Machine learning modeling of the atomic structure and physical properties of alkali and alkaline-earth aluminosilicate glasses and melts

The first version of the machine learning greybox model i-Melt was trained to predict latent and observed properties of K$_2$O-Na$_2$O-Al$_2$O$_3$-SiO$_2$ melts and glasses. Here, we extend the model compositional range, which now allows accurate predictions of properties for glass-forming melts in the CaO-MgO-K$_2$O-Na$_2$O-Al$_2$O$_3$-SiO$_2$ system, including melt viscosity (accuracy equal or better than 0.4 log$_{10}$ Pa$\cdot$s in the 10$^{-1}$-10$^{15}$ log$_{10}$ Pa$\cdot$s range), configurational entropy at glass transition ($\leq$ 1 J mol$^{-1}$ K$^{-1}$), liquidus ($\leq$ 60 K) and glass transition ($\leq$ 16 K) temperatures, heat capacity ($\leq$ 3 \%) as well as glass density ($\leq$ 0.02 g cm$^{-3}$), optical refractive index ($\leq$ 0.006), Abbe number ($\leq$ 4), elastic modulus ($\leq$ 6 GPa), coefficient of thermal expansion ($\leq$ 1.1 10$^{-6}$ K$^{-1}$) and Raman spectra ($\leq$ 25 \%). Uncertainties on predictions also are now provided. The model offers new possibilities to explore how melt/glass properties change with composition and atomic structure.Comment: Accepted version. Changes were made following reviews. New figures (4, 5, 6) were added. Text in section 3.1 has been modified accordingly. Figures 7 and 8 were improved. Small modifications of the text were made to correct typos, to add references or to clarify the text. Two lines were added in Table 1 to document errors on glass transition temperature and entrop

Atomic structure and physical properties of peridotite glasses at 1 bar

Earth’s mantle, whose bulk composition is broadly peridotitic, likely experienced periods of extensive melting in its early history that formed magma oceans and led to its differentiation and formation of an atmosphere. However, the physical behaviour of magma oceans is poorly understood, as the high liquidus temperatures and rapid quench rates required to preserve peridotite liquids as glasses have so far limited their investigation. In order to better characterize the atomic structure and estimate the physical properties of such glasses, we examined the Raman spectra of quenched peridotite melts, equilibrated at 1900 °C ± 50 °C at ambient pressure under different oxygen fugacities (fO2), from 1.9 log units below to 6.0 log units above the Iron-Wüstite buffer. Fitting the spectra with Gaussian components assigned to different molecular entities (Q-species) permits extraction of the mean state of polymerisation of the glass. We find that the proportions of Q1 (0.36–0.32), Q2 (0.50–0.43), and Q3 (0.16–0.23) vary with Fe3+/FeTOT (FeTOT = Fe2+ + Fe3+), where increasing Fe3+/FeTOT produces an increase in Q3 at the expense of Q2 at near-constant Q1. To account for the offset between Raman-derived NBO/T (2.06–2.27) with those determined by assuming Fe2+ exists entirely as a network modifier and Fe3+ a network former (2.10–2.44), ∼2/3 of the ferric iron and ∼90% of the ferrous iron in peridotite glasses must behave as network modifiers. We employ a deep neural network model, trained to predict alkali and alkaline-earth aluminosilicate melts properties, to observe how small variations in the atomic structure of peridotite-like melts affect their viscosity. For Fe-free peridotite-like melts, the model yields a viscosity of ∼ −1.75 log Pa s at 2000 °C, similar to experimental determinations for iron-bearing peridotite melts. The model predicts that changes in the peridotite melt atomic structure with Fe3+/FeTOT yield variations in melt viscosity lower than 0.1 log Pa s, barely affecting the Rayleigh number. Therefore, at the high temperatures typical of magma oceans, at least at 1 bar, small changes in melt structure from variations in oxidation state are unlikely to affect magma ocean fluid dynamics

Water and magmas: insights about the water solution mechanisms in alkali silicate melts from infrared, Raman, and 29 Si solid-state NMR spectroscopies

