Crystalline Albite Research Articles

Metasomatic Replacement of Albite in Nature and Experiments

Replacement of albite by sodium-rich, secondary phases is a common phenomenon, observed in different geological settings and commonly attributed to alkaline metasomatism. We investigated growth of nepheline and sodalite on albite in time series experiments between two and 14 days. A total of 42 hydrothermal experiments were performed in cold-seal hydrothermal vessels at a constant pressure of 4 kbar and 200–800 °C in the system SiO2–Al2O3–NaCl–H2O. To allow for fluid flow and material transport, a double-capsule technique was used; hereby, a perforated inner Pt capsule was filled with cleavage fragments of natural albite, whereas the shut outer Au capsule was filled with γ-Al2O3 and the NaCl–H2O solution. Complete overgrowth of albite by sodalite and nepheline occurred after just two days of experiments. At high salinity (≥17 wt % NaCl) sodalite is the stable reaction product over the whole temperature range whereas nepheline occurs at a lower relative bulk salinity than sodalite and is restricted to a high temperature of ≥700 °C. The transformation of albite starts along its grain margins, cracks or twin lamellae. Along the reaction front sodalite crystallizes as small euhedral and highly porous grains forming polycrystalline aggregates. Coarse sodalite dominates in the outermost domains of the reaction zones, suggesting recrystallization. Sodalite may contain fluid inclusions with trapped NaCl-rich brine, demonstrating that the interconnected microporosity provides excellent pathways for fluid-assisted material transport. Highly porous nepheline forms large, euhedral crystals with rectangular outline. Sodalite and nepheline in natural rock samples display only minor porosity but fluid and secondary mineral inclusions, pointing to coarsening of a previously present microporosity. The reaction interface between sodalite and albite in natural rock samples is marked by open channels in transmission electron microscopy. In many of the experiments, a zone of Si–H-rich, amorphous material is developed at the reaction front, which occurs at a temperature of up to of 750 °C as nanometer to 350 µm wide reaction zone around albite. This change in composition corresponds with the abrupt termination of the crystalline feldspar structure. The presence of sodalite as micro- to nanometer-sized, euhedral crystals within the amorphous zone demonstrates, that both the sodalite reaction rim and the amorphous material allow for fluid-assisted material transport between the crystalline albite (release of Si, Al) and the bulk fluid (H2O, Na, Cl). This texture, moreover, suggests that the amorphous phase represents a metastable interstage reaction product, which is progressively replaced by sodalite and nepheline. Remarkably, product sodalite, nepheline, and the amorphous material largely inherit the trace element budget of the respective ancestor albite, indicating that at least part of the trace elements remained fixed during the reaction process. The observed reaction textures in both natural and experimental samples indicate an interfacial dissolution–reprecipitation mechanism. Results of our study bear important implications with respect to mineral replacement in the presence of a fluid phase, especially regarding the interpretation of trace element patterns of the product phases.

Effect of local materials on the silver sorption and strength of ceramic water filters

In this paper, we present a systematic evaluation of the effects of local clays and the manufacturing process on the performance of ceramic water filters (CWFs) impregnated with silver compounds, which are used for point-of-use water treatment in developing countries. Mineral composition, silver sorption/desorption, and strength are the important characteristics that influence effectiveness and durability of CWFs during transport and use. Laboratory tests were conducted on ceramic samples obtained from five CWF factories around the world to determine their mineral composition, silver sorption/desorption, and flexural strength. The results of this study showed that clays that contain traces of crystalline albite or crystalline pyroxene have better sorption of silver species than those that do not. The results showed that the Freundlich model provided the best fit for both ionic silver and silver nanoparticles for all of the ceramic materials that were tested. Thus, this model can be used to optimize the manufacturing process and the application of silver. Silver nanoparticles were desorbed more slowly than ionic silver, so they last longer in the ceramic material. Water that contains a high concentration of divalent ions is not recommended for preparing solutions of silver nanoparticles due to aggregation of the particles, which limits their sorption by the ceramic materials. In this study, the mineralogy of the source materials was found to have the most significant influence on the strength of ceramic filters.

