Network-forming Cations Research Articles

29Si MAS–NMR studies of Q n structural units in metasilicate glasses and their nucleating ability

The purpose of this work is to verify the possible existence of a relationship between the similarity of the local structure of the network-forming cation Si 4+ (Q n units and chemical shifts) in glasses and isochemical crystals and the nucleating ability of these glasses. Four metasilicate glasses with widely different volume nucleation rates: Na 2Ca 2Si 3O 9 and Na 4CaSi 3O 9 (very large), CaSiO 3 (intermediate) and CaMgSi 2O 6 (undetectably small) were chosen. We present magic angle spinning nuclear magnetic resonance spectroscopy (MAS–NMR) data for Na 2Ca 2Si 3O 9 and Na 4CaSi 3O 9 glasses and for their respective isochemical crystalline phases for the first time. Additionally, we repeat NMR measurements of glasses and crystals previously studied by other authors (CaSiO 3 and CaMgSi 2O 6) to test the consistency of our experimental techniques and method of analysis. Different central chemical shifts of Q 2 resonances in parent glasses and their isochemical crystals were measured, indicating structural differences. The relative amount of Q n groups in each glass was obtained from the deconvolution of the 29Si MAS–NMR spectra. The shape of the Q n distribution for each system was considered as a measure of the similarity of the connectivities of SiO 4 tetrahedra in each glass with respect to its isochemical crystal (which has only Q 2 groups). A correlation was found between the shape of the Q n distribution and the nucleation tendency of these glasses, indicating that similarities between the tetrahedra connectivities in glass and isochemical crystal has a role in determining the internal nucleation tendency of the metasilicate glasses studied.

Carbon dioxide mediated dissolution of Ca-feldspar: implications for silicate weathering

Experimental studies on the dissolution kinetics of anorthite under N 2(g) and CO 2(g) atmospheres at 25°C in electrolyte solutions, pH range 5.5 < pH < 8.5, have been performed using laboratory flow-through reactors. Aluminum release from anorthite is accelerated in the neutral to near-basic pH region. Because Al is the slowest dissolving network-forming cation under these conditions, we propose that accelerated Al release corresponds to a long-term acceleration of anorthite dissolution. The rate of Al release from anorthite correlates with solution concentration of carbonate ion according to a fractional-order empirical rate law; k = (1.1 ± 1) × 10 −7 mol 0.76 m −152 h −1: [Display omitted] We propose a two-step reaction mechanism where inorganic carbon is rapidly adsorbed, forming a reactive bi-dentate surface Al–carbonate complex that is released to solution in a much slower, irreversible step. The rate expression formally derived from this dissolution mechanism is consistent with the observed dependence of Al release rate on the master variables PCO 2 and pH. A comparison with published data on the weathering kinetics of plagioclase in solutions of oxalic acid shows that the reactivity of carbonate is similar to that of the organic ligand oxalate. Because of pH effects on the speciation of these ligands in solution and on mineral surfaces, carbonate promoted weathering is especially important at near-neutral and basic pH. We propose that the generally termed “carbonation weathering”, the effect of elevated subsurface PCO 2 to accelerate rock weathering through suppression of pH, be extended to include the speciation and reactivity of inorganic carbon at neutral and near-basic pH.

Multiple‐Quantum Magic‐Angle Spinning 17O NMR Studies of Borate, Borosilicate, and Boroaluminate Glasses

Multiple‐quantum magic‐angle spinning (MQMAS) 17O NMR spectroscopy has been applied to study boron oxide, sodium borate, sodium potassium borosilicate, and sodium boroaluminate glasses. Up to eight distinct oxygen sites are identified, with the chemical shift and quadrupolar coupling parameters determined for each. Previous assignment of the B‐O‐B and Si‐O‐B resonances has been found to be incorrect. In contrast to standard models of glass structure, three‐coordinated boron mixes with the silicon oxide network to a great extent. Sodium borosilicate glasses with low‐sodium content are likely to be phase‐separated on the nanoscale. Those with intermediate sodium content form homogeneous glasses with boron atoms distributed evenly in the SiO2 network. A boron avoidance rule analogous to the aluminum avoidance rule may apply in this region, when BO4 groups are abundant. Adding excess sodium cations to the system may again lead to compositional heterogeneity. The existence of diborate group as the basic building unit in glasses with appropriate sodium content is not well supported. The network‐forming cations in sodium boroaluminate glasses are well mixed, with no Al‐O‐Al resonance observed by MQMAS. The effect of cation type and thermal history on glass structures is also discussed.