Degassing of water during the ascent of hydrous magma in a volcanic edifice produces dramatic changes in the magma density and viscosity. This can profoundly affect the dynamics of volcanic eruptions. The water exsolution history, in turn, is driven by the water solubility and solution mechanisms in the silicate melt. Previous studies pointed to dissolved water in silicate glasses and melts existing as molecules (H2Omol species) and hydroxyl groups, OH. These latter OH groups commonly are considered bonded to Si4+ but may form other bonds, such as with alkali or alkaline-earth cations, for instance. Those forms of bonding influence the structure of hydrous melts in different ways and, therefore, their properties. As a result, exsolution of water from magmas may have different eruptive consequences depending on the initial bonding mechanisms of the dissolved water. However, despite their importance, the solution mechanisms of water in silicate melts are not clear. In particular, how chemical composition of melts affects water solubility and solution mechanism is not well understood. In the present experimental study, components of such information are reported via determination of how water interacts with the cationic network of alkali (Li, Na, and K) silicate quenched melts. Results from 29Si single-pulse magic-angle spinning nuclear magnetic resonance (29Si SP MAS NMR), infrared, and Raman spectroscopies show that decreasing the ionic radius of alkali metal cation in silicate melts results in decreasing fraction of water dissolved as OH groups. The nature of OH bonding also changes as the alkali ionic radius changes. Therefore, as the speciation and bonding of water controls the degree of polymerization of melts, water will have different effects on the transport properties of silicate melts depending on their chemical composition. This conclusion, in turn, may affect volcanic phenomena related to the viscous relaxation of hydrous magmas, such as for instance the fragmentation process that occurs during explosive eruptions

Percolation channels:A universal idea to describe the atomic structure and dynamics of glasses and melts

Understanding the links between chemical composition, nano-structure and the dynamic properties of silicate melts and glasses is fundamental to both Earth and Materials Sciences. Central to this is whether the distribution of mobile metallic ions is random or not. In silicate systems, such as window glass, it is well-established that the short-range structure is not random but metal ions cluster, forming percolation channels through a partly broken network of corner-sharing SiO4 tetrahedra. In alumino-silicate glasses and melts, extensively used in industry and representing most of the Earth magmas, metal ions compensate the electrical charge deficit of AlO4? tetrahedra, but until now clustering has not been confirmed. Here we report how major changes in melt viscosity, together with glass Raman and Nuclear Magnetic Resonance measurements and Molecular Dynamics simulations, demonstrate that metal ions nano-segregate into percolation channels, making this a universal phenomenon of oxide glasses and melts. Furthermore, we can explain how, in both single and mixed alkali compositions, metal ion clustering and percolation radically affect melt mobility, central to understanding industrial and geological processespublishersversionPeer reviewe

A combined Fourier transform infrared and Cr K-edge X-ray absorption near-edge structure spectroscopy study of the substitution and diffusion of H in Cr-doped forsterite

