Anhydrous Glasses Research Articles

Estimating the viscosity of volcanic melts from the vibrational properties of their parental glasses

The numerical modelling of magma transport and volcanic eruptions requires accurate knowledge of the viscosity of magmatic liquids as a function of temperature and melt composition. However, there is growing evidence that volcanic melts can be prone to nanoscale modification and crystallization before and during viscosity measurements. This challenges the possibility of being able to quantify the crystal-free melt phase contribution to the measured viscosity. In an effort to establish an alternative route to derive the viscosity of volcanic melts based on the vibrational properties of their parental glasses, we have subjected volcanologically relevant anhydrous glasses to Brillouin and Raman spectroscopic analyses at ambient conditions. Here, we find that the ratio between bulk and shear moduli and the boson peak position embed the melt fragility. We show that these quantities allow an accurate estimation of volcanic melts at eruptive conditions, without the need for viscosity measurements. An extensive review of the literature data confirms that our result also holds for hydrous systems; this study thus provides fertile ground on which to develop new studies of the nanoscale dynamics of natural melts and its impact on the style of volcanic eruptions.

Retrieving dissolved H2O content from micro-Raman spectroscopy on nanolitized silicic glasses: Application to volcanic products of the Paraná Magmatic Province, Brazil

Determining volatile content in silicate glasses has critical importance to evaluate the original water content of magmas and its evolution from storage to eruption. Here we present a Raman spectroscopic study devoted to defining an optimized analytical procedure for the determination of dissolved water content of nano-crystallized ancient silicic glasses, belonging to the Paraná Magmatic Province (PMP) volcanism in South Brazil (131 to 134 Ma). A set of anhydrous glasses was prepared under atmospheric pressure by melting and quenching of natural rocks. These glasses were later hydrated under high pressure in an Internally Heated Pressure Vessel (IHPV). Even after rapid quenching, most of the dry and hydrous glasses showed prominent Raman peaks at ca. 670–690 cm−1, which are associated with the presence of Ti-magnetite nanolites. Although large Raman peaks are observed, the nanolites are invisible to standard Scanning Electron Microscopy (SEM) due to their small size and not quantifiable by X-ray diffraction (XRD) because of their low abundance. Available models and procedures to estimate water content in glasses are calibrated with crystal-free glasses and were proved inadequate for water measurement in nanolite-bearing glasses. Here we provide new calibration strategies to determine the water content in nanolite-bearing glasses. These strategies are based on the normalization of the intensity and area of the water band (3550 cm−1) to the individual vibrational bands of the silicate framework of the glass, at 350–500 cm−1 and 860–1240 cm−1. Our new calibrations were used to estimate the dissolved water content of glass matrixes of natural dacitic vitrophyres from the Caxias do Sul volcanic suite, whose Raman spectra show nanolite peaks of variable intensity. Our work constitutes a further step ahead toward the definition of strategies to determine the volatile content in natural multicomponent and multiphase glasses.

X-ray absorption study of 3d transition-metals and Mg in glasses and analogue crystalline materials in AFe3 + Si2O6 and A2X2 + Si5O12, where A = K, Rb, or Cs and X = Mg, Mn, Fe, Co, Ni, Cu, or Zn