Phosphorus Diffusion in Silicon Oxide and Oxynitride Gate Dielectrics

Evidence is presented that silicon oxynitride gate dielectrics suppress phosphorus diffusion, as compared to pure silicon dioxide dielectrics. Furthermore, the implantation of fluorine into the polycrystalline silicon gate enhances phosphorus diffusion. Both effects are similar to what has been observed with boron diffusion in silicon oxide and oxynitride. These results suggest a general model for diffusion in oxynitrides, in which network-forming cations (A = B, P, As, Si, Ge) diffuse substitutionally for Si as AOx, and the role of nitrogen is to block diffusion by impeding the rearrangement of SiO4 tetrahedra. © 1999 The Electrochemical Society. S1099-0062(99)04-105-X. All rights reserved.

Elastic properties of aluminate glasses via Brillouin spectroscopy

Sound wave velocities have been measured for CaAl 2O 4 glass and for two compositions along the Y 2O 3–Al 2O 3 join by Brillouin scattering spectroscopy. The sound speeds in these aluminate glasses range from 6.96 ± 0.16 km/s ( v L ) and 3.81 ± 0.05 km/s ( v T ). The corresponding bulk moduli are K=79 GPa for CaAl 2O 4 glass, and 113 ± 5 GPa for the Y 2O 3–Al 2O 3 glasses. The latter moduli are much larger than for most silicate or aluminosilicate glasses. The low compressibility is due to the lack of traditional network-forming cations in the yttrium aluminate glass so that all Al–O and Y–O bonds contribute equally in resisting compression.

Trace element diffusion in jadeite and diopside melts at high pressures and its geochemical implication

Diffusivities of geochemically important trace elements (eleven rare earth elements (REE), Rb, Sr, Ba, and Y) in jadeite and diopside melts and those of Zr, Nb, Th, and U in jadeite melt have been measured at pressures between 7.5 and 20 kbar and at temperatures 50–200° above the liquidus, using diffusion couples and ion microprobe analysis. The concentrations of these elements in the experimental charges are close to those in natural igneous rocks. In the jadeite melt which is nearly fully polymerized (NBO/T ∼ 0), (1) diffusivities of REE increase with increasing ionic radii, (2) diffusivities of tri- and tetravalent elements increase with increasing pressure at constant temperature, whereas those of mono- and divalent elements do not change significantly with pressure, and (3) diffusivities of these elements decrease with increasing their ionic charge at constant pressure and temperature. In the diopside melt which is considerably less polymerized (NBO/T ∼ 2), (1) diffusivities of these trace elements depend mainly on their ionic radii rather than ionic charges; the diffusivities of mono- and divalent ions decrease with increasing ionic radii at constant pressure and temperature, and (2) diffusivities of REE are nearly the same as that of Ca and decrease with increasing pressure at constant temperature. The behavior of these trace elements is correlated with that of major elements; in the jadeite melt, tri- and tetravalent elements behave similarly to network-forming cations Al and Si, whereas mono- and divalent elements behave as network-modifying cations similarly to Na. In the diopside melt, the diffusion behavior of all these trace elements is similar to that of network-modifying cations Ca and Mg. The results of the present experiments suggest that the abundance of some trace elements in igneous rocks may have been affected by diffusion process at the magmatic stage. In the case of REE, for example, if two different magmas with high and low REE concentrations become in contact one another by multiple intrusion, and a zoned magma chamber is formed, diffusion begins to take place between them, and near the interface, the REE-enriched magma will become more light REE-depleted, whereas REE-depleted magma will become more light REE-enriched, and in addition, if magmas are reduced, the former will show a negative Eu anomaly, whereas the latter will show a positive Eu anomaly.

