Jadeite Glass Research Articles

A First‐Principles Study on the Electronic, Vibrational, and Thermodynamic Properties of Jadeite and its Tentative Low‐Density Polymorph

AbstractThe high‐pressure clinopyroxene mineral jadeite (chemical composition NaAlSi2O6) was studied by density‐functional theory with respect to its electronic, vibrational, and thermodynamic properties, correctly reproducing the available experimental data. At a larger‐than‐normal volume, however, a low‐density alumosilicate phase with tetrahedral instead of octahedral Al–O coordination was identified. This low‐density phase was investigated theoretically, too, and the results were compared to jadeite and experimentally observed properties of what has been dubbed jadeite glass. It turned out that the theoretically obtained properties of this hypothetical polymorph, such as the bulk modulus, the molar volume and the vibrational frequencies, agree well with the corresponding properties of the glass phase. Hence, from an inverse structure‐property relationship we propose to model jadeite glass with the aforementioned low‐density alumosilicate phase and tetrahedral Al–O motifs, as suggested from first principles and corroborated from experiment. The possibilities as well as the limitations of electronic, phonon and thermodynamic properties calculations applied to such a polymorph are also discussed.

High-resolution solid-state NMR study of the effect of composition on network connectivity and structural disorder in multi-component glasses in the diopside and jadeite join: Implications for structure of andesitic melts

The structural evolution of andesitic melts with varying compositions remains one of the unsolved questions in high-temperature geochemistry and petrology. In this article, we report the structural details of model andesitic glasses [CaO–MgO–Na2O–Al2O3–SiO2 (CMNAS)] in the diopside (CaMgSi2O6) and jadeite (NaAlSi2O6) join using high-resolution, multi-nuclear, solid-state nuclear magnetic resonance (NMR). The 27Al NMR spectra of CMNAS glasses confirm that [4]Al is dominant. While a minor fraction of [5]Al is observed, its presence is only prevalent in the glasses with higher Ca–Mg content. Topological disorder in the glass network also tends to increase with Ca–Mg content as evidenced by the increase in the quadrupolar coupling constant (Cq) of [4]Al for glasses with increasing diopside contents (XDiopside). Despite the complex nature of the glasses studied here (with five oxide components), the 17O 3QMAS NMR spectra resolve diverse bridging oxygens (BOs) and non-bridging oxygens (NBOs), from which the degree of Al avoidance among framework cations (Si and Al) and preferential proximity among non-network cations (Ca2+, Mg2+, and Na+) and each oxygen site can be estimated: presence of Al–O–Al in jadeite glass implies a violation of the Al-avoidance rule in the glasses and the decrease in the fraction of NBOs with increasing XDiopside is consistent with a decrease in their viscosity. Analysis of the peak position of {Ca, Mg}-mixed NBOs, along with the absence of Na-NBO peak, and the peak shape of Si–O–Al reveals preferential partitioning of Ca2+ and Mg2 into NBOs and the proximity of Na+ to BOs. The fraction of highly coordinated Al has been linked to thermodynamic and transport properties of the melts. Considering all the experimental Al coordination environments available in the literature, together with the current experimental studies, we attempt to establish the relationship between the fractions of highly coordinated Al and composition, particularly average cationic potential of non-network forming cations (<c/r>ave, defined as cationic potential normalized by the mole fraction of each non-network cation). The fraction of highly coordinated Al increases nonlinearly with increasing <c/r>ave. The fraction of [5,6]Al is negligible up to <c/r>ave=∼1.7, then it increases above <c/r>ave>∼1.7 regardless of changes in other compositional variables (e.g., Si/Al, NBO content). This indicates the presence of a threshold <c/r>ave value for the formation of [5,6]Al. The current experimental results with the changes in network polymerization, coordination environments, and the degree of disorder in the CMNAS glasses can improve understanding of the structure–property (particularly, configurational thermodynamic properties) relationships of multi-component natural silicate melts, including andesitic melts and glasses.