Crystalline and glassy phases in the ‘leucite’ systems (K,Rb,Cs)Fe3+Si2O6 and (K,Rb,Cs)2(Mg,Mn,Fe,Co,Ni,Cu,Zn)2+–Si5O12 have been studied using K-edge X-ray Absorption Spectroscopy (XAS) for 3d transition elements and Mg. Crystalline samples of known structure are used as model compounds for deducing the local structures of the equivalent glasses. ‘Ideal’ tetrahedral framework X2+–O distances are: Mn2+–O 2.02, Fe2+–O 1.98; Co2+–O 1.96, Ni2+–O 1.95, Cu2+–O 1.92, Zn2+–O 1.93 and Mg2+–O 1.93Å. The quenched glasses have Extended X-ray Absorption Fine Structure (EXAFS) first-shell distances and coordination numbers consistent with Mg and 3d elements occurring in tetrahedral coordination in most cases. The X-ray Absorption Near Edge Structure (XANES) spectra of the glasses are basically the same as those for the crystalline samples showing that they have similar medium range order, consistent with most of the Mg and 3d elements acting as network formers along with the associated alkalis as X2Y2+O2 complexes. Anhydrous glasses in the system CaO–MgO–Al2O3–SiO2 together with a basalt glass from Hawaii show examples of Mg in octahedral- (2.07Å, basalt), tetrahedral- (~1.87Å, siliceous glass) and 5-coordinated (2.01–2.04Å) sites. Although most Earth scientists assume that Mg and Fe2+ act as network modifiers in natural magmas, ultra-potassic, peralkaline compositions ((Na+K)/(Al+Fe3+)>1) could have had some Fe2+ and Mg complexes with alkalis in the melt network as K2(Mg,Fe2+)O2 and the implications of this to melt density and viscosity are considered.

Water and the compressibility of silicate glasses: A Brillouin spectroscopic study

The compressional and shear wave velocities of seven series of hydrous aluminosilicate glasses were measured by Brillouin scattering at room temperature and pressure as a function of water content. The glasses were quenched from high temperature and 0.2 or 0.3 GPa pressure. The dry end-members range from highly polymerized albitic and granitic compositions, to depolymerized synthetic analogues of mantle-derived melts. For each set of glasses, the adiabatic shear and bulk moduli have been calculated from the measured sound velocities and densities. These moduli are linear functions of water content up to 5 wt% H 2 O, the highest concentration investigated, indicating that both are independent of water speciation in all series. For anhydrous glasses, (∂V/∂P) S changes from about -0.3 to -0.7 × 10 −15 m 3 /(Pa·mol) with increasing degree of polymerization. For the same compositions, the partial molar (∂V/∂P) S of the water component changes from near zero to -0.8 × 10 −15 m 3 /(Pa⋅mol) with increasing degree of polymerization, such that dissolved water amplifies the differences in rigidity between the anhydrous glasses. This strong variation indicates that the solubility mechanisms of water depend strongly on silicate composition. A simple linear model reproduces (∂V/∂P) S to within measurement uncertainty, with glasses becoming more compressible as NBO/T decreases and as alkalinity increases. The dissolved water component also becomes more compressible as NBO/T decreases, but less compressible as alkalinity increases. In strongly depolymerized structures, water causes an increase in sound velocities even though it induces a decrease in density. The general rule according to which sound velocities correlate with density is, therefore, violated, for in these cases water dissolution results in a decrease of compressibility. Barring any strongly anomalous effect of water on configurational compressibility, the trends observed for glasses should be still more divergent for melts. Consequently, hydrous mafic and ultramafic liquids are predicted to have a very low compressibility. The shear modulus of hydrous glasses varies little with bulk composition or water content, supporting the use of a single composition-independent pre-exponential factor in models of melt viscosity.

Comment on “Study of dielectric relaxations of anhydrous trehalose and maltose glasses” [J. Chem. Phys. 134, 014508 (2011)

Very recently Kwon et al. [H.-J. Kwon, J.-A. Seo, H. K. Kim, and Y. H. Hwang, J. Chem. Phys. 134, 014508 (2011)] published an article on the study of dielectric relaxation in trehalose and maltose glasses. They carried out broadband dielectric measurements at very wide range of temperatures covering supercooled liquid as well as glassy state of both saccharides. It is worth to mention that authors have also applied a new method for obtaining anhydrous glasses of trehalose and maltose that enables avoiding their caramelization. Four relaxation processes were identified in dielectric spectra of both saccharides. The slower one was identified as structural relaxation process the next one, not observed by the others, was assigned as Johari-Goldstein (JG) β-relaxation, while the last two secondary modes were of the same nature as found by Kaminski et al. [K. Kaminski, E. Kaminska, P. Wlodarczyk, S. Pawlus, D. Kimla, A. Kasprzycka, M. Paluch, J. Ziolo, W. Szeja, and K. L. Ngai, J. Phys. Chem. B 112, 12816 (2008)]. In this comment we show that the authors mistakenly assigned the slowest relaxation process as structural mode of disaccharides. We have proven that this relaxation process is an effect of formation of thin layer of air or water between plate of capacitor and sample. The same effect can be observed if plates of capacitor are oxidized. Thus, we concluded that their slowest mode is connected to the dc conduction process while their β JG process is primary relaxation of trehalose and maltose.