International audienceSingle crystals of synthetic Cr-doped forsterite (Cr:Mg2SiO4) containing both Cr3+ and Cr4+ were partially hydroxylated in piston-cylinder apparatuses at 750-1300 degrees C and pressures from 0.5 to 2.5 GPa, with P(H2O) approximate to P-total. The oxygen fugacity (fO(2)) was buffered by graphite-water, Ni-NiO, Re-ReO2, Fe2O3-Fe3O4 or Ag-Ag2O, and the silica activity (a SiO2) was buffered by powdered forsterite plus either enstatite (Mg2Si2O6), periclase (MgO) or zircon-baddeleyite (ZrSiO4-ZrO2). Profiles of OH content versus distance from the crystal edge were determined using Fourier transform infrared (FTIR) spectroscopy, and profiles of the oxidation state and coordination geometry of Cr were obtained, at the same positions, using K-edge X-ray absorption near-edge structure (XANES) spectroscopy. The techniques are complementary - FTIR spectroscopy images the concentration and nature of O-H bonds, where Cr K-edge XANES spectroscopy shows the effect of the added H on the speciation of Cr already present in the lattice. Profiles of defect-specific absorbance derived from FTIR spectra were fitted to solutions of Fick's second law to derive diffusion coefficients, which yield the Arrhenius relationship for H diffusion in forsterite: log(10)(D) over tilde ([001]) = -2.5 +/- 0.6 + -(224 +/- 12 + 4.0 +/- 2.0 P)/2.303 RT , where (D) over tilde is the measured diffusion coefficient in m(2) s(-1), valid for diffusion parallel to [001] and calibrated between 1000 and 750 degrees C, P and T are in GPa and K, and R is 0.008314 kJK(-1) mol(-1). Diffusivity parallel to [100] is around 1 order of magnitude lower. This is consistent with previous determinations of H diffusion associated with M-site vacancies. The FTIR spectra represent a variety of Cr-bearing hydrous defects, along with defects associated with the pure Mg-Si-O-H system. It is proposed that all of the defects can form by interaction between the dry lattice, including Cr3+ and Cr4+, and fully hydroxylated M-site vacancies. The initial diffusive wave of hydroxylation is associated with neither reduction nor oxidation of Cr but with Cr4+ changing from tetrahedral to octahedral coordination. Superimposed on the H diffusion and concomitant change in Cr4+ site occupancy, but at a slower rate, producing shorter profiles, is reduction of Cr4+ to Cr3+ and potentially of Cr4+ and Cr3+ to Cr2+. In addition, by comparing FTIR data to trace element contents measured by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), constraints can be placed on absorption coefficients used for converting absorbance to H2O contents - our data support either wavenumber- or defect-dependent values of absorption coefficients. We estimate absorption coefficients of between 60 200 and 68 200 L mol(-1) cm(-1) for OH- associated with octahedral Cr3+ and an M-site vacancy and 18 700 to 24 900 L mol(-1) cm(-1) for two OH- associated with octahedrally coordinated Cr4+ and a Si vacancy (i.e. a clinohumite-type point defect)

Spinel Harzburgite-Derived Silicate Melts Forming Sulfide-Bearing Orthopyroxenite in the Lithosphere. Part 1: Partition Coefficients and Volatile Evolution Accompanying Fluid- and Redox-Induced Sulfide Formation

We report abundances of major trace and volatile elements in an orthopyroxenite vein cutting a sub-arc, mantle-derived, spinel harzburgite xenolith from Kamchatka. The orthopyroxenite contains abundant sulfides and is characterized by the presence of glass (formerly melt) both interstitially and as inclusions in minerals, comparable with similar veins from the West Bismarck arc. The glass formed by quenching of residual melts following crystallization of abundant orthopyroxene, amphibole, and minor olivine and spinel. The interstitial glass has a low-Ti, high-Mg# andesite composition, with a wide range of H2O and S contents but more limited F and Cl variations. We calculate trace element partition coefficients using mineral and glass data, including those for halogens in amphibole, which agree with experimental results from the literature. Despite having a similar, high-Mg# andesite composition, the orthopyroxene-hosted glass inclusions usually contain much more H2O and S than the interstitial glass (4–7 wt% and ∼2,600 ppm, respectively). The initial vein-forming melts were oxidized, recording oxygen fugacity conditions up to ∼1.5 log units above the fayalite–magnetite–quartz oxygen buffer. They intruded the sub-arc mantle lithosphere at ≥1,300°C, where they partially crystallized to form high-Mg# andesitic derivative melts at ca. 1,050–1,100°C. Comparison with literature data on glass-free orthopyroxenite veins from Kamchatka and the glass-bearing ones from West Bismarck reveals fundamental similarities indicating common parental melts, which were originally produced by low-degree melting (≤5%) of spinel harzburgite at ≥1,360°C and ≤1.5 GPa. This harzburgite source likely contained ≤0.05 wt% H2O and a few ppm of halogens. Volatile evolution inferred from glass compositions shows that (i) redox exchange between S6+ in the original melt and Fe2+ in the host mantle minerals, together with (ii) the formation of an S-bearing, (H2O, Cl)-rich hydrothermal fluid from the original melt, provides the conditions for the formation of abundant sulfides in the orthopyroxenites during cooling. During this process, up to 85% of the original melt S content (∼2,600 ppm) is locally precipitated as magmatic and hydrothermal sulfides. As such, melts derived from spinel harzburgite sources can concentrate chalcophile and highly siderophile metals in orthopyroxenite dykes and sills in the lithosphere