Disorder among network-modifier cations in silicate glasses; new constraints from triple-quantum17O NMR

The state of ordering among network-modifier cations in molten silicates has a potentially large effect on their overall configurational entropy, on free energies of mixing, and on viscosity and diffusivity. Most models of thermodynamic and transport properties assume random mixing, but there is relatively little direct microstructural information to constrain the real extent of this disorder. Two-dimensional 17 O NMR can produce spectra that are free of quadrupolar broadening and thus in which peak widths for non-bridging oxygen sites directly reflect the extent of disorder in the local structural environment. In this report, we describe new data from triple-quantum magic-angle-spinning (3QMAS) NMR for a series of barium and calcium silicate glasses. Results are best explained by completely random mixing of Ba and Ca, confirming conventional modeling assumptions. Other recent data show, however, that significant ordering may be present (at least at the temperature of the glass transition) for modifier cations with greater differences in size or charge, and among network-forming cations.

Diffusion in silicate melts: I. Self diffusion in CaO[sbnd]Al 20 3[sbnd]SiO 2 at 1500°C and 1 GPa

Self diffusion coefficients of calcium (DCa), aluminum (DAl), silicon (DSi), and oxygen (DO) were measured in molten CaOAl 20 3SiO 2 at 1500°C and 1 GPa over a range of melt compositions, using the isotope tracer method. For all but one composition, the measured self diffusion coefficients decrease in the order DCa > D Al > D O > D Si, with D Ca ∼ 4–19 D Si , D Al ∼ 2D Si, and D O ∼ 1–2 D Si. The relative uncertainties, based on replicated experiments, are 8% for DCa, 27% for DAl, 28% for DSi, and 18% for DO. Although the self diffusion coefficients of calcium, aluminum, silicon, and oxygen increase with the decrease of melt viscosity, they do not obey the Stokes-Einstein equation or the Eyring equation. The compositional dependence of the self diffusion coefficients of calcium, aluminum, and oxygen were parameterized in terms of melt viscosity (Pa · s) via the power law relation: DCa = (115.141/viscosity 0.389) × 10 −12m 2s −1DA1 = (47.302/viscosity 0.800) × 10 −12m 2s −1DO = (33.080/viscosity 0.722) × 10 −12m 2s −1The compositional dependence of the self diffusion coefficients of Si was parameterized in terms of the degree of melt polymerization (NBO/T) DSi = [0.669 exp(4.195 NBO/T)] × 10 −12m 2s −1There is an abrupt change in the diffusion behavior of Al and Si between melts of more and less polymerized compositions, occurring at a NBO/T value of approximately 0.6, which may be indicative of a major change in melt structure. The hypothesized structural transition, the nature of which remains to be identified, may control other aspects of diffusion such as the sign of the activation volume for self diffusion of network-forming cations and the magnitude of the activation energy.The self diffusion coefficients of calcium, aluminum, silicon, and oxygen were used to calculate electrical conductivities of molten CaOAl 2O 3SiO 2 at 1500°C and 1 GPa using the generalized NernstEinstein relation. The estimated electrical conductivity implies that calcium, silicon, and oxygen contribute nearly equally to the total conductance: 31 % from calcium, 29% from silicon, and 30% from oxygen, which is quite different from what one would expect for alkali-bearing melts in which Na and K are believed to be the dominant conducting species.

Coordination chemistry of Ti(IV) in silicate glasses and melts: II. Glasses at ambient temperature and pressure