Sulfur K-edge XANES analysis of natural and synthetic basaltic glasses: Implications for S speciation and S content as function of oxygen fugacity

XANES analyses at the sulfur K-edge were used to determine the oxidation state of S species in natural and synthetic basaltic glasses and to constrain the fO 2 conditions for the transition from sulfide (S 2−) to sulfate (S 6+) in silicate melts. XANES spectra of basaltic samples from the Galapagos spreading center, the Juan de Fuca ridge and the Lau Basin showed a dominant broad peak at 2476.8 eV, similar to the spectra obtained from synthetic sulfide-saturated basalts and pyrrhotite. An additional sharp peak at 2469.8 eV, similar to that of crystalline sulfides, was present in synthetic glasses quenched from hydrous melts but absent in anhydrous glasses and may indicate differences in sulfide species with hydration or presence of minute sulfide inclusions exsolved during quenching. The XANES spectra of a basalt from the 1991 eruption of Mount Pinatubo, Philippines, and absarokitic basalts from the Cascades Range, Oregon, USA, showed a sharp peak at 2482.8 eV, characteristic of synthetic sulfate-saturated basaltic glasses and crystalline sulfate-bearing minerals such as hauyne. Basaltic samples from the Lamont Seamount, the early submarine phase of Kilauea volcano and the Loihi Seamount showed unequivocal evidence of the coexistence of S 2− and S 6+ species, emphasizing the relevance of S 6+ to these systems. XANES spectra of basaltic glasses synthesized in internally-heated pressure vessels and equilibrated at fO 2 ranging from FMQ − 1.4 to FMQ + 2.7 showed systematic changes in the features related to S 2− and S 6+ with changes in fO 2. No significant features related to sulfite (S 4+) species were observed. These results were used to construct a function that allows estimates of S 6+/ΣS from XANES data. Comparison of S 6+/ΣS data obtained by S Kα shifts measured with electron probe microanalysis (EPMA), S 6+/ΣS obtained from XANES spectra, and theoretical considerations show that data obtained from EPMA measurements underestimate S 6+/ΣS in samples that are sulfate-dominated (most likely because of photo-reduction effects during analysis) whereas S 6+/ΣS from XANES provide a close match to the expected theoretical values. The XANES-derived relationship for S 6+/ΣS as a function of fO 2 indicates that the transition from S 2− to S 6− with increasing fO 2 occurs over a narrower interval than what is predicted by the EPMA-derived relationship. The implications for natural systems is that small variation of fO 2 above FMQ + 1 will have a large effect on S behavior in basaltic systems, in particular regarding the amount of S that can be transported by basaltic melts before sulfide saturation can occur.

Influence of glass polymerisation and oxidation on micro-Raman water analysis in alumino-silicate glasses

