1000°C and moves to higher Na 4Al
2O 5 content with increasing temperature. The
observation of a viscosity minimum along the join Na 2Si
2-O 5-Na 4Al 2O 5
is not predicted based on earlier viscosity data for the system Na
2O-Al 2O 3-SiO 2 (RlEBLlNG,
1966) or based on calculation methods derived from this and other data
(Bottinga and Weill, 1972). This unexpected behavior in melt
viscosity-temperature relations emphasizes the need for a more complete
data set in simple silicate systems. Previous spectroscopic
investigation of melts on the join Na 22Si 2O
5-Na 4Al 2O 5 offer a
structural explanation for the observed viscosity data in terms of a
disproportionation reaction involving polyanionic units.
Macroscopically, the viscosity data may be qualitatively reconciled with
the configurational entropy model for viscous flow ( RICHET, 1984).
The results of high-resolution 23Na, 27Al and 29Si NMR spectroscopy of aluminosilicate glasses with MO/Al2O3 or ) shows that these glasses have a fully polymerized tetrahedral framework structure with only a defect level of non-bridging oxygens. The chemical shifts of the peak maxima for all three nuclides become less shielded (less negative or more positive) with decreasing Si/ (Si + Al). For 29Si and 27Al, this variation parallels the variation of framework crystalline silicates. The 23Na chemicAl shifts for the glasses becomes less shielded (less negative) with decreasing Na/(Na + K), opposite to the trend for crystalline alkali feldspars. Few data exist for the 23Na chemical shifts of crystalline samples, and the structural causes of variation in 23Na chemical shifts are not well understood. The 27Al and 29Si chemical shifts of the glasses do not vary significantly with different large (modifier) cations. The 29Si chemical shifts provide estimates of average bond angles and bond distances and the 27AL and 29Si chemical shifts and peak breadths are consistent with a decrease in the tetrahedralring order (number of tetrahedra per ring) with decreasing Si/(Si + Al). The data presented here for fully polymerized glasses form a base from which the data for aluminosilicate glasses containing both fully polymerized sites and less polymerized sites can be interpreted.
The Raman spectrum of sodium tetrasilicate (Na2Si4O9) glass has been obtained as a function of pressure at 298 K to approximately 50 GPa. The scattering intensity of bands associated with silica tetrahedra containing nonbridging oxygens (Q3 species) decreases with increasing pressure and these bands are absent above 20 GPa. The spectral changes observed over this pressure interval are consistent with the formation of high-coordinate Si species at the expense of nonbridging oxygens. Above 33 GPa, the mode of change of the Raman spectrum with pressure changes abruptly. The main Raman band markedly broadens and weakens, accompanied by a twofold increase in the pressure derivative of the peak maximum. These spectral changes indicate the onset of a second densification mechanism operative at higher pressures. We conjecture that above 33 GPa, high coordinate silicon species are formed through the involvement of bridging oxygens (i.e., formation of IIIO species). The changes in the Raman spectrum associated with this latter mechanism are found to be reversible on decompression. However, spectral changes observed at lower pressures (
A molecular-kinetic theory, which explains the temperature dependence of relaxation behavior in glass-forming liquids in terms of the temperature variation of the size of the cooperatively rearranging region, is presented. The size of this cooperatively rearranging region is shown to be determined by configuration restrictions in these glass-forming liquids and is expressed in terms of their configurational entropy. The result of the theory is a relation practically coinciding with the empirical WLF equation. Application of the theory to viscosimetric experiments permits evaluation of the ratio of the kinetic glass temperature Tg (derived from usual ``quasistatic'' experiments) to the equilibrium second-order transition temperature T2 (indicated by either statistical-mechanical theory or extrapolations of experimental data) as well as the hindrance-free energy per molecule. These parameters have been evaluated for fifteen substances, the experimental data for which were available. Hindrance-free energies were found to be of the magnitude to be expected from consideration of molecular interaction energies. The values of Tg/T2 thus obtained for these fifteen widely differing materials were found to be nearly the same, i.e., 1.30±8.4%. Values for Tg/T2 of nearly the same magnitude were derived by Bestul and Chang from calorimetric data.