Fractionation of alkali elements during laser ablation ICP-MS analysis of silicate geological samples

Data on elemental fractionation of alkali elements during laser ablation ICP-MS analysis of silicate reference glasses (NIST-610, BCR-2G, alkali element-doped andesite glasses) and crystalline mineral albite (NaAlSi3O8) are reported. Laser ablation ICP-MS and SIMS were used to determine differences in sample composition before and after laser interaction with the samples. The fractionation trends of the alkali elements are different from those of other lithophile elements, such as Ca and the rare earth elements (REEs). The rate of fractionation varies for different sample matrices and for different alkali elements in the same matrix. Data from SIMS analyses of the ejecta blanket deposited adjacent to the laser ablation crater in silicate NIST-610 glass and crystalline albite suggest a matrix dependent fractionation of alkali elements for different particle size fractions in the ablated aerosol. The extent of fractionation varies for different alkali elements and is independent of their ionic radii. Ion probe depth profiling into the bottom of laser craters showed laser ablation-induced chemical changes in the sample that involved alkali elements and major matrix elements including Si and Ca. This suggests that a combination of thermally-driven diffusion and size-dependent particle fractionation are responsible for the observed fractionation of alkali elements during laser ablation of silicate samples.

Raman spectroscopy and vibrational analyses of albite: From 25 C through the melting temperature

Raman spectra were collected for crystalline albite from 25 °C to above the 1118 °C melting temperature, where vibrational assignments for the crystal spectra were determined by lattice dynamics (LD). The Raman spectra and associated vibrational assignments are reported for triclinic albite (NaAlSi3O8) at 25 °C and monoclinic albite at 1060 °C. The 25 °C calculations determined that localized T-O stretch and O-T-O bend modes are above 900 cm –1 (where T = Si,Al), while motions from the aluminosilicate tetrahedral cage mixed with Na displacements occur in modes as high as 814 cm –1 . Vibrational modes for the most prominent peaks in the spectrum, between 350 and 550 cm –1 , are dominated by four-membered tetrahedral ring deformations. For completeness, calculated infrared mode frequencies and their atomic displacements are reported for the 25 °C structure and compared with normal mode calculation results and observed infrared mode frequencies presented by von Stengel (1977). At higher temperatures, modes above 550 cm –1 broaden and shift to lower frequencies by 15 to 27 cm –1 ; modes below 550 cm –1 broaden, but experience little, if any frequency shifts. Albite melted sluggishly, was completely liquid at 1320 °C, and remained amorphous upon cooling to room temperature. At frequencies above 550 cm –1 , the crystalline albite peaks, and possibly their vibrational assignments, can be correlated to Raman bands for albite glass. Spectral differences below 550 cm –1 between crystal and glass correspond to changes of average tetrahedral ring type upon melting, as shown by Taylor and Brown (1979).

The Dissolution Behaviour of Crystalline and Amorphous Albite in Acidic and Basic Environments at 25°C

The dissolution behaviour of crystalline feldspar in the laboratory has been studied extensively for many years (Blnm and Stillings, 1995). From these studies, three well-known models evolved to describe the mechanisms of feldspar dissolution. The first model, which has generally been discredited, is based on diffusioncontrolled kinetics in which the feldspar dissolution rate is controlled by the diffusion of reaction products through an altered layer on the mineral surface. The second model is based on surface-reaction controlled kinetics in which reactions at the solid/solution interface control the rate of feldspar dissolution. A third model, which combines ideas from the other two models, is based on reactions at the surface and within the leached layer that control the overall dissolution behaviour of the feldspar (Hellmann, et al., 1990; Brantley and Stillings, 1996). The resistance to dissolution of some sodiumaluminosilicate glasses (including the albite composition) make them excellent matrices for the immobilization of radioactive fission products. Because the dissolution of crystalline albite is controlled by a surface-reaction mechanism, whereas the dissolution of albite glass is controlled by diffusion through an altered surface layer (Zellmer and White, 1986), comparisons of the kinetics of these two phases should shed light on the mechanism of dissolution. Few studies compare the dissolution kinetics of crystalline albite with amorphous albite and none of these studies were conducted at room temperature. Zellmer=s study indicates that the dissolution rate of albite glass is slightly faster than crystalline albite over a pH range of 2 -10 at 70~ (Zellmer and White, 1986). In this study, the leaching and dissolution behaviour of crystalline and amorphous albite was investigated in acid and base at 25~