The development of an accurate analytical procedure for determination of dissolved water in complex alumino-silicate glasses via micro-Raman analysis requires the assessment of the spectra topology dependence on glass composition. We report here a detailed study of the respective influence of bulk composition, iron oxidation state and total water content on the absolute and relative intensities of the main Raman bands related to glass network vibrations (LF: ∼490 cm −1; HF: ∼960 cm −1) and total water stretching (H 2O T: ∼3550 cm −1) in natural glasses. The evolution of spectra topology was examined in (i) 33 anhydrous glasses produced by the re-melting of natural rock samples, which span a very large range of polymerisation degree (NBO/T from 0.00 to 1.16), (ii) 2 sets of synthetic anhydrous basaltic glasses with variable iron oxidation state (Fe 3+/Fe T from 0.05 to 0.87), and (iii) 6 sets of natural hydrous glasses ( C H 2 O T from 0.4 to 7.0 wt%) with NBO/T varying from 0.01 to 0.76. In the explored domain of water concentration, external calibration procedure based on the H 2O T band height is matrix-independent but its accuracy relies on precise control of the focusing depth and beam energy on the sample. Matrix-dependence strongly affects the internal calibrations based on H 2O T height scaled to that of LF or HF bands but its effect decreases from acid (low NBO/T, SM) to basic (high NBO/T, SM) glasses. Structural parameters such as NBO/T (non-bridging oxygen per tetrahedron) and SM (sum of structural modifiers) describe the matrix-dependence better than simple compositional parameters (e.g. SiO 2, Na 2O + K 2O). Iron oxidation state has only a minor influence on band topology in basalts and is thus not expected to significantly affect the Raman determinations of water in mafic (e.g. low SiO 2, iron-rich) glasses. Modelling the evolution of the relative band height with polymerisation degree allows us to propose a general equation to predict the dissolved water content in natural glasses: C H 2 O T = ( TOT N ) × ( I H 2 O N ) k where C H 2 O T is the total water content (in wt%) dissolved in glass; TOT N represents the computed I LF/ I HF variation as a function of the calculated NBO/T and SM parameters; I H 2 O N is the H 2O band height scaled to ratio of the reference bands; k is the linearity spectrometer response on the H 2O T band in function of water content. The water concentrations of the reference glasses are reproduced using this equation with a standard deviation of 0.06 wt%. The adopted parameterisation provides a useful tool towards the characterisation of composition dependence of micro-Raman procedures for silicate glasses. We show, based on the widest range of glass compositions so far investigated, that accurate evaluation of dissolved water content is achieved by micro-Raman spectroscopy.

Micro-Raman determination of iron redox state in dry natural glasses: Application to peralkaline rhyolites and basalts

The position, scaled intensity and topology of the Raman stretching envelope at ~ 1000 cm − 1 (HF) have been established in complex anhydrous glasses having variable Fe 3+/ΣFe ratios and contrasting compositions of peralkaline rhyolite (pantellerite; FeO T: 7.8 wt.%), iron-poor and iron-rich basalts (FeO T: 7.6 and 10.1 wt.%). Sensitivity of glass structure to iron redox decreases with increasing glass basicity. Raman spectra of pantellerite glasses (Fe 3+/ΣFe: 0.04–0.97) evidence the sharp increase of a band at ~ 970 cm − 1 with the progressive entry of 4-fold Fe 3+ in alkali-bonded Q 3 units. The spectral evolution of basalts with increasing iron oxidation (Fe 3+/ΣFe: 0.05–0.87) is more subtle and is likely determined by formation of less polymerized units. The moderate to low influence of redox state changes on the structure of natural basalt glass, compared with the high sensitivity of their synthetic analogues, can be attributed to strong competition between network-forming cations (Al, Fe) in the absence of an excess of alkalis. We demonstrate that Raman calibration against reference glasses allows precise determination of Fe 2+/Fe 3+ratio but is highly composition dependent. As a consequence of the variable link existing between Fe 2+/Fe 3+ and glass polymerization, specific calibration procedures based on deconvolution, shift or intensity of HF band are required for compositions with high (acid glasses), moderate or poor (basic glasses) sensitivity to iron redox changes, respectively.