Processes involving melting have been of fundamental importance in the origin and evolution of the Earth and other terrestrial planets, but thermodynamic data and physical properties of liquids necessary to calculate phase relationships are not well known, particularly at high pressures. We present revised fusion curves of the silicate diopside and the aluminosilicate albite to 30 kbars. The fusion curve of diopside is in agreement with that calculated with the Clapeyron equation. Conversely, we were able to calculate the fusion curve of albite only by considering the effects of pressure and temperature in lowering the activity of NaAlSi3O8 component in the liquid, which presumably results from partial dissociation of this component into other species. Our experimental data corroborate that thermodynamic and, presumably, structural changes occur in aluminosilicate liquids with changes in pressure as well as with temperature, and they provide evidence that the metastable persistence of high-enthalpy structures in aluminosilicate glasses quenched from temperatures above the solidus produces glasses that are unreliable models of liquids in stable equilibrium with crystalline solids. These experiments also reveal for the first time that metastable aluminosilicate glasses prepared at very high temperatures can persist when reheated above the liquidus temperature and fail to convert instantly to the structure of the stable liquid. These observations provide a basis for extracting thermodynamic data for liquids presently unobtainable by other methods.-Authors
The viscosities of melts NaAlSi2O6 (jadeite) and Na2Si3O7 compositions have been determined at pressures between 5 and 24 kbar, using the falling sphere method and graphite capsules 10 mm long in solid media piston-cylinder apparatus. The viscosity of NaAlSi2O6 melt decreases by a factor of less than 3 from 1 atm to 20 kbar at 1175°C. These results indicate that the remarkable decrease in viscosity of NaAlSi2O6 melt with increasing pressure is largely due to the presence of Al in the melt. It is most probable that a part of Al in the melt changes from four- to six-fold coordination at high pressures. The density of glass of NaAlSi2O6 composition increases with increasing pressure of quenching from 2.42 g/c3 at 5 kbar to 2.58 at 21 kbar, indicating that the melt also changes to a denser structuer with the coordination change of Al at high pressures. It is suggested that most magmas undergo similar structural changes in the upper mantle and have higher density and lower viscosity at greater depths.
The shear viscosities of forty melts in the system Na2OAl2O3SiO2P2O5 have been determined in the temperature range 1652−1052°C using the concentric cylinder method. Six P-free compositions containing ∼67 mol% SiO2 varying in molar from 0.70 (peralkaline) to 0.44 (peraluminous) were studied, to each of which successive additions of up to 7 mol% (13 wt%) P2O5 were made. At a fixed temperature, viscosities in the P-free system show a maximum, not at the ‘charge-balanced’ metaluminous composition ( ), but at . Addition of P to peralkaline melts results in an increase in viscosity. With progressive additions of P to mildly peralkaline melts ( ), there is a maximum in melt viscosity that occurs at lower P content as the peralkalinity of the melt decreases. In contrast, the addition of P to the metaluminous and peraluminous melts causes a decrease in melt viscosity. The magnitude of this decrease is identical for the metaluminous, and mildly peraluminous ( ) compositions, but smaller for the most peraluminous melt ( ).
The Raman spectrum of calcium aluminate (CaAl204) glass has been obtained as a function of pressure at ambient temperature to ∼ 15 GPa, and as a function of temperature at room pressure to 1650 K. On increasing pressure at 300 K, the Raman spectrum changes drastically between 8 and 10 GPa. The intensity of the overall spectrum weakens, the intensity of the 580-cm−1 band decreases markedly, and a new broad band centered near 750 cm−1 begins to dominate the spectrum. We interpret these spectral changes as due to the formation of highly coordinated A1 species through the involvement of bridging oxygens (i.e. formation of [3]O species). Upon decompression, the pressure-induced spectral changes are mostly reversible. However, the decompressed samples do not relax completely to their normal ambient state, and the spectra reflect important changes in the inter-tetrahedral AlOA1 linkages. These changes are consistent with some high-coordinate species being retained in the decompressed sample. On increasing temperature through the glass transformation range (Tg = 1178 K) at room pressure, our Raman spectra show evidence for important configurational changes, after vibrational effects are removed. The major spectral change consists of growth of a broad band between 600 and 1100 cm−1 appearing immediately above Tg. We suggest that these configurational changes concern the removal of O2− ions from the fully polymerized aluminate framework, with the resulting formation at high temperature of [3]O species (“triclusters”), coordinated by three tetrahedral aluminate groups.
Viscosity and density data were obtained for several series of alkali aluminosilicate melts between 1200° and 1700°C with a counterbalanced sphere viscometer. Property comparisons are presented as functions of the SiO2 content, Na2O content, and constant Al/Na ratios at 1500° and 1700°C.
The viscosity and expansion information support the formation of three-dimensional cristobalitelike liquids on the Al/Na=1.0 line with some of the aluminum (III) in the form of AlO6 octahedra for Al/Na>1.0 melts.
The molar volume models that coincide with experiment for Al/Na≤1.0 melts involve AlO4 tetrahedra and v̄Na2O values that increase with increase of the Al2O3 content. The model that best agrees with experiment for melts with Al/Na>1.0 involves only some of the excess aluminum ions in octahedral coordination as well as a changing v̄Na2O. Melt molar volume dependencies on composition can resemble those observed for similar glasses, which suggests structural similarities. The present liquid structure picture differs somewhat from previous concepts of alumina-rich glass structures in this ternary system.
