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The Raman spectra of glasses containing 0 to 30 mol% Cs2O have been fit successfully with line shapes of dominantly Lorentzian character for the Q3 species, allowing quantification of Q3 and Q4 species intensities (Q represents a Si tetrahedron and the superscript indicates the number of bridging oxygen atoms, BOs, bonded to Si.) The intensity of the Q4 species A1 symmetric stretch is exceptionally weak in vitreous silica (v-SiO2) but it increases dramatically with addition of small amounts of Cs2O to the glass. We propose that Cs, where in close proximity to BO of Q4 species, promotes the polarizability of Q4 tetrahedra and these primed Q4 species (Q4-p) produce a strong Q4 signal. There are, therefore, two variants of the Q4 species, a Q4-p species which produces a strong signal, and an unprimed species (Q4-u) which yields a very weak signal. The increase in the abundance of the primed Q4 species (Q4-p) can be modelled as a function of alkali content using a simple, upper-bounded growth model: XQ4-p = (1 − e−kx) where XQ4-p is the fraction of polarizable Q4 species, k is a constant and x is the mol% counter oxide in the glass (here Cs2O). Comparison of calculated XQ4-p values with experimental results indicates that its cross-section is similar to that of the Q3 species. There is no evidence for a ~1050 cm−1 band in the 5 mol% Cs2O glass spectrum but in the 30 mol% Cs2O glass spectrum about 11% of spectral intensity is observed at about this frequency. The intensity likely results from development of asymmetry on the Q3 band, which increases with Cs2O content of the glass. The asymmetry results from weakened Si O force constants of some Q3 tetrahedra due to charge transfer via Cs BO bonds. As evidence, Si 2p and O 1s X-ray Photoelectron Spectroscopic (XPS) studies demonstrate that the electron density over Si and BO atoms of Q4 species increases with Cs2O content. With charge transfer to tetrahedra, the negative charge accumulates preferentially on Si atoms thus decreasing Si O coulombic interactions, weakening Si O force constants, and shifting the Q3 A1 symmetric stretch vibrational frequencies to lower values (e.g., from ~1100 cm−1 to ~1050 cm−1). The fraction of affected Q3 species increases with alkali content, as does the Q3 peak asymmetry. The Raman shifts of the Q4 species are also affected by increased Cs2O contents.

Many crystallization processes of great importance, including frost heave, biomineralization, the synthesis of nanomaterials, and scale formation, occur in small volumes rather than bulk solution. Here, the influence of confinement on crystallization processes is described, drawing together information from fields as diverse as bioinspired mineralization, templating, pharmaceuticals, colloidal crystallization, and geochemistry. Experiments are principally conducted within confining systems that offer well-defined environments, varying from droplets in microfluidic devices, to cylindrical pores in filtration membranes, to nanoporous glasses and carbon nanotubes. Dramatic effects are observed, including a stabilization of metastable polymorphs, a depression of freezing points, and the formation of crystals with preferred orientations, modified morphologies, and even structures not seen in bulk. Confinement is also shown to influence crystallization processes over length scales ranging from the atomic to hundreds of micrometers, and to originate from a wide range of mechanisms. The development of an enhanced understanding of the influence of confinement on crystal nucleation and growth will not only provide superior insight into crystallization processes in many real-world environments, but will also enable this phenomenon to be used to control crystallization in applications including nanomaterial synthesis, heavy metal remediation, and the prevention of weathering.

We present the first Li K-edge XANES study of a varied suite of lithium silicate (LS) and lithium alkaline-earth silicate (LMS) glasses. The LS series contains three glasses with compositions LS4 (20 Li2O–80 SiO2), LS2 (33 Li2O–67 SiO2) and LS1.5 (40 Li2O–60 SiO2) and the LMS series is comprised of four glasses of compositions 20 Li2O–20 M2O–60 SiO2 where M = Mg, Ca, Sr and Ba. The spectra were compared to those of known salts and minerals, particularly to lithium metasilicate (Li2SiO3) and lithium carbonate (Li2CO3). Crystalline lithium metasilicate has a sharp, strong intensity absorption edge whereas the lithium silicate glasses all have a weak intensity edge feature, similar to that of lithium carbonate (Li2CO3). The intensity of the edge is governed by the existence of a 2p electron and can be correlated with the ionicity of the Li O bond. Therefore, the bonding environment in LS glasses differs considerably from their crystalline counterparts. The area of the absorption edge peak increases with the lithium content of the LS glasses. As for the LMS glasses the edge peak changes drastically depending on the alkaline-earth present. The peak area of the LMS glasses decreases with increasing charge density (Z/r2) from barium to magnesium. The area of the absorption edge peak in the LS glasses is larger than that of the LMS glasses. Thus, the addition of alkaline-earth elements to lithium silicate glasses modifies the glass network differently depending on the specific element, which in turn creates an altered lithium bonding environment as evidenced by their Li K-edge XANES spectra.