Sodium Environments in Dry and Hydrous Albite Glasses: Improved 23Na Solid State NMR Data and Their Implications for Water Dissolution Mechanisms

The sodium environments in albite glasses with water concentrations ranging from 0 to 60 mol% were studied using 23Na off-resonance quadrupole nutation and magic angle spinning (MAS) NMR spectroscopy. Crystalline albite was used as a model compound to demonstrate that off-resonance nutation is a suitable method for determination of the quadrupole coupling constant (C q) for 23Na. Off-resonance nutation experiments gave a mean C q = 1.75 ± 0.2 MHz for all the albite glasses studied here. MAS NMR experiments were performed at three magnetic fields, 7.05 T, 9.4 T, and 14.1 T in order to deduce the mean isotropic chemical shift, δ iso, and to provide an independent measurement of the values of C q. The mean isotropic chemical shift is a strong function of dissolved water concentration, but the mean C q is essentially constant at 2.1-2.2 ± 0.2 MHz over the water concentration range studied. The distributions of both chemical shift and quadrupolar interactions decreases markedly with increasing water concentration, consistent with earlier suggestions that the hydrous glasses have a much more ordered structure. These new data using off-resonance nutation and faster MAS combined with higher applied magnetic fields supersede the 23Na NMR data of Kohn et al. 1989a and should be used in preference in devising or testing models for water dissolution mechanisms in albite melts and glasses. Our revised data provide no evidence for a change in water dissolution mechanism at 30 mol% H 2O, but the other conclusions of Kohn et al. 1989a and the principal features of the dissolution mechanism developed by Kohn et al. 1989a, Kohn et al. 1992, Kohn et al. 1994 are essentially unchanged.

Structural Study of Molten Albite (NaAlSi3O8) by the High Temperature Energy Dispersive X-ray Diffraction (EDXD) Method

The structure of molten albite (NaAlSi 3 O 8 ) at 1460K has been determined by the high temperature energy dispersive X-ray diffraction (EDXD) method, Tetrahedral local ordering structure of SiO 4 and AlO 4 is suggested as a fundamental local unit of molten albite. The linkage of this structural unit is also discussed in comparison with the structure of crystalline albite.

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.

Scanning calorimetric determinations of the heat contents of diopside-rich systems during melting and crystallization

The results of scanning calorimetric measurements on crystalline albite, diopside, diopsideanorthite, and diopside-wollastonite-enstatite are reported in the temperature range of 1300 K to 1775 K. The results for albite agree with those obtained previously using a drop-calorimetric technique and confirm that melting takes place during a 300 K temperature interval starting near the accepted equilibrium melting point of 1373 K. The heat content, H(T), of anorthite-diopside mixtures is calculated numerically based on existing thermodynamic data. The results are compared to measurements on a mechanical mixture of 10 mol% anorthite-90 mol% diopside containing 3% of second phase impurities. Due to the presence of these impurities, the onset of premelting or melting occurs at 150 K below the An-Di eutectic and the heat of fusion at the liquidus (1665 K) is 114 ± 6 kJ/mol, which is 6% less than calculated for the pure mixture. Measurements of H(T) on a sample of 13 mol% wollastonite-83 mol% diopside are also presented. The measured heat of fusion at 1665 K is 125 ± 7 kJ/mol, in excellent agreement with the value of 124.8 kJ/mol calculate from existing thermodynamic data. Our data place au upper limit of 7 kJ/mol on the heat of mixing in this system at 1665 K.