The coordination environment of Ti(IV) in a number of Ti-silicate and Ti-aluminosilicate glasses has been determined by x-ray absorption fine structure (XAFS) spectroscopy at the Ti K-edge at ambient temperature and pressure. These glasses contain 2.7–30.5 wt% TiO 2 and varying amounts of Na 2O, K 2O, or CaO (5.0–38.7 wt%) and Al 2O 3 (0–11.9 wt%), and have NBO/T ratios ranging from 0.07–0.81. Quantitative analysis of the Ti XANES spectra, based on ab initio multiple-scattering calculations for a variety of Ti-containing clusters, and anharmonic analysis of the normalized XAFS oscillations suggest the presence of three types of atoms around Ti: O first neighbors, (Si, Ti)-second neighbors, and alkali third neighbors. Five-coordinated Ti, [5]Ti, is the dominant Ti species in the glasses most concentrated in Ti (> 16 wt% TiO 2) and is located in distorted square pyramids ( [5]TiO)O 4), with one short Ti O titanyl distance (1.67–1.70 ± 0.03Å) and four long Ti O distances (1.94–1.95 ± 0.02Å). In addition, minor amounts of [4]Ti were detected, the proportion of [4]Ti increasing in the order: Na glasses < K glasses. [4]Ti is the dominant Ti species in the potassic glasses with the lowest TiO 2 contents (≈3–6 wt%) and highest NBO/T ratio. The relative amount of [4]Ti increases in the order: Ca glass < K glass. Finally, [6]Ti is a minor species (< 20%) when detected in these glasses. The presence of Ti-(Si, Ti) correlations near 3.2–3.4 ± 0.1Å, as in crystalline Na 2( [5]TiO)SiO 4, is consistent with [5]TiO 5 and SiO 4/TiO 5 polyhedra sharing corners in these glasses, with Ti O-(Si, Ti) angles of ≈120°–130° ± 10°. Quantitative analysis of the Ti K-edge XANES for the K-bearing glasses suggests the presence K around Ti, in good agreement with bond-valence predictions, which indicate that [5]Ti is most likely to bond to both nonbridging oxygens (one O in short Ti O titanyl bonds) and bridging oxygens (four O in long Ti O bonds), thus can act as a new type of Q 4 specie with one additional nonbridging oxygen. Then, we propose [5]Ti to behave simultaneously a network former and a network modifier, with the network former role dominant. Bond valence models explain why the relative proportions of [4]Ti and [5]Ti change when the type of low field strength cation or the type of network-forming cation (Si vs. P) changes in oxide glasses. These models also provide a structural basis for the study of glasses and melts at higher temperatures (see Part III of this study).

Diffusion of silicon and gallium (as an analogue for aluminum) network-forming cations and their relationship to viscosity in albite melt

Self-diffusion of Si and tracer diffusion of Ga have been measured in an albitic melt, Na0.97Al0.99Si3.02O8, at 1 atmosphere between 1160 and 1558°C. Diffusion of these network-forming elements was studied for comparison with viscous transport and Nuclear Magnetic Resonance (NMR) relaxation parameters measured in albite melts and glasses. Gallium was used as an analogue for Al and estimated diffusion parameters for both Ga and Al are presumed to be equivalent. Diffusion can be described by Arrhenius lines for both Si and Ga (with 1-standard errors): +6.58 × 10−3Dsi = 1.37 × 10−4exp(−337.4 ± 52.6/RT)−1.35 × 10−4 +7.97 DAl ∼ DGa = 3.55 × 10−1 exp (−424.6 ± 42.5 /RT)−3.39 × 10−1, where diffusivities are in m2s−1 and activation energies are in kJ/mol. Comparison of Si and Ga diffusivities with Si/Al diffusivities in crystalline albite indicates relatively small differences between activation energies for diffusion of 3+ and 4+ network-forming cations in crystalline and in molten albite. Apparently, the energy barrier to diffusion of these network-forming cations is influenced significantly only by nearest neighbours, and is not strongly affected by changing the dominant structure from four-membered rings of (Si, Al)O4 tetrahedra in crystalline albite to three-, four-, and six-membered rings of tetrahedra in molten albite. Comparison of Na diffusion in albite crystals and glass demonstrates significant differences in activation energies for diffusion. The behaviour of oxygen diffusion appears to be intermediate between Na and network-forming cations, however, this observation is based upon limited oxygen diffusion data for rhyolitic, not albitic, melts. Activation energies for Ga diffusion in albite melt are similar to those measured in rhyolite melt at 1.0 GPa. The silicon activation energy for diffusion is three times higher in albite melt than in rhyolite melt at 1.0 GPa and does not correlate with the activation energy for the inverse of the NMR spin-lattice relaxation. The differences between Si activation energies in albite and rhyolite melts are suggested to be due to the presence of a mixed alkali-like effect in the latter melt. Calculated diffusivities based upon measured viscosities for albite melt and the Eyring equation agree almost within error with Ga diffusivities, but are typically 3–10 times greater than Si diffusivities. This correlation suggests that diffusion of Al in albite melt controls viscous transport. The activation energy for oxygen diffusion in crystalline albite and the estimated activation energy for oxygen diffusion in the melt are much lower than the activation energy for viscous transport and argue against a relationship between oxygen diffusion and viscous transport in this aluminosilicate melt. The more rapid diffusion of Ga and A1 compared to Si in albite melt is proposed to be due to the more frequent formation of five- or six-coordinated Ga-O and Al-O transition state complexes than similar Si-O complexes. The previously observed relationship between viscosity and chemical diffusivities during interdiffusion (Baker, 1992b) suggests that diffusion of Al also controls chemical diffusion in basaltic to rhyolitic melts.