Raman spectra at 298 K of the SiO4 symmetric stretch region (800–1200 cm−1) for low alkali (M) silicate glasses (5 and 10 mol% M2O, for M = Cs, K) yield intense well-resolved Q3 peaks at ~1100 cm−1 with mostly Lorentzian character, in contrast to previous Gaussian fits for silicate glasses. The spectra of 5 mol% Na and Li glasses show an additional Q3 peak. It results from close approach of alkalis (M) to bridging oxygen (BO) which splits the Q3 band into two peaks. All linewidths (FWHM) of fitted Q1, Q2 and Q3 species peaks are similar (~35–55 cm−1) at 298 K for all the above glasses. The Q3 species FWHM of 5 and 10 mol% Cs2O silicate glasses show a T dependence similar to those of crystal silicate spectra: both increase by 35–45 cm−1 from 298 K to 1200 K. The T dependence of the FWHM of these Lorentzian dominated peaks are consistent with theory. The more complex spectra of the 30 mol% K2O and 50 mol% Na2O glass spectra are then readily fit with two or three Q2 or Q3 peaks, constraining the % Lorentzian character of all peaks from the fit to the resolved peak in those spectra. The 50 mol% Na2O spectrum shows that Q3 > Q1, indicating ~4.5 mol% free oxygen in the glass, a value in good agreement with previous 29Si NMR and O 1s XPS results. Lorentzian/Gaussian lineshapes now need to be considered for many more silicate glass/melt Raman spectra.

This study investigated a series of caesium bearing silicate glasses with compositions ranging from 5 to 35 mol. % Cs2O using Raman spectroscopy. The proportions of 4- and 3-fold SiO4 rings are estimated by using the relative intensities of the D1 and D2 vibrational bands in each spectra. An increase in the abundance of 3-fold SiO4 rings is apparent and confirms previous studies which proposed such a mechanism to be present with increasing alkali content in alkali silicate glasses. The behaviour of the D1 and D2 bands correlates with the density measurements lending credence to the importance of the formation 3-fold SiO4 rings with increasing molar volume in silicate glasses. The high frequency envelopes (∼800–1300 cm−1) were modeled using Gaussian lineshapes in order to isolate the separate contributions to the convoluted peak shape. These Gaussian bands are interpreted as being representative of the Qn species and are linked to the depolymerization reaction. We observe an increase in Q2 species coupled with a decrease in Q4 species with increasing Cs2O content. We propose that proximity to modifier cations drastically changes the vibrational character of the different Qn species and due to this effect we have assigned our curve-fit bands to both Qn and QnM species. High-temperature measurements were also conducted to investigate pre-melting and melting effects on the structure of these glasses. We observed similar changes in the high frequency envelope and Qn species distribution with increasing temperature as we did for increasing alkali content. The creation of Q2 species above the melting point leads us to conclude that depolymerization occurs at the solid-liquid transition.