Effect of current density on the electron microprobe analysis of alkali aluminosilicate glasses

In a previous work (Morgan and London 1996), we proposed an optimized procedure for electron microprobe analysis (EMPA) of rhyolitic glasses using a broad (20 μm diameter), low-current (2 nA) fixed beam. Some important applications for EMPA of glass, such as vitreous inclusions in minerals and experimental run products, require smaller beam diameters that produce greater areal current densities (expressed as nA/μm 2 ). For these situations, we have assessed the effect of areal current density on the migration of Na and its concomitant effects on other elements and their ratios during EMPA of granitic glasses. Anhydrous and hydrous glasses of a haplogranite composition (Ab 38.23 Or 29.31 Qtz 33.37 C 0.10 ) were analyzed at 20 kV accelerating potential, using 2.50 nA beam currents, fixed beam diameters of 2.20 μm, and counting times scaled to yield similar analytical uncertainty at each condition (~2.6% relative for Na 2 O). There is almost no loss of Na (≤1.7.2.7% relative) using a current density of 0.006 nA/μm 2 , minor (7.9%) Na loss for current densities up to 0.1 nA/μm 2 , and increasing Na loss with higher current densities that becomes severe at >0.5 nA/μm 2 (e.g., 48.63% relative loss from hydrous glass at 50 nA and 2 μm during 3.6 s of irradiation). Sodium migration is more pronounced in hydrous glasses than in anhydrous ones, with significant loss from hydrous glass occurring during the first second of irradiation. The migration of Na results in increased concentrations of Al and Si, but little or no change in the concentration of K; if not fully corrected for, these effects produce systematic errors in important elemental ratios. With current densities <0.01 nA/μm 2 , anhydrous glasses or crystalline materials are suitable standards and data correction may not be needed. Significant Na loss using current densities up to ~0.1.0.2 nA/μm2, especially in hydrous glasses, requires data correction or primary standardization utilizing a glass having composition and water content similar to the unknown. Current densities ≥0.5.1.0 nA/μm 2 are not suitable for EMPA of glass because of large and uncertain corrections (~25% to >100% of the Na 2 O value obtained). The correlation of analytical condition (beam current and diameter) with current density and EMPA results provided here allows analysts to select beam conditions that optimize the quality of analyses. When current densities >~ 0.01 nA/μm 2 must be used (e.g., with beam spot sizes <20 μm), the results can lead to improved estimates of the systematic errors due to alkali migration. Natural and some experimental glasses contain a variety of other minor components among which Ca and Fe are important, and so the discussion of analytical methods is extended to more complex compositions. For example, Na migration is accelerated as glass structures become less polymerized than those of simple tectosilicate stoichiometry (e.g., due to increasing alkalinity and/or the presence of fluxing components such as F, Cl, B). Analysis using 20 kV accelerating voltage, as opposed to 15 kV, both slightly decreases Na migration and improves limits of detection and statistical accuracies for minor components such as Fe while providing reasonable beam penetration depths.

Structural implications of water and boron dissolution in albite glass

A combined nuclear magnetic resonance, infrared and Raman spectroscopic study on the effect of water dissolution on the structure of B-bearing aluminosilicate glasses is presented. The base composition was albite (NaAlSi 3O 8) to which different amounts of B 2O 3 (4.8, 9.1, 16.7 wt%) were added. Hydrous glasses containing 4.4 ± 0.1 wt% water were synthesized at pressures of 2000 bar. The results show that B dissolves in both dry and hydrous glasses by forming predominantly trigonal BO 3 groups although some tetrahedral BO 4 is also present. In anhydrous glasses prepared at high pressures (above 10 kbar) the fraction of BO 4 increased. The hydrous glasses contain more BO 4 groups compared to the dry counterparts, suggesting that this species is stabilized by water. The Raman and NMR ( 17O, 27Al, 29Si) spectra show that B interacts with the aluminosilicate network by formation of Si–O–B and probably Al–O–B units. In the hydrous glasses the water speciation changes significantly towards higher hydroxyl concentrations with increasing B-content. The NIR peaks, which are related to OH groups and molecular H 2O, develop additional shoulders, suggesting that possibly B–OH complexes are formed.

