Calcium Aluminosilicate Melts Research Articles

Effect of calcium and potassium oxide addition on the viscosity and fragility of a calcium aluminosilicate melt

AbstractFragility is commonly quantified as the magnitude of change in viscosity at a temperature close to the glass transition temperature (Tg). It is a critical characteristic of melts used in scientific and industrial applications. The fragility of silicate melts generally increases with the depolymerization of silicate anions upon the addition of alkali or alkaline earth oxides. However, the effects of oxide additives on the fragility of aluminosilicate melts remain unclear. In this study, the effect of CaO or K2O addition on the viscosity of the 36CaO–51SiO2–13Al2O3 (mol.%) melt for the wide viscosity range of 10−1–1012 Pa s was studied. The relationship between the logarithmic viscosity and Tg‐scaled temperature indicated that the melt fragility increased with the addition of CaO, whereas the addition of K2O reduced the fragility when the additive content of CaO or K2O was less than 10.8 mol.%. The effect of the addition of K2O on fragility cannot be explained by the depolymerization of silicate anions alone. Raman and 27Al nuclear magnetic resonance spectroscopies of the glasses indicated that a decrease in the level of distortion of the AlO4 tetrahedra decreased the fragility of the aluminosilicate melt.

Influence of Al2O3 and SiO2 on the structure and viscosity of iron-compound bearing calcium-aluminosilicate slags

The effect of the addition of Al 2 O 3 on the viscosity of the iron-compound bearing CaO-SiO 2 -Al 2 O 3 -MgO slags with different SiO 2 content (= 30, 40, and 50 wt%) was investigated at high temperatures. The addition of Al 2 O 3 increased the viscosity of iron-compound bearing calcium-aluminosilicate melts. The addition of Al 2 O 3 to the slag at low-SiO 2 content (= 30 wt%) increased the fraction of Fe 2+ at a fixed iron content because Fe 3+ cations prefer to exist as tetrahedral unit, i.e., Fe 3+ [IV], in the melt. On the other hand, the addition of Al 2 O 3 to the slag at high-SiO 2 content (= 50 wt%) increased the fraction of Fe 3+ at a fixed iron content due to the preference of octahedrally coordinated Fe 3+ [VI] in the slag. Although the redox equilibrium reaction of iron in the CaO-SiO 2 -Al 2 O 3 -Fe t O-MgO system was strongly affected by the basicity of the melt, the addition of Al 2 O 3 polymerized silicate networks regardless of the SiO 2 content in the slags. The activation energy for the viscous flow of the slags had a linear correlation with the non-bridged oxygen per tetrahedron (NBO/T) in the aluminosilicate network. Therefore, the viscosity of the iron-compound bearing calcium-aluminosilicate melt was determined by the degree of polymerization within the aluminosilicate networks. • Viscosity-structure relationship of iron-compound bearing calcium-aluminosilicate system was experimentally confirmed. • Al2O3 increased melt viscosity due to polymerization reaction and redox equilibria of iron-compound. • Activation energy of viscous flow was dominantly affected by degree of polymerization of slag.

Neutron diffraction of calcium aluminosilicate glasses and melts

The combination of neutron diffraction with aerodynamic levitation and laser heating, pioneered by Neville Greaves and co-workers about 15years ago, is an important tool for studying the structure of liquid melts. Since the first work on liquid Al2O3 published in 2001, the technique has been largely improved and experiments are now routinely performed at neutron sources, providing interesting structural information on various materials.In this paper, the structure of glass-forming compounds in the system CaO-Al2O3-SiO2 was measured by applying neutron diffraction with aerodynamic levitation. Results obtained in the liquid state above the melting point and from the glass at room temperatures are presented. Various compositions were studied by increasing the silica content and by changing the ratio CaO/Al2O3. As observed using other methods, the main structural changes relate to modification of the Al-O short range order.