KNOWLEDGE of the structure of liquid silicates is essential to understanding the properties of materials ranging from magmas in lava flows to melts in glass processing. At 1 atmosphere pressure, a wide range of evidence indicates that most silicon cations in these systems are coordinated by four oxygens in a tetrahedral configuration (SiIV). Molecular dynamics computer simulations of these liquids have, however, predicted that defect complexes (of relatively low abundance) consisting of silicon with five oxygen neighbours (SiV) are of key importance in the mechanism by which viscous flow takes place1–5. I present here direct experimental evidence from 29Si NMR studies of K2Si4O9 glass that SiV does exist in silicate liquids at low pressures, and that the abundance of this species increases with temperature, supporting the idea that SiV defects contribute to 'weakening' of the structure of molten silicates.
Using a recently developed NMR technique, which involves aerodynamically levitating the sample and heating it with a CO2 laser in the bore of a conventional NMR spectrometer, we have obtained Al-27 NMR spectra of liquids along the silica-alumina binary join at temperatures above 2000-degrees-C. All of the NMR spectra obtained contain a single Lorentzian shaped line with FWHM on the order of 10(2)-10(3) Hz. Isotropic chemical shifts become slightly more positive (less shielded) with increasing Al2O3 from 56.3 ppm for the 10 mol % Al2O3 liquid to 59.5 ppm for pure Al2O3 liquid. Molecular dynamics simulations of SiO2-Al2O3 liquids indicate an increase in the average coordination number of oxygen about aluminum with increasing Al2O3, with significant proportions of AlO5 groups in the alumina-rich liquids. The slight variation in chemical shift is most probably due to opposing effects of average Al coordination and Si/(Si + Al) ratio (next-nearest-neighbor environment). For the high alumina containing liqUids (mol % Al2O3 > 50), Al-27 NMR lines are fully narrowed (FWHM 100-200 Hz). Below 50 mol % Al2O3, lines broaden with increasing SiO2 content, to near 700 Hz for the 20 mol % Al2O3 liquid. This observation is interpreted as due to decreasing spin-lattice relaxation times as the silica content is increased. Correlation times (tau(c)) for spin-lattice relaxation calculated from the observed NMR linewidths are in good agreement with shear relaxation times (tau(v)) determined from viscosity measurements of these liquids especially at high alumina content.
New experimental data at 750–1000°C, 2–5 kbar, and for peraluminous melt compositions show a dramatic increase of apatite solubilities and phosphorus melt contents when compared to metaluminous or peralkaline compositions (Harrison and Watson, 1984). The formation of alumino-phosphate units in peraluminous melts qualitatively explains our data. A new apatite solubility model, incorporating the data for the peraluminous region, is presented. Application of this model to examples of peraluminous felsic suites is discussed.
Hydrothermal experiments in the temperature range 750–1020°C have defined the saturation behavior of zircon in crustal anatectic melts as a function of both temperature and composition. The results provide a model of zircon solubility given by: In DZrzircon/melt= −3.80−[0.85(M−1)]+12900/T where DZrzircon/melt is the concentration ratio of Zr in the stoichiometric zircon to that in the melt, T is the absolute temperature, and M is the cation ratio (Na + K + 2Ca)/(Al · Si). This solubility model is based principally upon experiments at 860°, 930°, and 1020°C, but has also been confirmed at temperatures up to 1500°C for M = 1.3. The lowest temperature experiments (750° and 800°C) yielded relatively imprecise, low solubilities, but the measured values (with assigned errors) are nevertheless in agreement with the predictions of the model.For M = 1.3 (a normal peraluminous granite), these results predict zircon solubilities ranging from ∼ 100 ppm dissolved Zr at 750°C to 1330 ppm at 1020°C. Thus, in view of the substantial range of bulk Zr concentrations observed in crustal granitoids (∼ 50–350 ppm), it is clear that anatectic magmas can show contrasting behavior toward zircon in the source rock. Those melts containing insufficient Zr for saturation in zircon during melting can have achieved that condition only by consuming all zircon in the source. On the other hand, melts with higher Zr contents (appropriate to saturation in zircon) must be regarded as incapable of dissolving additional zircon, whether it be located in the residual rocks or as crystals entrained in the departing melt fraction. This latter possibility is particularly interesting, inasmuch as the inability of a melt to consume zircon means that critical geochemical “indicators” contained in the undissolved zircon (e.g. heavy rare earths, Hf, U, Th, and radiogenic Pb) can equilibrate with the contacting melt only by solid-state diffusion, which may be slow relative to the time scale of the melting event.
