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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 K 2 O-Na 2 O-Al 2 O 3 -SiO 2 system, including configurational entropy, viscosity, optical refractive index, density, and Raman signals. Viscosity can be predicted in the 100–1015 log 10 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 log 10 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 K 2 O-Na 2 O-Al 2 O 3 -SiO 2 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, T g , can be predicted to within 19 K, while the melt configurational entropy at the glass transition, Sconf(T g) , 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. [ABSTRACT FROM AUTHOR]

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

• Experimental stick-slip on SiO 2 glass surfaces causes localised frictional melting. • Systematic changes in molecular structure of glass recorded following slip. • Structural changes caused by high pressures and fast cooling rates on asperities. • Thermomechanical history alters future strength and behaviour of interface. New insights into the pressure and temperature conditions on fault surfaces during seismic slip are provided by Raman-active vibrational modes of SiO 2 glass. We performed triaxial stick-slip experiments at room temperature and high normal stresses on pre-ground, high-purity silica glass surfaces. During slip, velocities exceed 0.32 m s−1 over durations of less than one millisecond, generating frictional heat and locally melting the fault surfaces. Temperature increases permit structural rearrangement within the melt; these changes are preserved by rapid quenching. Using Raman spectroscopy, we analyse melt-welded regions and show that these areas exhibit systematic changes in the spectra of silica. Changes result from a decrease in the inter-tetrahedral Si-O-Si bond angle and are correlated to increasing silica glass density in the slip regions. Densification results from both rapid cooling rates and exposure to very high pressures at asperity contacts. We use data from other experiments to calibrate these effects, estimating quench temperatures up to 1800 K and pressures of ∼180 MPa. These results provide the first quantitative evidence for the effects of quench rates and high inter-asperity pressures on the physics of melting and quenching during seismic slip and its impact on fault behaviour. [ABSTRACT FROM AUTHOR]

In this study, the effect of Ti4+ on the structure of nepheline glass (NaAlSiO 4) is investigated as SiO 2 is systematically replaced with TiO 2. Traditionally, TiO 2 is considered to be a nucleating agent for silicate crystallization but can also be incorporated into the glass network in relatively large amounts as either a network former or modifier depending on its coordination with oxygen. To determine the effect of Ti4+ on the structure of nepheline glass, X-ray and neutron pair distribution function (PDF) analysis paired with Empirical Potential Structure Refinement (EPSR) were conducted and are supplemented with Raman spectroscopy, electron probe microanalysis, and X-ray absorption spectroscopy (including Extended X-ray Absorption Fine Structure, EXAFS). Through these methods, it has been found that up to 15 mol% (16 wt%) TiO 2 can incorporate into the glass network as a four-fold coordinated species, with a minor contribution of higher coordinated Ti. Between NaAlTi 0.1 Si 0.9 O 4 and NaAlTi 0.2 Si 0.8 O 4 , EXAFS suggests a local structure change in the second coordination sphere of Ti, which changes from Ti-Ti to Ti-Al. Raman spectroscopy also suggests that as Ti content increases, the Na environment becomes more ordered. These results suggest that the Ti activity coefficient and its isotopic fractionation for magnetite and other Ti-bearing minerals should be fairly constant in polymerized melts, such as metaluminous and peraluminous rhyolites. [ABSTRACT FROM AUTHOR]

The speciation of water dissolved into and reacted with hydrous alumino-silicate glasses (of NaAlSi 3 O 8 and "rhyolitic" compositions) quenched from high temperature is re-investigated where the predominant species are expected to be "X(Si and Al)-OH" and "H 2 O". Only, two analytical methods are capable of assessing such speciation: Near InfraRed (NIR) spectroscopy and solid-state 1H Nuclear Magnetic Resonance (NMR) spectroscopy. It is observed that the apparent water speciation, as a function of total water content, as determined by NIR spectroscopy is nearly the opposite from what the 1H NMR data reveal. Deuterium (2H) NMR and silicon (29Si) NMR report consistent trends in apparent speciation (depolymerization) with those indicated by the 1H NMR data. Compared with four previous NMR studies of hydrous NaAlSi 3 O 8 glasses it is shown that whereas NIR data always report the same apparent systematic variation in the intensity of the 4500 ("X-OH") and 5200 ("H 2 O") cm−1 bands with total water content, multiple 1H NMR studies of hydrous NaAlSi 3 O 8 report a wide range in OH/H 2 O. The discrepancy between the various NMR studies likely reflects differences in how the various glasses were made. Specifically, quench rate (fast or slow) and synthesis pressure (higher or lower), might impose a strong effect on observed water speciation in glasses via 1H NMR. It is concluded that the application of NIR spectroscopy, specifically the use of the intensities of the 4500 ("X-OH") and 5200 ("H 2 O") cm−1 NIR bands, does not provide an accurate assessment of water speciation in hydrous alumino-silicate glasses. NIR spectroscopy does remain a very valuable analytical tool for determination of total water content. [ABSTRACT FROM AUTHOR]

To better understand the solution of volatile species in a reduced magma ocean, we identify via Raman spectroscopy the nature of C-O-H-N volatile species dissolved in a series of reduced basaltic glasses. The oxygen fugacity (ƒ O2) during synthesis varied from highly reduced at two log units below the iron-wustite buffer (IW-2.1) to moderately reduced (IW-0.4), spanning much of the magmatic ƒ O2 conditions during late stages of terrestrial accretion. Raman vibrational modes for H 2 , NH 2 –, NH 3 , CH 4 , CO, CN–, N 2 , and OH– species are inferred from band assignments in all reduced glasses. The integrated area of Raman bands assigned to N 2 , CH 4 , NH 3 and H 2 vibrations in glasses increases with increasing molar volume of the melt, whereas that of CO decreases. Additionally, with increasing ƒ O2 , CO band areas increase while those of N 2 decrease, suggesting that the solubility of these neutral molecules is not solely determined by the melt molar volume under reduced conditions. Coexisting with these neutral molecules, other species as CN–, NH 2 – and OH– are chemically bonded within the silicate network. The observations indicate that, under reduced conditions, (1) H 2 , NH 2 –, NH 3 , CH 4 , CO, CN–, N 2 , and OH– species coexist in silicate glasses representative of silicate liquids in a magma ocean (2) their relative abundances dissolved in a magma ocean depend on melt composition, ƒ O2 and the availability of H and, (3) metal-silicate partitioning or degassing reactions of those magmatic volatile species must involve changes in melt and vapor speciation, which in turn may influence isotopic fractionation. [ABSTRACT FROM AUTHOR]