Effect of Zirconium on the Structure and Congruent Crystallization of a Supercooled Calcium Aluminosilicate Melt

The influence of zirconium as a nucleating agent on the congruent crystallization and relevant physical properties of a supercooled calcium aluminosilicate melt of a composition close to CaAl2SiO6 has been investigated up to 6 mol% ZrO2. Zirconium marginally affects rheological and structural properties, decreasing the viscosity of the Zr‐free melt by no more than 0.25 log unit and, as observed by Raman spectroscopy, not changing significantly the polymerization state of the material. Whereas the Zr‐free melt crystallizes congruently and heterogeneously from the sample surface to yield yoshiokaite, a stuffed derivative of the nepheline structure, addition of zirconia promotes instead bulk crystallization of tetragonal ZrO2 and then of yoshiokaite. The latter process takes place in two stages: dissolved Zr first promotes homogeneous precipitation of zirconia before yoshiokaite crystallizes congruently from a Zr‐depleted volume of melt around zirconia precipitates. This process makes zirconium, and probably other poorly soluble oxides, valuable to control congruent crystallization in silicate glass‐ceramics. From the recorded thermograms, an enthalpy of crystallization of 40 and 46 kJ/mol has been determined at 1060 and 1140 K, respectively, for CaAl2SiO6 yoshiokaite, a very low value that is likely due to the extensive atomic disorder of crystals precipitating at high degrees of supercooling.

The effect of melt composition on the partitioning of oxidized sulfur between silicate melts and magmatic volatiles

Experiments were conducted at 500MPa and 1240°C in a piston cylinder apparatus to assess the effect of melt composition on the melt/volatile partition coefficient of sulfur (DSmelt/volatile), which was used as a measure of the silicate melt’s capacity to dissolve oxidized sulfur species. Iron-free, three- and four-component silicate melts were equilibrated with H2O–S fluids with sulfur concentrations ⩽2mol% at an oxygen fugacity imposed by the Re–ReO2 buffer (1.4 log units above the Ni–NiO buffer). At these conditions, SO2 (S4+) is predicted to be the dominant sulfur species in the volatile phase and sulfate (S6+) is the dominant sulfur species in the silicate melt. The values of DSmelt/volatile were calculated by mass balance. The results show that DSmelt/volatile values increase exponentially with decreasing the degree of polymerization of the silicate melt structure. For example, in calcium-aluminosilicate melts, DSmelt/volatile changes from 0.005 to 0.3 as the degree of melt polymerization changes from the equivalent of a rhyolite to the equivalent of a basalt. At a constant degree of melt polymerization, DSmelt/volatile in equilibrium with sodium-aluminosilicate (NAS) melts is more than an order of magnitude higher than in equilibrium with calcium-aluminosilicate (CAS) melts, and more than two orders of magnitude higher than in equilibrium with magnesium-aluminosilicate (MAS) melts. The value of DSmelt/volatile changes from 0.014 in MAS glasses to 3.4 in NAS glasses for the most depolymerized compositions in each series. Potassium has a similar effect on sulfate dissolution to that of Na. The variation of DSmelt/volatile in equilibrium with various calcium–sodium aluminosilicate (CNAS), magnesium–sodium aluminosilicate (MNAS) and magnesium–potassium aluminosilicate (MKAS) melts indicates that alkalis are only available for sulfate complexation when they are present in excess compared to the required amount to charge balance for the Si4+ to Al3+ substitution in the melt structure. Calcium has a moderate, Mg has a very minor affinity to replace alkalis in this charge balancing role. Apparent equilibrium constants are provided to predict the available amount of various network modifiers for sulfate complexation and, therefore, the presented data can be used to predict DSmelt/volatile for geologically relevant melt compositions.