On the glass formation in the Na2O-SiO2-H2O system

The glass formation in the Na2O–SiO2–H2O system is considered in the framework of the free volume theory of glasses. The boundaries of the glass transition range in the given system are determined from the dependences of the specific volume and the degree of connectivity on the water content in the system. The differences in the structure of hydrated glasses prepared by different methods are revealed. It is demonstrated that the molecular packing coefficients of the structures formed by drying of sodium silicate solutions are smaller than those of glasses prepared by hydration of anhydrous glasses. The densities are determined and the molar volumes are calculated for hydrated glasses in the Na2O–SiO2–H2O system with a water content of 12–23 wt % (33–51 mol %) at a constant molar ratio SiO2 : Na2O = 2.8. The correlation relationship for the partial molar volume of water as a component of hydrated glasses is proposed as a function of the water content in the system. The use of this dependence allows one to calculate the molar volumes (densities) of hydrated glasses with different water contents.

Liquid Crystal Structures in a Vitreous Acetates of Alkali Metals

The transformation of anhydrous melt glasses of alkali metals acetates in nematic and smectic LC-mesophases have been studied. A change of electric conductivity of samples over the phase transformation has been observed. Textures corresponding to the nematic and smectic mesophases were revealed with the use of polarizing microscopy. It was studied the influence of external pulse unidirectional electric field at the mesophases formation. The role of LC-state as the fundamental energetic level over the going from the melt to the crystalline phase is discussed.

Solution mechanisms of phosphorus in quenched hydrous and anhydrous granitic glass as a function of peraluminosity

Solution mechanisms of P in metaluminous to peraluminous quenched, hydrous (≈9 wt% H 2O) and anhydrous glasses in the system CaONa 2OK 2OAl 2O 3SiO 2P 2O 5 have been examined with microRaman spectroscopy. The principal aim was to examine relative stability of phosphate complexes as a function of bulk chemical composition. Increasing peraluminosity was accomplished by increasing Al 3+ and Ca 2+ proportions with constant SiO 2 content. The molar ratio Al 2O 3/(CaO + Na 2O + K 2O) (A/ CNK) ranged from 1 (metaluminous) to ∼1.3 (peraluminous). In all compositions P 5+ is bonded to Al 3+ to form AIPO 4 complexes. The principal solution mechanism is one where depolymerized species (Q 3), involving Al 3+ both within and outside the aluminosilicate network, interact with P to form the AIPO 4 complex together with Q4 species. The mechanism does not involve alkali metals or alkaline earths. In anhydrous compositions, the spectra are interpreted to suggest SiOP cross-linking in the structure. In hydrous compositions, evidence for SiOP bonding is less evident. In such glasses, there is, however, possible spectroscopic evidence for SiOH bonding and possibly POH bonding resulting from breakage of cross-linking SiOP bonds existing in the anhydrous glasses. Therefore, the water content of peraluminous aluminosilicate melts is likely to affect the solubility behavior of P, and conversely, the solubility behavior of H 2O is affected by P in such melts.

Optimizing the electron microprobe analysis of hydrous alkali aluminosilicate glasses

The time-dependent loss of NaKa X-ray intensity during electron-beam irradiation of hydrous alkali alumino silicate glasses is apparently more significant during the initial few seconds of beam exposure than it is for anhydrous glasses, and it is pronounced for incident beam currents > 2-5 nA (using 15-20 /.Lmbeam diameters). Exponential fits of NaKa intensity vs. time show a progressive decrease in the apparent zero-time intercepts for incident beams from 2 to 20 nA, and thus methods for correcting Na concentrations solely on the basis of curve fitting and extrapolation to zero-time values may underestimate Na contents by almost 10% (relative) for higher beam currents. Similar exponential fits to the intensity-time data for AlKa and SiKa show that is greater for Al than for Si. For incident currents ~ 5 nA, the magnitudes of all intensity changes also increase with total H20 content of glass. On the basis of these observations, the optimal conditions for analysis of hydrous alkali aluminosilicate glasses include a 2 nA beam with 20 /.Lmdiameter and counting times of 20-40 s for the analysis of alkali aluminosilicate components, with Na and Al analyzed first (simultaneously, if possible). These methods minimize Na loss and grow-in for Al and Si to the point that little or no correction is needed, provide good statistical accuracy, and work with a wide variety of standard materials (i.e., glass standards with compositions and H20 contents comparable to the unknowns are not needed). For complete analysis of more complex multicomponent systems, two beam conditions are recommended: an initial 2 nA, 20 /.Lmdiameter beam for analysis of alkali alumino silicate components, followed by a 20 nA, 20 /.Lmdiameter beam for analysis of all other components. With the use of these methods, the H20 contents of hydrous glasses (H20 as the only unknown) can be determined by difference with uncertainties mostly 5 nA, corrections for Na loss ignoring Al (and Si) grow-in underestimate H20 contents by about 10-50% of concentration.