Influence of network-forming cations on ionic conduction in sodium silicate glasses

Abstract The substitution of Si by Al or B in Na2O · 3SiO2 glass has different effects on ionic conductivity. Whereas the activation energy of ionic conduction Eσ, decreases with the amount of Al in sodium aluminosilicate (SAS) glasses, it shows a shallow minimum with respect to B in sodium borosilicate (SBS) glasses. This difference is explained in terms of the strain energy, ΔEs, and bond energy, ΔEb, components of Eσ, which are deduced from recent studies of chemical bonding and local structure of these glasses. It is found that ΔEs increases and ΔEb decreases with increasing Al and B in bond SAS and SBS glasses. The theory of polar covalence has been applied to explain the variation of ΔEb in both glass systems. The dielectric constant is important for evaluating the ionic part of the bond energy, rather than to account for the partially covalent character of the bonds.

Structural study of Rb and (Rb,Ag) germanate glasses by EXAFS and XPS

Abstract The interatomic distance, R , coordination number, CN, and degree of disorder, Δσ 2 , around the mobile (Rb,Ag) as well as the network-forming (Ge) cations are obtained using EXAFS (extended X-ray absorption fine structure) of x Rb 2 O·(1− x )GeO 2 and mixed rubidium-silver germanate 0.2[ y Ag·(1− y )Rb] 2 O·0.8GeO 2 glasses with x = 0.01, 0.02, 0.05, 0.10, 0.15, 0.20, 0.30, 0.40 and y = 0, 0.25, 0.50, 0.75, 1.00. The non-bridging oxygen (NBO) and bridging oxygen (BO) concentrations are determined by X-ray photoelectron spectroscopy. With the addition of Rb 2 O, both GeO 6 and NBOs are produced for all x ≤ 0.2. For x > 0.2, NBOs continue to increase at the expense of GeO 6 units. A comparison of molar volume with various bond lengths indicates the presence of an ‘unoccupied volume’ which may vary with composition without affecting various interatomic distances. Replacement of Rb by Ag in mixed germanate glasses has the same effect on R Rb−O and Δσ Rb −O 2 as the reduction of Rb content in single Rb germanate glasses. The R Ag−O and Δσ Ag−O 2 show a trend opposite to that of the bond around Rb when Ag and Rb replace each other.

Color and Selected Properties of PbO─BiO1.5─GaO1.5 Glasses

The density; molar volume; thermal expansion coefficient; dissolution rate in water, HC1, and NaOH; glass transition and crystallization temperatures; and the absorption edge in the ultraviolet‐visible and infrared were measured for PbO─BiO1.5─GaO1.5 glasses. The range of compositions investigated was xPbO (100 −(x + y))BiO1.5. yGaO1.5 for x between 20 and 60 cat% and y of 20, 25, 30, and 35 cat%. The glass‐forming tendency increased with increased GaO1.5 and decreased with increased PbO or BiO1.5. The compositional dependence of these properties was consistent with the weight, size, charge, and bond strength of the cations. The Ga3 + ions in these glasses are believed to act primarily as network‐forming cations, whereas the majority of the Bi3+ and Pb2+ ions behave as network‐modifying cations. It is suggested that a small friction of the lead ions are present as Pb4+. Depending upon melting conditions, these glasses ranged in color from brown to yellow. Various attempts, including containerless melting, were made to obtain colorless glasses, but no conditions were found which totally eliminated the color. The least color (pale yellow) was obtained when the glasses were melted in an air or nitrogen atmosphere in an alumina or gold crucible.