Structure-Viscosity Relationship of Low-silica Calcium Aluminosilicate Melts

The structure-viscosity relationship of the low-silica (SiO2 ≤ 10 wt%) calcium aluminosilicate melts, which represent the secondary refining ladle slag systems, was investigated by employing the rotating-cylinder viscosity measurement in conjunction with the Raman spectroscopy measurement for linking the macroscopic thermophysical property and molecular (ionic) structural information. Furthermore, the influence of CaF2 on the structure-property relationship was explored. The viscosity of low-silica calcium aluminosilicate melts decreased with increasing both CaO/Al2O3 and CaO/SiO2 ratios. However, the effect of the former on the viscosity of low-silica calcium aluminosilicate melts was larger than that of the latter. By employing the Neuville’s structure model, in which the silicate structural units with various NBO, i.e. QnSi are located at the boundary of the AlO4 aluminate, and the Raman scattering data of the glass samples, it was demonstrated that the aluminate and silicate units are more effectively modified by increasing the CaO/Al2O3 ratio at fixed silica content. The addition of small amounts of CaF2 (~5 wt%) to the low-silica calcium aluminosilicate melts decreased the viscosity of the melts. From the analysis of Raman scattering data, the liberation of SiO44– (Q0Si) units from the AlO4 aluminate structure by addition of CaF2 was understood. However, the effect of CaF2 addition on the viscosity became less discernible at higher CaF2 content (≥ 10 wt%) region, where the F ions simply substitute for the non-bridging oxygen ions in AlO4 tetrahedra.

Interplay between non-bridging oxygen, triclusters, and fivefold Al coordination in low silica content calcium aluminosilicate melts

In the present work the structural properties of low silica content calcium aluminosilicate melts with concentration ratio CaO/Al2O3 = 1 are investigated in the liquid and undercooled states by neutron diffraction experiments and ab initio molecular dynamics simulations. The results show the presence of AlO5 units and triclusters as well as non-bridging oxygen in the fully charge balanced compositions. Moreover, our findings allow us to identify a possible interplay between these structural units. Finally, we discuss the influence of these defective structural units on the properties of tetrahedral network and more particularly their implication on the evolution of the viscosity and the fragility.

Compositional dependence of fragility and glass forming ability of calcium aluminosilicate melts

Abstract The compositional dependence of glass forming ability (GFA) of two series of calcium aluminosilicate melts is studied by measuring their viscous behavior and crystallization tendency. The first series consists of five compositions on the joining line between the eutectic point of anorthite–wollastonite–tridymite and that of anorthite–wollastonite–gehlenite. The series includes the eutectic compositions as end members. The second series consists of five compositions on a line parallel to the joining line on the alumina rich side. In the present work, GFA is described in terms of glass stability. The Hruby parameter ( K H ) [A. Hruby, Czech. J. Phys. B 22 (1972) 1187] obtained from calorimetric data is used as a measure of glass stability. From the viscous behavior of the undercooled melts studied by viscometry the fragility ( m ) is derived. The results show that m is inversely proportional to K H for the two series of melts, proposing m as an indirect measure of GFA. However, this proportionality is only valid for comparison of the glasses in the series of systematic substitutions of components. In general, the melts on the joining line between the two eutectic compositions are found to have superior GFA. Within each series GFA increases as a function of the network polymerization.

Anisotropic Dissolution of α-Alumina (0001) and (1120) Surfaces into Adjoining Silicates

The dissolutions of the (0001) and (1120) orientations of α-Al2O3 into calcium silicate, aluminosilicate, and calcium aluminosilicate melts were modeled using molecular dynamics simulations. In all cases, it was found that the (1120) surface of the crystal destabilizes and melts at a lower temperature than does the (0001) surface. This anisotropy in dissolution counters the anisotropy in grain growth, in which the outward growth of the (1120) surface occurs more rapidly than that on the (0001) surface, causing platelets. However, anisotropic dissolution occurred only at a certain temperature range, above which dissolution behavior was isotropic. The presence of calcium in the contacting silicate melt plays an important role in this anisotropic dissolution, similar to its role in anisotropic grain growth observed previously. However, anisotropic dissolution also occurs in the silicate melts not containing calcium, indicating the importance of the different surface energies. In combination with previous simulations of anisotropic grain growth in alumina, these simulations reveal a complex kinetic competition between preferential adsorption and growth versus preferential dissolution of the (1120) orientation in comparison with the (0001) orientation as a function of temperature and local composition. This, in turn, indicates potential processing variations in which to design morphology in alumina.