Systematics of Si O Raman stretching frequencies in the range 800–1200 cm − 1 , and Si 2p and O 1s XPS binding energies (BE) are examined for a large number of alkali and alkaline earth silicate glasses, melts, and crystals. Several relationships in the Raman and X-ray Photoelectron spectral (XPS) data lead to new insights concerning charge delocalization over Si tetrahedra and to recognition of controls on both Raman stretching frequencies, and Si 2p and O 1s XPS chemical shifts. The Raman stretching frequencies of the five Q species and the Si 2p and O 1s XPS chemical shifts, vary linearly with the alkali and alkaline-earth content of glasses over the range 0–50 mol% counter oxide (M 2 O or MO). The relationships demonstrate that the frequencies and chemical shifts are controlled by the same property, the valence band electron densities on the atoms of silicate tetrahedra. Specifically, where M-O ion interactions are strongly ionic, the number of NBO's attached to the central Si atom most affects the electron density of Si and O in tetrahedra, and is the major control on Raman symmetric stretching frequencies. Also, the Raman stretching frequency of individual Q species are effectively independent of the mass of the alkali counter cation (M Na, K, Rb, Cs). Addition of alkali oxides to vitreous SiO 2 produces Si 2p and O 1s XPS chemical shifts. The magnitude of the chemical shifts increase with counter oxide abundance through transfer of valence ‘s’ electrons from alkali atoms to NBO of tetrahedra. The negative charge acquired by the NBO is redistributed (delocalized) over all atoms of the tetrahedron via Si O bonds thereby increasing valence band electron densities over the constituent Si and O atoms. The greater the number of NBOs associated with a tetrahedron the greater the enhancement of electron density on all atoms of the tetrahedron, thus explaining the Si 2p and O 1s XPS chemical shifts. With respect to Raman spectra, the increased valence electron density causes the central Si atom to become appreciably less positive, which diminishes the strength of Coulombic interactions between Si and O atoms, and weakens Si O force constants. In turn, the weakened force constants result in decreases to Si O stretching frequencies. The weakening of force constants and decrease in stretching frequencies occur in a stepwise manner and relate directly to the number of NBO attached to the tetrahedra. The Si O force constant for the Q 4 species is much larger than the force constant for the Q 0 species, causing their stretching frequencies to decrease (stepwise) from ~ 1200 cm − 1 to ~ 850 cm − 1 . The mathematical relationships among alkali oxide compositions, Raman stretching frequencies and XPS chemical shifts are quantified.

Foam formation during vesiculation of hydrous magmatic melts at 1 atm was studied in situ by synchrotron X-ray tomographic microscopy at the TOMCAT beamline of the Swiss Light Source (Villigen, Switzerland). Four different compositions were studied; basaltic, andesitic, trachyandesitic and dacitic hydrous glasses were synthesized at high pressures as starting materials and then laser heated on the beamline. The porosity, bubble number density, size distributions of bubbles and pore throats, as well as the tortuosity and connectivity of bubbles in the foams, were measured in three dimensions based on tomographic reconstructions of sample volumes. The reconstructed volumes were also used in lattice-Boltzmann simulations to determine viscous permeabilities of the samples. Connectivity of bubbles by pore throats varied from ~100 to 105 mm-3, and for each sample correlated positively with porosity and permeability. Although permeability increased with porosity, the relationship is complex; consideration of the results of this and previous studies of the viscous permeabilities of aphyric and crystal-poor magmatic samples demonstrated that at similar porosities the permeability could vary by many orders of magnitude, even in similar composition samples. More than 90 % of these permeabilities are bounded by two empirical power laws, neither of which identifies a percolation threshold. Comparison of the permeability relationships from this study with previous models (Degruyter et al. 2010; Burgisser et al. 2017) relating porosity, characteristic porethroat diameters and tortuosity demonstrated good agreement. However, modifying the Burgisser et al. (2017) model by using the maximum measured pore-throat diameter, instead of the average diameter, as the characteristic diameter produced a model that reproduced the lattice-Boltzmann permeabilities to within 1 order of magnitude. Measured correlations between the average bubble diameter and the maximum pore-throat diameter as well as between porosity and tortuosity in our experiments produced relationships that allow application of the modified Burgisser et al. model to predict permeability based only upon the average bubble diameter and porosity. The experimental results are consistent with previous studies suggesting that increasing bubble growth rates result in decreasing permeability of equivalent porosity foams. The effect of growth rate on permeability is hypothesized to substantially contribute to the multiple orders-of-magnitude variations in the permeabilities of natural magmatic samples at similar porosities.

The first comprehensive LiK-edge XANES study of a varied suite of Li-bearing minerals is presented. Drastic changes in the bonding environment for lithium are demonstrated and this can be monitored using the position and intensity of the main LiK-absorption edge. The complex silicates confirm the assignment of the absorption edge to be a convolution of triply degeneratep-like states as previously proposed for simple lithium compounds. The LiK-edge position depends on the electronegativity of the element to which it is bound. The intensity of the first peak varies depending on the existence of a 2pelectron and can be used to evaluate the degree of ionicity of the bond. The presence of a 2pelectron results in a weak first-peak intensity. The maximum intensity of the absorption edge shifts to lower energy with increasing SiO2content for the lithium aluminosilicate minerals. The bond length distortion of the lithium aluminosilicates decreases with increasing SiO2content, thus increased distortion leads to an increase in edge energy which measures lithium's electron affinity.