The albite-water system: Part II. The time-evolution of the stoichiometry of dissolution as a function of pH at 100, 200, and 300°C

Albite feldspar was hydrolyzed over a wide-range of pH conditions at 100, 200, and 300°C. The release rates of Na, Al, and Si were measured as a function of time from the initial, pre-steady-state phase to the attainment of steady-state, congruent dissolution conditions. Leached layers were developed during the initial stages of dissolution due to the preferential release of Na with respect to Al and Si at nearly all pH and temperature conditions. The preferential release of Na is due to the higher rate of ion exchange reactions vs. hydrolysis reactions associated with the release of Si and Al. The depths. of Na preferential leaching show a pH dependence. Maximum depths on the order of 1500 and 1200 Å were recorded at acid and basic pH conditions, respectively, whereas minimum depths were observed in the neutral pH range. Leached layers deficient in Al (with respect to Si) were recorded at acid and neutral pH conditions. At mildly basic pH conditions, Al and Si were congruently released, or alternatively, either Al or Si was preferentially released. At more extreme basic pH conditions, only Al was preferentially released. The depths of Al preferential leaching were not determinable under all conditions due to the precipitation of an Al surface phase; a maximum recorded depth of 250 Å was determined at basic pH conditions. The preferential release behavior of Al and Si is ascribed to the pH dependency of the speciation of AlOH and SiOH groups at the surface and within the leached layers. However, at very basic pH conditions, where AlO − and SiO − groups are postulated to predominate, the preferential release of Al is probably due to the intrinsically greater reactivity of Al-bridging oxygen bonds. Calculations of diffusion coefficients for Na diffusion within the leached layers suggest that leached layers are structurally more open and porous than unaltered, crystalline albite. In addition, the calculated diffusion coefficients show a strong pH dependence. This result implies that leached layers formed at acid and basic pH conditions undergo a greater degree of structural modification since they are more open than those formed at neutral pH. The pH-dependent nature of the structural and transport properties of leached layers can be considered to be a contributing factor to the overall pH dependence of feldspar dissolution rates. The formation of leached layers is an important factor in providing hydrolyzing molecules (H +, H 2O, OH −) access to Na + exchange sites and SiOSi and AlOSi hydrolysis sites deep within the structure. Evidence for this is shown by the positive relationship between the overall rates of dissolution and the depths of Na and Al leaching. The data from this study suggest that reactions at the surface and within leached layers control the overall dissolution behavior of feldspars and other similar, multi-oxide silicates. It is therefore proposed that a “leached layer-surface reaction” model more accurately describes the dissolution process at elevated temperatures than the traditionally accepted “surface reaction” model.

Experimental study of [formula omitted] partitioning between crystalline albite, albitic glass and CO 2 gas

Oxygen isotope partitioning between gaseous CO 2 (~ 1 bar) and crystalline albite and albitic glass has been measured at 750–950°C, using the gas-mineral exchange techniques of O'Neil and Epstein (1966) and Stolper and Epstein (1991). Convergence of oxygen isotope ratios of CO 2 and avoidance of surface-correlated fractionation effects is achieved in long runs (>200 days), using relatively coarse grain sizes. Equilibrium CO 2-crystalline albite oxygen isotope fractionation factors are: 4.74 ± 0.22 at 750°C, 3.77 ± 0.23 at 850°C and 3.36 ± 0.21 at 950°C. These values compare well with calculations based on the experimental calcite-albite data of Clayton et al. (1989) and the CO 2-calcite fractionation factors determined by Chacko et al. (1991) and Rosenbaum (1994). Our results, thus, provide independent support for the high pressure calcite-mineral fractionation factors of Clayton et al. (1989). An estimate of the reduced partition function ratio for albite derived from the CO 2-albite data using the reduced partition function ratio of CO 2 ( Richet et al., 1977) differs by ~2% from that proposed by Clayton and Kieffer (1991). CO 2-albite exchange experiments of relatively short duration give disequilibrium fractionation factors. Oxygen diffusion coefficients calculated from these experiments, however, are comparable with previous determinations of oxygen diffusion in feldspars under nominally anhydrous conditions and support the hypothesis that isotopic exchange is diffusion-controlled. Equilibrium oxygen isotope fractionation factors determined for CO 2-albitic glass are identical within experimental uncertainty to those determined for CO 2-crystalline albite, thus indicating that fractionation between crystalline and glassy albite is unresolvable at the 0.1 %. level. In contrast, additional measurements of oxygen partitioning between CO 2 and silica glass confirm the results of Stolper and Epstein (1991) and imply that at equilibrium silica glass is 18O-enriched relative to quartz.