Solution properties of rare earth elements in silicate melts: Inferences from immiscible liquids

The partitioning behavior of rare earth elements (REE) between immiscible silicate liquids was investigated as a function of temperature and total REE oxide concentration. REE preferentially partition into the end-member liquids that are rich in network-modifying cations, with two-liquid partition coefficients greater than any other cation. The two-liquid partition coefficients of REE in a given experiment are essentially identical to one another. No evidence was seen to suggest that the solution mechanisms of REE in high-silica liquids vary with differences in their ionic radii. “Two-lattice” models of trace element solution behavior fail to remove the compositional dependence of REE two-liquid partition coefficients due to the assumption that all network-forming and network-modifying cations mix ideally with one another. As a result, it is probably incorrect to derive physical interpretations from the results of the application of two-lattice models. The results are applied to the origins of the LREE-enriched phenocryst-matrix partition coefficients seen for some ferromagnesian minerals in high-silica rhyolites. Since liquids with extremely high concentrations of T-O-T linkages do not strongly discriminate between the REE on the basis of their ionic radii, the anomalous REE crystal-matrix partition coefficient patterns are best explained as the result of contamination of these minerals by LREE accessory phases.

Raman study of some melilites in crystalline and glassy states

Raman spectra are reported for crystalline akermanite (Ak, Ca2MgSi2O7), hardystonite (Har, Ca2ZnSi2O7), gehlenite [Geh, Ca2Al(AlSi)O7], sodium melilite (SM, CaNaAlSi2O7) and for glasses of corresponding compositions. The spectra of melilites are dominated by the vibrational modes of pyrosilicate (T2O7) units (T = Si or Al) and not by the sheet-like structure formed by interconnected TMO4 tetrahedra (M = Mg, Zn and Al). The frequency of vs(T-O-T), t symmetric stretching mode of the bridging oxygen in the pyrosilicate unit, is directly related to the angle of the T-O-T linkage. The symmetric stretching bands of nonbridging oxygens vs(T-O−) appear in the spectral range characteristic of T-O− stretching in pyrosilicate units. The intensity of the vs(T-O−) band is, however, affected by th presence of A13+ in tetrahedral sites adjacent to the pyrosilicate units. The lowering of the intensities of nonbridging oxygen stretching bands in the spectra of Geh and SM is attributed to a change in the degree of covalency of T-O− bonds in the pyrosilicate unit resulting from substitution of A13+ for Mg2+ in the adjacent tetrahedral sites. Comparison of the spectra of crystals with glasses indicates that most Al3+ ions act as network-forming cations, whereas Mg2+, Zn2+ and Ca2+ retain their role as network modifiers. Spectra of glasses of Ak and Har composition show SiO4−4 and Si2O6−7 bands, indicating a redistribution of silicate species among monomer, dimer, trimer and tetramer chains. Glasses of Geh and SM compositions are more highly polymerized than their respective crystalline counterparts because of the role of Al3+ as network-forming cations in the glass structures.

An evaluation of individualized educational services for the prevention of industrial back injuries

Using synchrotron radiation-based analytical (SRXRF) and spectroscopic (XAS) non-destructive techniques, a study was carried out on lead-rich, tin-opacified yellow glazes (silica-lime-alkali type glasses) decorating ancient tiles (17–19th century).These glasses have a rather complex chemistry, being currently assumed that the yellow pigment used for centuries – a pyrochlore-type double oxide of lead and antimony – prevails within the glaze, despite Sb(3+) being recognized as a network-forming cation in glasses. Minerals and synthetics with known crystal structure were used as model compounds to interpret X-ray absorption spectroscopy data at Sb K-edge and Pb L3-edge collected from ancient glazes. Theoretical modelling of Sb 1s XANES spectra was applied to demonstrate that antimony alone may be responsible for the yellow colour through a finely dispersed pyrochlore-type Sb-oxide, while lead remains hosted in the glassy matrix.