The melting mechanism of Na 2 SiO 3 , a crystal with pyroxene structure, includes three distinct reactions. All are driven by heating with each reaction commencing at a different temperature. The first two reactions proceed within the crystal at temperatures well below the melting point and are expressed by distinctive crystallographic, calorimetric, and Raman spectroscopic changes to the crystal. With the reactions identified and explained for Na 2 SiO 3 (c) and the melting mechanism elucidated, the Na 2 SiO 3 system becomes the “Rosetta Stone” by which to decipher the melting mechanisms of all pyroxenes and other silicate minerals. The first reaction produces itinerant Na + within the crystal. Itinerancy results from dissociation of some NBO-Na bonds due to heating, with dissociation commencing at ~770 K. The reaction proceeds according to: Si-O-Na → Si-O – + Na + . The Si-O − moiety remains attached to its SiO 3 chain and it is charged because one of its NBO atoms has no associated Na ion. The second reaction is characterized by the appearance of a Q 3 band in Raman spectra of the crystal at temperatures >770 K. It is produced via a polymerization reaction involving the Si-O − species, a product of the first reaction, and a Q 2 species of an adjacent SiO 3 chain according to: 1 Na 2 -Q 2 + 1 ( Na 1 -Q 2 ) − → 2 Na 1 -Q 3 + Na + + O 2 − . Na atoms are included with each Q species to preserve mass balances and (Na 1 -Q 2 ) − is equivalent to the Si-O − species. The produced Q 3 species form cross-chain linkages that affect the crystallographic properties of the crystal. They are responsible for the cessation of thermal expansion of the Na 2 SiO 3 unit cell in the a – b axial plane at T >770 K, and the near-constancy of a and b unit-cell parameters between ~770 and ~1300 K. The presence of Q 3 species in Raman spectra and the inhibited expansion in the a – b axial plane provide exceedingly strong evidence for this reaction. The third reaction commences at ~1200 K where Q 1 Raman band first appears. It can be produced only through depolymerization of Q 2 chains according to: 2 Na 2 -Q 2 + 2 Na + + O 2 − → 2 Na 3 -Q 1 where Na + and O 2− are itinerant species produced by the second reaction. With conversion of Q 2 to Q 1 species, SiO 3 chains are ruptured, long-range order is lost, and melt is produced at 1362 K. The last two reactions proceed by nucleophilic substitution where Si centers are attacked to form fivefold-coordinated activated complexes. Si-O − acts as nucleophile in the second reaction (producing Q 3 species), and O 2− acts as nucleophile in the third reaction (producing Q 1 ). Taken in reverse, these mechanisms describe the formation of nuclei in crystallizing melts and in addition provide insight into the elusive changes that occur at the glass transition. Elucidation of the melting mechanism could thus provides a unified framework within which melting, crystallization, and the glass transition can be understood.

Chlorine-hydroxyl diffusion was measured in pargasitic amphibole from Yunnan province, China at 1.0 GPa, 625 to 800 °C. Experiments were performed by immersing unoriented crystals in water-bearing NaCl in a piston cylinder for durations from 100 to 454 h. Diffusion profiles were on the order of greater than tens of micrometers in length, and electron microprobe analysis allow us to extract semi-quantitative diffusivities from these experiments. The preliminary diffusion coefficients for chlorine in amphibole in the water-bearing experiments are 2.6 × 10 −16 m 2 /s at 625 °C, 4.9 × 10 −16 m 2 /s at 650 °C, 7.6 × 10 −16 m 2 /s at 700 °C, 1.8 × 10 −15 m 2 /s at 750 °C, 2.8 × 10 −15 m 2 /s at 800 °C. For temperatures between 625 and 800 °C, the Arrhenius relation for chlorine-hydroxyl diffusion has an activation energy of 106.6 ± 7.8 kJ/K mol and a D 0 of 4.53 (+7.3, −2.8) × 10 −10 m 2 /s. Our measurements do not show evidence of anisotropy in the diffusion of Cl-OH into amphibole, but future experiments need to better investigate this possibility.