SO2‐rock interaction on Io: Reaction under highly oxidizing conditions

Laboratory simulations have been carried out to test the possibility that interactions of SO2 and silicates can produce Na‐S compounds which can account for the observed surface enrichment of Na relative to Si on Io. A wide variety of silicate compositions were heated under oxidizing conditions with a SO2/O2 = 1 gas phase at a mid‐level crustal temperature for Io (1123K). All experiments produced sulfate deposits on the silicate surfaces. The nature of the sulfate changed systematically with the silicate Ca/Na ratio, with mixtures of CaSO4 and Na‐rich sulfate formed by basaltic compositions having higher Ca/Na but only alkali‐rich sulfates formed for more granitic (low Ca/Na) compositions. For crystalline albite and an albite‐orthoclase eutectic glass composition, K and Al‐rich sulfates were formed. Assuming that burial of SO2 to the temperatures of our experiments is plausible on Io and that somewhat less oxidizing conditions do not qualitatively change the results, SO2‐rock interactions producing Na‐rich sulfates could account for the Io surface enrichment in Na. Observations on minor elements, such as K or Ca, in the atomic cloud or in magnetospheric ion spectra could be used to identify sulfates as Io surface phases and, conceivably, even define Ionian crustal magma types. Many analyses show significant S excesses which are best interpreted as due to the presence of bisulfate (HSO4.) components, and NaHSO4 deserves consideration as an Io surface mineral. The possibility of elemental S on Io can be regarded as a totally separate issue from the problem of the surface Na enrichment.

Crystallization of Anorthite‐Seeded Albite Glass by Solid‐State Epitaxy

Stoichiometric albite glass (NaAlSi3O8) was seeded with 5 wt% crystalline anorthite (CaAl2Si2O8) to make albite glass‐ceramics. The epitaxial crystallization of the albite glass to the glass‐ceramics was investigated by X‐ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectrometry (EDS). High albite was observed as the major crystallization product over the temperature range of 800–1200°C. No crystalline albite could be crystallized from pure albite glass without seeds. Small amounts of nepheline (NaAlSiO4), however, crystallized along with albite after heat treatments at temperatures lower than 1000°C. The platelike microstructure of albite crystals was revealed in the seeded glasses. The albite blades grew epitaxially from the anorthite seeds, and the Ca content decreased in the direction away from the seeds. The degree of crystallization and the grain size were dependent upon the heat treatment conditions. By increasing the particle size of the seed, the crystallization process was retarded and the resultant microstructure was degraded. The seeding efficiency was also lowered by adding nonisostructural hexagonal anorthite seeds which produced less albite but more nepheline crystals. Crystallization of albite glass by seeding with 5 wt% anorthite is much greater than with the surface nucleation which takes place in a homogeneous 95 wt% albite + 5 wt% anorthite glass.

Hydrothermal reaction of albite and a sodium aluminosilicate glass: A solid-state NMR study