Raman spectroscopy study of C-O-H-N speciation in reduced basaltic glasses: Implications for reduced planetary mantles

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 (f 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 f O2 conditions during late stages of terrestrial accretion. Raman vibrational modes for H2, NH2 – , NH3, CH4, CO, CN– , N2, and OH– species are inferred from band assignments in all reduced glasses. The integrated area of Raman bands assigned to N2, CH4, NH3 and H2 vibrations in glasses increases with increasing molar volume of the melt, whereas that of CO decreases. Additionally, with increasing f O2, CO band areas increase while those of N2 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– , NH2 – and OH– are chemically bonded within the silicate network. The observations indicate that, under reduced conditions, (1) H2, NH2 – , NH3, CH4, CO, CN– , N2, 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, f 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.CD and MH acknowledge support from the National Science Foundation grant AST1344133. SDJ acknowledges support from NSF EAR-1853521 and the David and Lucile Packard Foundation

Iron cation vacancies in Pt(iv)-doped hematite

Platinum-doping of hematite (α-Fe2O3) is a popular method to increase the performance of hematite in photoelectrochemical applications. The precise mode of Pt incorporation is however unclear, as it can occur as Pt0, Pt2+ or Pt4+, either on the surface, as dispersed inclusions, or as part of the hematite crystal lattice. These different Pt-doping varieties can have major effects on the hematite performance. Here, we employ a high-pressure synthesis method assisted by silicate liquid flux to grow Pt-doped hematite crystals large enough for elemental analysis by wavelength dispersive spectroscopy (WDS). We find that the total cations are lower than the expected 2 atoms per formula unit, and together with Fe, they are inversely correlated with Pt contents. Linear regressions in compositional space reveal that the slopes are consistent with 4Fe3+ = 3Pt4+ + VFe as the charge-balanced substitution mechanism. Therefore, Pt4+-doping of hematite at high oxygen fugacities, which does not allow Fe2+ to form, will lead to removal of Fe and formation of cation vacancies. Our hematite also contains significant Al3+, Ti4+ and Mg2+, raising the possibility of fine tuning the hematite properties by co-doping with other elements. Photoelectrochemical performance of cation vacancy bearing hematite is experimentally understudied and is a potentially promising future field of study

Percolation channels: A universal idea to describe the atomic structure and dynamics of glasses and melts

Understanding the links between chemical composition, nano-structure and the dynamic properties of silicate melts and glasses is fundamental to both Earth and Materials Sciences. Central to this is whether the distribution of mobile metallic ions is random or not. In silicate systems, such as window glass, it is well-established that the short-range structure is not random but metal ions cluster, forming percolation channels through a partly broken network of corner-sharing SiO4 tetrahedra. In alumino-silicate glasses and melts, extensively used in industry and representing most of the Earth magmas, metal ions compensate the electrical charge deficit of AlO4 − tetrahedra, but until now clustering has not been confirmed. Here we report how major changes in melt viscosity, together with glass Raman and Nuclear Magnetic Resonance measurements and Molecular Dynamics simulations, demonstrate that metal ions nano-segregate into percolation channels, making this a universal phenomenon of oxide glasses and melts. Furthermore, we can explain how, in both single and mixed alkali compositions, metal ion clustering and percolation radically affect melt mobility, central to understanding industrial and geological processes.CLL thanks support from the ARC Laureate Fellowship FL130100066 to Hugh O’Neill (Research School of Earth Sciences, ANU, Australia) and the Carnegie Postdoctoral Fellowship (Carnegie Institution of Washington, USA) during the redaction of this manuscript