With the configurational entropy theory of relaxation processes of Adam and Gibbs (1965), one predicts that the viscosity depends on temperature according to log , where Sconf is the configurational entropy of the liquid. Thermochemical calculations of Sconf performed for some mineral compositions show the importance of non-configurational contributions to the entropy differences between amorphous and crystalline phases. Except for the case of SiO2, the available thermodynamic data indicate that the above equation for viscosity accounts quantitatively for the experimentally determined temperature dependence of the viscosity of silicate melts. The Adam and Gibbs theory also provides a simple rationale for the non linear variation of the logarithmic viscosity with composition in mixed alkali silicate liquids at low temperatures, the minimum of viscosity resulting from the contribution of the entropy of mixing to Sconf.
Results of a series of molecular dynamics simulations indicate that the Al:Si ratio (and to a lesser extent the type of charge-balancing countercation) present in aluminosilicate melts induces a “chemical pressure” that significantly influences the temperature dependence of the melt viscosity. The temperature dependence of the T-O bond length and T-O-T angle in the melt has been calculated for albite, anorthite, nepheline, and MgAl2Si2O8 (a magnesium analog of anorthite) at their experimental melt densities at atmospheric pressure and for pure silica at two different densities corresponding to . These simulations show that melt fragility can be correlated to increasing T-O length and decreasing T-O-T angle, both of which are characteristics of silicate melts under increasing pressure. Framework cations with coordination numbers greater than four are observed in these aluminosilicate melts and are shown to be related to fragile behavior. Finally, for nepheline and anorthite we infer a large contribution to fragility from configurational disordering of Al-O-Si links which are known to be dominant in the glasses but are found to be absent from the simulated melts.
Solar furnace melting and fast-quench techniques have been used to prepare calcium aluminate glasses from 75 mol% CaO to 82 mol% Al2O3, which have been studied by Raman spectroscopy. The CaAl2O4 glass spectrum may be interpreted in terms of a fully-polymerized network of tetrahedral aluminate units, which is depolymerized on addition of CaO component analogous to binary silicate systems. The spectra of glasses with higher alumina content than CaAl2O4 may not be simply interpreted and a structural model is proposed which would be consistent with the glass spectra and with observed crystal structures along the CaAl2O4Al2O3 join. This model suggests formation of highly condensed aluminate tetrahedral on initial addition of alumina, with the appearance of aluminate polyhedra of higher average coordination at higher alumina content. Similar high coordination polyhedral are also suggested for a limited composition range along the CaOCaAl2O4 join. These interpretations are compared with the results of a previous study in the SiO2Al2O3 glass system.
The solubility of rutile has been determined in a series of compositions in the K2O-Al2O3-SiO2 system ( + Al2O3) = 0.38–0.90), and the CaO-Al2O3-SiO2 system (). Isothermal results in the KAS system at 1325°C, 1400°C, and 1475°C show rutile solubility to be a strong function of the K∗ ratio. For example, at 1475°C the amount of TiO2 required for rutile saturation varies from 9.5 wt% (K∗ = 0.38) to 11.5 wt% (K∗ = 0.48) to 41.2 wt% (K∗ = 0.90). In the CAS system at 1475°C, rutile solubility is not a strong function of C∗. The amount of TiO2 required for saturation varies from 14 wt% (C∗ = 0.48) to 16.2 wt% (C∗ = 0.59).The solubility changes in KAS melts are interpreted to be due to the formation of strong complexes between Ti and K+ in excess of that needed to charge balance Al3+. The suggested stoichiometry of this complex is K2Ti2O5 or K2Ti3O7. In CAS melts, the data suggest that Ca2+ in excess of A13+ is not as effective at complexing with Ti as is K+. The greater solubility of rutile in CAS melts when C∗ is less than 0.54 compared to KAS melts of equal K∗ ratio results primarily from competition between Ti and Al for complexing cations (Ca vs. K).TiKβ x-ray emission spectra of KAS glasses (K∗ = 0.43–0.60) with 7 mole% added TiO2, rutile, and Ba2TiO4, demonstrate that the average Ti-O bond length in these glasses is equal to that of rutile rather than Ba2TiO4, implying that Ti in these compositions is 6-fold rather than 4-fold coordinated. Re-examination of published spectroscopic data in light of these results and the solubility data, suggests that the 6-fold coordination polyhedron of Ti is highly distorted, with at least one Ti-O bond grossly undersatisfied in terms of Pauling's rules.
29Si MAS NMR investigation of the Na2OAl2O3SiO2 glasses
- Maekawa
Physikalische Eigenschaften und strucktureller Feinbau von Natrium-Aluminosilicatgläsern und -schmelzen
- Hunold
Aluminum in glasses and melts
- Lacy
Oxygen self diffusion study of polymerised silicate melts at high pressures
- Poe
A structural investigation of high alumina content glasses in the CaOAl2O3SiO2 system via Raman and MAS NMR
- Sato
SnO2 solubility: Experimental results in peraluminous and peralkaline high silica glasses
- Naski
Experimental confirmation of five-coordinated silicon in a silicate glass at 1 atmosphere pressure
- Stebbins