Contrasting behavior of intermediate-range order structures in jadeite glass and melt

Structures of jadeite glasses and melt are studied by the multi-angle energy-dispersive X-ray diffraction technique under high pressures and temperatures. The temperature dependence of X-ray structure factors, S(Q), is obtained at experimental conditions from 4.5GPa and 300K to 6.3GPa and 2173K. When the glass sample is heated at high pressures, the first sharp diffraction peak (FSDP) in the structure factor becomes sharper and shifts to higher-Q with increasing temperature to 473K. At higher temperatures to 873K, both shape and position of the FSDP change only slightly. The structure factor of jadeite melt at 2173K and 6.3GPa clearly differs from that of the glass at 873K (just below melting). The FSDP of the melt is broader and located at lower-Q (corresponding to higher real-space atomic distance). In addition, a small peak (second sharp diffraction peak – SSDP) appears in the S(Q) of the melt between 3.0 and 3.2Å−1, which is known to be a sign for change of short-range ordering. Although the pressure drops to 3.7GPa upon quenching from the melt, the quenched glass exhibits densified features in the structure factor, with the FSDP located at a higher-Q than that of glass before melting at 873K and 5.3GPa. This implies that the intermediate-range order structure of the quenched glass (300K, 3.7GPa) is more densified by thermally-induced modification, although this quenched glass does not show clear SSDP in its S(Q). Similar to previous studies, the glass decompressed to ambient condition remains denser than the glass before compression and heating. The FSDP position changes from 2.02Å−1 in the starting material to 2.17Å−1 in the recovered glass, suggesting that the volume of recovered glass decreases about 20% in volume. The present result implies that glasses quenched under ambient pressure do not accurately represent structural behavior of melts under high pressure, while glasses quenched under high pressure can be treated as good analogs to melts at corresponding high pressure ranges, at least from the structural behavior point of view.

The influence of H2O and CO2 on the glass transition temperature: insights into the effects of volatiles on magma viscosity

CO 2 can play an important role in eruptive processes; in particular, it has the potential to reach saturation at lower concentrations than H 2 O and initiate degassing. The effect of such CO 2 loss on magma viscosity is not well constrained, especially compared to the established effects of H 2 O loss. In terms of understanding the CO 2 solubility mechanism, recent spectroscopic studies have shown that CO 2 speciation is strongly temperature dependent and that CO 2 speciation preserved in quenched glasses below T g is different from the true CO 2 speciation observed in the melts. However, the effect of CO 2 on the glass transition temperature, and by inference the viscosity, has not been previously established. In this study, calorimetric measurements were conducted on synthetic H 2 O- and CO 2 -bearing phonolite and jadeite glasses in order to investigate the volatile’s effect on the glass transition interval, by defining a single glass transition temperature ( T g onset ). The samples were synthesised in a piston-cylinder apparatus between 1300 and 1550 °C, at 1.0 to 2.5 GPa, and contained up to 2.29 wt.% CO 2 and up to 5.49 wt.% H 2 O. For both compositions, H 2 O has a large effect in reducing T g onset , but CO 2 appears to have little or no effect. For the entire range of H 2 O contents, T g onset decreases exponentially with H 2 O content from 870 to 523 K and 1036 to 636 K for phonolite and jadeite, respectively, regardless of the CO 2 content. No measurable effect of CO 2 on T g onset was observed. These results suggest that compared to H 2 O, CO 2 contributes little to changes in the physical properties of the melt. They also provide strong evidence for the decoupling of CO 2 speciation from the bulk silicate melt structural relaxation process at T g .