Raman study of the structure of glasses along the join SiO 2GeO 2

In order to better understand the distribution of tetrahedra in multicomponent tetrahedral network structures of melts and glasses, we have investigated the Raman spectra of binary SiO 2GeO 2 glasses. We compare the Raman spectral features of the end-member glasses and discuss their vibrational origins. The mixing of GeO 2 and SiO 2 melts results in a continuous random network structure of TO 4 tetrahedra (T  Si, Ge) in the glass. Raman bands corresponding to the asymmetric stretch ( v as) of oxygen in GeOGe, SiOSi and SiOGe bonds are observed in the glasses having intermediate compositions along the SiO 2GeO 2 join. The presence of three distinct v as (TOT) bands in the spectrum of a glass having Si/Ge one reveals that a considerable degree of SiGe disorder exists in the glass. The presence of a single symmetric oxygen stretching band in the spectra of binary SiO 2GeO 2 glasses indicates that the symmetric stretch modes ( v s) of oxygen in SiOSi, SiOGe and GeOGe bonds are strongly coupled. An observed decrease in the halfwidth of the v s (TOT) band in the spectra of SiO 2GeO 2 glasses with increasing concentration of GeO 2 may be attributed to a decrease in the average TOT bond angle and a predominance of six-membered ring structures. Results of the present study support the assignment of the bands in the 900–1200 cm −1 region of the alumino-silicate glasses, spectra to the v as(AlOSi) and v as(SiOSi) modes. In contrast to the alumino-silicate glasses, however, the SiO 2GeO 2 glasses have a much higher degree of disorder of the network-forming cations.

Effect of pressure on the diffusivity of network-forming cations in melts of jadeitic compositions

The diffusivities of network-forming cations (Si 4+, Al 3+, Ge 4+ and Ga 3+) in melts of the jadeitic composition NaAl(Si, Ge) 2O 6 and Na(Al, Ga)Si 2O 6 have been measured at pressures between 6 and 20 kbar at 1400°C. The rates of interdiffusion of Si 4+-Ge 4+ and Al 3+-Ge 3+ increase with increasing pressure at constant temperature. The results are consistent with the ion-dynamics computer simulations of Jadeite melt by Angell et al. (1982, 1983). The coefficient measured for the Si 4+-Ge 4+ interdiffusion is between 8 × 10 −10 and 2.5 × 10 −8 cm 2 sec at 6 kbar, depending on the composition of the melt, whereas at 20 kbar it is between 7 × 10 −9 and 2 × 10 −7 cm 2 sec . The effect of pressure is greater for more Si-rich compositions ( i.e., closer to NaAlSi 2O 6 composition). The coefficient measured for the Al 3+-Ga 3+ inter- diffusion is between 9 × 10 −10 and 3 × 10 −9 cm 2/sec at 6 kbar and between 3 × 10 −9 and 1 × 10 −8 cm 2 sec at 20 kbar. The rate of increase in diffusivity with pressure of Al 3+-Ga 3+ (a factor of 3–4) is smaller than that of Si 4+-Ge 4+ (a factor of 7–17). The Si 4+-Ge 4+ interdiffusion in melts of Na 2O · 4(Si, Ge)O 2 composition has also been measured at 8 and 15 kbar for comparison. The effect of pressure on the diffusivity in this melt is significantly smaller than that for the jadeitic melts. The increase in diffusivity of the network-forming cations in jadeitic melts with increasing pressure may be related to the decrease in viscosity of the same melt. The present results, as well as the ion-dynamics simulations, suggest that the homogenization of partial melts and mixing of magmas would be more efficient at greater depths.

Composition—property relationships in inorganic oxide glasses

By regarding glasses as oxygen polymers with a cross-linked network structure that can have different packing densities and varying degrees of cross-linking according to which network-forming elements and cations are present, the way in which many of their properties change with composition can be explained, at least qualitatively, in terms of the corresponding changes in cross-link density and network packing. The packing of an oxide network is conveniently measured by the oxygen density of the glass, and this and the cross-link density are readily calculated from the composition and actual density.

ATOMIC CONSIDERATION OF IMMISCIBILITY IN GLASS SYSTEMS*

An attempt is made to apply the ideas of crystal chemistry to the problem of immiscibility in glass systems. Miscibility is favored by the tendency of the networkforming cations, Si, B, and P, to bond with all available oxygens in the melt. Immiscibility is favored in compositions which do not allow the other cations, such as Na+ and Ca++, to be properly surrounded by unsaturated oxygens. The importance of this effect may be related to the valence and size of the cation. A qualitative discussion is given for a number of binary and ternary systems.