The failure of magmatic foams has been implicated as a fundamental process in eruptions occurring at open-conduit, basaltic volcanoes. In order to investigate the failure of magmatic foams we applied the fiber bundle model using global load sharing. The strengths of the fibers for the model were taken from bubble wall widths measured in four computer-simulated foams of low-porosity and from one very low-porosity and two high-porosity foams produced in the laboratory by heating hydrated basaltic glasses to 1200 °C. The relative strength of an individual fiber in the model was calculated from the square of a bubble wall's average width and absolute strengths of the foams were calculated based upon the correlation of the strength of one modeled foam with experimental data. The fiber bundle model is shown to successfully reproduce measured tensile strengths of porous volcanic rocks studied by other researchers and confirms previous findings of the primary importance of foam porosity, as well as the secondary importance of structural details that affect the number and size of bubble walls and permeability. Because of the success of the fiber bundle model in reproducing experimental foam failure, its results are compared to infrasonic measurements associated with bubbles at Stromboli (Italy) and demonstrate that within uncertainty the power-law exponents of the infrasonic energies and of the fiber bundle model energies are in agreement; both show a crossover from an exponent of 5/2 associated with the bursting of small bubbles in the infrasonic measurements to an exponent of 3/2 for normal Strombolian eruptions associated with infrasonic signals from meter-scale bubbles. The infrasonic signals for major explosions and a paroxysmal eruption at Stromboli fall near the extrapolation of the power law defined by the low-amplitude, bubble bursting events and are interpreted to reflect the bursting of multitudes of small bubbles, rather than a few large bubbles. The measurement of small-amplitude infrasonic events at Stromboli appears useful in predicting the recurrence interval of paroxysmal eruptions at this volcano and may also provide a tool that uses common, small-amplitude infrasonic events to constrain the frequency of larger eruptions at other volcanoes.

X-ray absorption near-edge structure (XANES) spectroscopy has been used to investigate the local structural environments around each of the elements within the haplobasaltic system. The XANES results reveal both, linear changes in Ca, O and Si whereas non-linear changes are also observed for Si as well as Al and Mg. Unexpectedly, both Si and Al spectra show complex edge structure that varies non-linearly across the join. The Si L 2,3 -edge results suggest that the Si environment in An100 is very similar to that in crystalline anorthite whereas the Di100 and diopside spectra show large offsets. The L 3 and L 2 peaks display a linear behavior whereas peak B of the Si L 2,3 spectra displays a non-linear behavior displaying a maximum position at the Di40 and Di100 compositions and decreasing in between. We estimate the Si O bond length from shift in the peak B position which suggests a change in bond length between 1.58 and 1.66 A across the join. The Al K -edge spectra suggests that there is a constant decrease in the inter-tetrahedral angle as Al is added. The local Mg environment displays non-linear changes in intensity and for compositions with An content greater than Di40 it appears that [4] Mg plays an important role whereas [5] Mg is dominant in diopside-rich melts, though octahedral coordinated Mg cannot be ruled out. The oxygen environment appears similar to crystalline anorthite in the haplobasaltic system. Calcium is present in highly distorted sites that are dominantly [7] Ca in anorthite-rich glasses whereas [8] Ca is more important in diopside-rich glasses. These results provide additional independent constraints on the atomic structure of haplobasaltic silicate melts and highlight the need for further investigation into the relationships between network formers, modifiers and the oxygen to which they are bound.

Corrigendum: A four-dimensional X-ray tomographic microscopy study of bubble growth in basaltic foam

Understanding the influence of bubble foams on magma permeability and strength is critical to investigations of volcanic eruption mechanisms. Increasing foam porosity decreases strength, enhancing the probability of an eruption. However, higher porosities lead to larger permeabilities, which can lessen the eruption hazard. Here we measure bubble size and wall thickness distributions, as well as connectivity, and calculate permeabilities and tensile strengths of basaltic foams imaged by synchrotron X-ray tomographic microscopy during bubble growth in hydrated basaltic melts. Rapid vesiculation produces porous foams whose fragmentation thresholds are only 5-6 MPa and whose permeabilities increase from approximately 1x10(-10) to 1x10(-9) m(2) between 10 and 14 s despite decreasing connectivity between bubbles. These results indicate that basaltic magmas are most susceptible to failure immediately upon vesiculation and at later times, perhaps only 10's of seconds later, permeability increases may lessen the hazard of explosive, basaltic, Plinian eruptions.