Annealing experiments on CO2-bearing jadeite glass: an insight into the true temperature dependence of CO2 speciation in silicate melts

AbstractThe thermodynamics and kinetics of CO2 speciation in silicate melts have been studied by measuring the concentration of CO2mol and carbonate in jadeite glass annealed at 575, 450 and 400°C. Assuming that the reaction is1where CO2mol..Obr represents a CO2 molecule weakly bonded to a bridging oxygen in the network and CO3 represents a bridging carbonate group with no net negative charge, ΔH for the reaction is –17 (+4/–8) kJ mol−1 and ΔS is –24 (+6/–9) J K−1 mol−1. The rate of equilibration of the species was measured at each temperature and the rate constants were deduced. The temperature dependence of the rate constants was used to determine the activation energy of the forward and reverse reactions which are 68 (+3/–31) kJ mol−1 and 86 (+1/–69) kJ mol−1 respectively. The data suggest that CO2mol may be much more abundant in silicate melts than previously assumed on the basis of studies of CO2-bearing glasses. Models of solubility, diffusion, and isotope fractionation should take this into account.

Pressure induced coordination change of Al in silicate melts from Al K edge XANES of high pressure NaAlSi2O6‐NaAlSi3O8 glasses

Aluminum K‐edge XANES spectra of high pressure and high temperature (4.4 GPa and 1575°C) glasses along the NaAlSi2O6 (Jd)‐NaAlSi3O8 (Ab) join are reported using synchrotron radiation, and shown to provide direct experimental evidence for the pressure‐induced coordination change of Al. Five‐ and six‐fold coordinated Al (5Al and 6Al), characterized by Al K‐edge positions at 1567.8 and 1568.7 eV, respectively, first appear in glass of composition Jd60Ab40 and increase in proportion progressively with increasing Jd content. The end‐member jadeite glass contains about 6% of each of 5Al and 6Al. The present direct measurements confirm literature suggestions for the important role of Al in controlling viscosity and diffusion in mantle melts.

The absence of 6-fold coordinated Al in jadeite melt near the jadeite liquidus

A 27Al MAS NMR study indicates that Jadeite and Jadeite glass produced on the liquidus at 1390° ± 25°C and 35 kbars had Al in two different coordinations. In the solid phase Al was 6-fold and in the glass it was 4-fold. If the glass is taken as a good model for the liquid phase, then the common assumption that the structure of precipitating crystals reflects the structure of the melt must not be automatically accepted.

EXAFS study of the coordination environment of aluminum in a series of silica-rich glasses and selected minerals within the Na 2OAl 2O 3SiO 2 system

Photoelectron total yield Al K-XANES and EXAFS spectra were collected for a series of Na 2OAl 2O 3SiO 2 minerals and glasses. The purpose of this study is to determine if EXAFS can accurately characterize these known Al environments in terms of AlO distances and coordination numbers, and to characterize the unknown Al environment in the glasses. Al XANES spectra were collected for corundum, sillimanite, albite, jadeite and nepheline, as well as jadeite glass, nepheline glass and a 75 mol% silica glass series having R ( = Al/Na) ranging from 0.28 to 1.61. Al EXAFS spectra were collected for corundum, sillimanite, jadeite and albite, as well as jadeite glass, albite glass and a 75 mol% silica glass with an R of 1.61. Al XANES spectra for our model compounds have two edge maxima at 1568 and 1572 eV for minerals having octahedral Al, while one edge maximum is present at 1566 eV for minerals with tetrahedral Al. The glasses all have a single edge maximum near 1566 eV, indicating tetrahedral-like Al sites in the glasses. The results of our Al EXAFS refinements of the model compounds are in agreement to within ±0.03 Å for AlO distances and to within ±23% for coordination numbers of recent crystal structure refinements in the literature. The EXAFS results for the glasses clearly indicate Al in tetrahedral sites, yielding AlO distances near 1.77 Å, well within the range of average AlO distances for aluminate tetrahedra of aluminosilicate minerals. Our XANES and EXAFS results indicate tetrahedral Al throughout our 75 mol% silica glass series, which may be consistent with the existence of tricluster units occurring in the Al-rich glasses in the sodium aluminosilicate system.