We present here a solid-state NMR study of the structure and chemical composition of the products and mechanisms of the reaction of crystalline low albite and a glass of nearly albite composition with aqueous solutions of pH from 1 to 11 at 250°C. For the crystalline albite, there are no detectable bulk or surface structural changes due to aqueous attack, consistent with the idea that both cation exchange and disruption of the aluminosilicate framework occur only near the mineral/solution interface and that the hydrated surface layer, if it exists, is not more than about 30 Å thick. This reaction occurs by solution/reprecipitation, and its rate decreases with increasing solution pH, supporting the idea that the dissolution of feldspar is initiated by cation-exchange. For the glass, the reaction proceeds by cation exchange of protons for Na +, incorporation of molecular water into the bulk glass, and a small amount of depolymerization of the aluminosilicate framework in the interior of the glass. Cation exchange becomes less important with increasing solution pH. The incorporation of molecular water and cation-exchange cause structural changes in the glass via solidstate adjustment without dissolution/reprecipitation. The large cations in the hydrated glass (Na and K) probably have a shell of water molecules around them, with a maximum average coordination number of six. The secondary phases formed from both albite and the glass are often amorphous and can be well characterized by NMR. The compositional and structural variations of the amorphous phases are important factors in these reactions and cannot be ignored in theoretical models of aluminosilicate dissolution. As expected, the aluminum coordination in the secondary phases changes from six-fold to four-fold as the solution pH increases.

Calorimetry of silicate melts at 1773 K: measurement of enthalpies of fusion and of mixing in the systems diopside-anorthite-albite and anorthite-forsterite

Transposed-temperature-drop calorimetry, using a Setaram HT 1500 calorimeter, was used to study directly the melting at 1773 K of mixtures of crystalline albite, anorthite, and diopside and of anorthite and forsterite. The enthalpy of albite at 1000–1773 K, starting with both crystalline and glassy samples, was also measured. The results confirm previously measured enthalpies of fusion of albite, diopside and anorthite (Stebbins et al. 1982, 1983; Richet and Bottinga 1984,1986). The new results use thermochemical cycles which completely avoid the glassy state by transforming crystals directly to melts. The enthalpy of fusion of forsterite is estimated to be 89±12 kJ/mol at 1773 K and 114±20 kJ/mol at its melting point of 2163 K. The data allow semiquantitative evaluation of heats of mixing in the molten silicates. Along the Ab-An join, enthalpies of mixing in the liquid at 773 K are the same or somewhat more negative than those in the glass at 986 K, whereas along Ab-Di and An-Di, enthalpies of mixing in the liquid are distinctly more positive than in the glass. These differences correlate with excess heat capacities in the liquids suggested by Stebbins et al. (1984).

Polymerization of silicate and aluminate tetrahedra in glasses, melts, and aqueous solutions—V. The polymeric structure of silica in albite and anorthite composition glass and the devitrification of amorphous anorthite

29Si MAS NMR experiments have been carried out to determine the silica species distribution (Q distribution) in albite, NaAlSi 3O 8, and anorthite, CaAl 2Si 2O 8, composition glasses (designated albite and anorthite glass). Our results indicate that the Q distribution of albite glass contains all five possible silica species and shows a tendency towards high Q 3 and Q 4 concentrations, whereas anorthite glass does not contain Q 4 and has a high Q 0 concentration. Rationalizations are made in terms of the observed Q distributions to explain differences in devitrification behavior of these two glasses. 27Al MAS NMR data for these glasses suggest that differences in devitrification behavior between these two glasses should be ascribed to small growth rates rather than small nucleation rates of crystalline albite from albite glass.

Glass reactivity in aqueous solutions

Surface analysis methods have been incorporated as part of several studies concerning the mechanisms and amount of glass leaching in aqueous solutions. The reactivity of simple glasses with water has been determined by measuring the uptake of D and 18O in soda-lime silicate glass, amorphous and crystalline silica, and amorphous and crystalline albite leached in D218O at 90 °C as a function of time. The concentrations and profiles of 18O and D going into the glass were determined using nuclear reaction analysis and compared to the material leached from the glass as determined by solution and Rutherford backscattering (RBS) analyses. The measured ratio of D/18O does not follow any one simple uptake model. Reaction layers formed on simulated nuclear waste glasses containing Pu have also been measured using RBS. Specimens were leached in deionized water or brine at 90 °C. The concentration of Pu on the specimens generally decreased with time. Analysis of the leaching solution indicated that the rate of Pu release was considerably less than that of the glass matrix. The results support the buildup of a surface layer covering the Pu.