Effects of atmospheric humidity on the infrared reflectivity of vitreous P 2O 5 and ultraphosphate glasses

Nearly anhydrous vitreous P 2O 5 and ultraphosphate glasses with up to 15 mol% MeO (Me = Ca, Zn) were prepared. All glasses were melted in evacuated and sealed SiO 2 glass tubes at 1050°C for 2 h and were cooled by switching off the oven. The infrared spectroscopic properties of the nearly anhydrous glasses are given and the decomposition of the glasses in atmospheric humidity was followed by time-resolved measurements of the infrared reflectance spectra. The resistance of the ultraphosphate glasses to humidity is correlated with the type of interaction between the cation and the ultraphosphate network.

Water solubility mechanisms in KAlSi 3O 8 melts at high pressure

27A1, 29Si MAS NMR, Raman and IR spectroscopic techniques were employed to investigate water solubility mechanisms in KAlSi 3O 8 melt. Anhydrous glasses were produced by isobarically quenching melts at 0.1 MPa and 2 GPa. Hydrous glasses with 2.5 wt% [H 2O] tot were quenched isobarically from 5 and 7 GPa. Hydrous glasses with 2.5, 5.0 and 7.5 wt% [H 2O] tot were quenched isobarically at 2 GPa. A peak near 900 cm −1 appears in the Raman and IR spectra of all hydrous KAlSi 3O 8 glasses. The intensity of the 900 cm −1 peak relative to the intensity of the high-frequency envelopes remains constant for all of the hydrous 2 GPa glasses. The intensity of the 900 cm −1 peak decreases with pressure at constant [H 2O] tot. The 27Al resonance is at more positive parts per million in the hydrous 2 GPa glasses relative to the anhydrous 2 GPa sample. Hydrous glasses have narrower linewidths than the 1 atm and 2 GPa anhydrous glasses. We suggest that this spectroscopic information indicates two stages to the water dissolution mechanism. In the initial stage, we propose that water interacts with the aluminosilicate network to form Al-(OH) terminal bonds. At higher water contents (>25–30 mol% [H 2O] tot), the solubility mechanism occurs solely by the exchange of H + for M + (M = Na, K, Li) and the formation of partial hydration complexes around the alkali cation.

Glass Transition and Infrared Spectra of Low‐Alkali, Anhydrous Lithium Phosphate Glasses

Anhydrous glasses in the series xLi2O + (1 ‐ x)P2O5 have been prepared and characterized in the range 0 ≤x≤ 0.5. FT‐IR spectroscopy and glass transition temperature measurements have been used to explore the structure and a key physical property of the low‐alkali phosphate glasses. The structure of v‐P2O5 is proposed to consist of a 3‐D network of trigonally connected tetrahedra decorated with a P=O unit. Contrary to what has long been proposed for these glasses, the addition of alkali degrades the 3‐D network through the generation of nonbridging oxygens rather than strengthen the network through the proposed alkali ion bridging. The Tg of v‐P2O5 is ∼653 K and decreases some 130 K with the addition of 10 mol% Li2O. Tg then reaches a minimum value at 20 mol% Li2O and increases with further Li2O additions. The increase in Tg, even though the fraction of nonbridging oxygens is still increasing, is interpreted in terms of an increasing entanglement of long‐chain PO2 groups in the glass. Such a structural transition from a 3‐D network of interconnected PO4 groups for P2O5 to a 1‐D chain structure for LiPO3 is one of the first examples of the importance of intermediate‐range order in governing the properties of glass.

Fluorine in silicate glasses: A multinuclear nuclear magnetic resonance study

Anhydrous nepheline, jadeite, and albite glasses doped with F as well as hydrous F-containing haplogranitic glasses were investigated using 19F combined rotation and multiple-pulse spectroscopy; 19F → 29Si cross-polarization/magic angle spinning (MAS); and high-power 19F decoupled 29Si, 23Na, and 27Al MAS nuclear magnetic resonance methods. Fluorine preferentially coordinates with Al to form octahedral AlF 6 3− complexes in all glasses studied. In addition, F anions bridging two Al cations, units containing octahedral Al coordinated by both O and F, or tetrahedral Al-F complexes might be present. The presence of Si-F bonds cannot be entirely ruled out but appears unlikely on the basis of the 19F → 29Si CP/MAS spectra. There is no evidence for any significant coordination of F with alkalis in the glasses studied. 23Na spectra are identical for the samples and their F-free equivalents and the spectra do not change upon decoupling of 19F. The speciation of F in the hydrous and anhydrous glasses appears to be very similar. Over the range of F contents studied ( up to 5 wt.% ), there seems to be hardly any dependence of F speciation on the concentration of F in the samples. The spectroscopic results explain the decrease of the viscosity of silicate melts with increasing F content by removal of Al from bridging AlO 4-units due to complexing with F, which causes depolymerization of the melt. The same mechanism can account for the shift of the eutectic point in the haplogranite system to more feldspar-rich compositions with increasing F content, and for the peraluminous composition of most F-rich granites. Liquid immiscibility in F-rich granitic melts might be caused by formation of (Na,K) 3AlF 6 units in the melt with little or no interaction with the silicate component. The presence of F in granitic melts might increase the solubility of high field strength cations by making nonbridging O atoms available which form complexes with these cations.

Stable silicate glasses containing up to 10 weight percent of water

Alkali silicate glasses containing other metal oxides such as PbO, ZnO, Al 2O 3, BaO, MgO, P 2O 5, B 2O 3, and ZrO 2 in amounts of 0–20 mol% total are hydrated in a steam atmosphere at elevated temperatures resulting in a water content of 3–30% by weight. Various species of water are shown to exist in the hydrated glasses. They consist of the free silanol groups, hydrogen-bonded silanol groups, monomeric molecular water and polymeric species of molecular water up to hexamer. These species of water are in true solution in the homogeneously hydrated silicate glasses. Four types of hydration kinetics are distinguished among the silicate glasses of various compositions. The properties of the resulting products are compared and the origin of the differences is discussed. The properties of hydrated glasses are studied with measurements of infrared absorption spectra, thermogravimetric analysis, durability, viscosity and stress-strain behavior. The relative polarizing power of cations in silicate glasses are calculable from infrared spectra. The overall polarizing power of all the cations in a silicate glass is proportional to the fraction of free silanol groups in the glass and is a function of the glass composition. If reflects the extent of the chemical reaction and chemical binding of the various species of water in the glass, and offers a clue in synthesizing hydrated glasses with no loosely bonded molecular water. Theories are proposed to explain various observed phenomena and properties of the hydrated glasses. The structure cohesion of hydrated glasses is distinguished from those of the anhydrous glasses, invert glasses and organic polymers. A flow mechanism is proposed to explain the lowering of viscosity due to the presence of water in glass. A dynamic structure of the bonding of water in glass is deduced from the combined data (infrared and others) of the present study. With this model, the unusual combination of properties may be understood.