Temperature effects on non-bridging oxygen and aluminum coordination number in calcium aluminosilicate glasses and melts

Configurational changes with temperature are important for the thermodynamic and transport properties of most aluminosilicate melts, but in general are not well understood. Here, we present high-resolution 27Al and 17O NMR data on several calcium aluminosilicate glasses prepared with varying quench rates and thus with fictive temperatures that span ranges up to about 200 K. In all compositions the content of five-coordinated aluminum increases with fictive temperature, in agreement with recent high temperature NMR data on melts. In a glass of CaAl 2Si 2O 8 (“anorthite”) composition, the content of non-bridging oxygens also increases with temperature; however this effect was not observed in a sample with a much higher CaO/Al 2O 3 ratio. We present a consistent notation for reactions among structural species in these systems that clarify why in some cases, high-coordinated network cations may appear on the same side of the reaction, while in others they occur on the opposite sides: the key difference is in accounting for all coordination changes for oxygens. Mixing of non-bridging oxygens and of high-coordinated aluminum make significant contributions to the overall configurational entropy and heat capacity of the melts, as does the mixing of various bridging oxygens and of tetrahedral network cations. Other, less well known, types of increase in disorder with temperature may be important as well.

In situ high temperature 27Al NMR study of structure and dynamics in a calcium aluminosilicate glass and melt

The temperature dependence (25–1400 °C) of 27Al NMR spectra and spin–lattice relaxation time constants T 1 have been studied for a calcium aluminosilicate (43.1CaO–12.5Al 2O 3–44.4SiO 2) glass and melt using an in situ high temperature probe, and the glass has been characterized by ambient temperature, high field MAS NMR. The peak positions and the line widths show a consistent behavior as motional averaging of the quadrupolar satellites increases with increasing temperature. The rate of decrease with temperature of T 1 drastically increases near the glass transition temperature T g, which suggests a change in NMR relaxation process from vibrational to translational motions. Above the T 1 minimum (≈1200 °C), NMR correlation times obtained from T 1 are in good agreement with shear relaxation times estimated from viscosity, suggesting that microscopic nuclear spin relaxation is controlled by the same dynamics as macroscopic structural relaxation, and thus that atomic-scale motion is closely related to macroscopic viscous flow.

Immiscibility of calcium fluoride and aluminosilicate melts in ongonite from the Ary-Bulak intrusion, Eastern Transbaikal region

The addition of fluorides of Na, K, Ca, and other elements into silicate melts lowers the degree of their homogeneity owing to the formation of sybotaxic groups enriched in fluorine and cations-modifiers [1]. Many fluoride–silicate systems are characterized by a microheteregoneous structure in the hyperliquidus region related to immiscibility [2]. Under certain conditions, the evolution of natural magmatic systems gives rise to the formation of fluoride melts. Such examples are not numerous and mostly pertain to the effects of immiscibility in sodic fluoride melts (melts– brines) associated with peralkaline granitic magmas. The existence of calcium fluoride melts is confirmed by inclusions of fluoritic glass in mantle xenoliths captured by alkali basalt in New Zealand [3]. As was suggested previously, the residual glasses in ongonite from the Ary-Bulak intrusion were formed as a result of quenching of a calcium alumino-silicofluoride melt consisting of Ca, F, Al, and Si with an admixture of Na and K [4]. In this communication, we present new data on the active participation of calcium fluoride melts in crystallization of this rock.

Temperature independent thermal expansivities of calcium aluminosilicate melts between 1150 and 1973 K in the system anorthite–wollastonite–gehlenite (An–Wo–Geh): A density model

The thermal expansivities of 10 compositions from within the anorthite–wollastonite–gehlenite (An–Wo–Geh) compatibility triangle have been investigated using a combination of calorimetry and dilatometry on the glassy and liquid samples. The volumes at room temperature were derived from densities measured using the Archimedean buoyancy method. For each sample, density was measured at 298 K using glass that had a cooling-heating history of 10–10 K min −1. The thermal expansion coefficient of the glass from 298 K to the glass transition interval was measured by a dilatometer and the heat capacity was measured using a differential scanning calorimeter from 298 to 1135 K. The thermal expansion coefficient and the heat flow were determined at a heating rate of 10 K min −1 on glasses which were previously cooled at 10 K min −1. Supercooled liquid density, molar volume and molar thermal expansivities were indirectly determined by combining differential scanning calorimetric and dilatometric measurements assuming that the kinetics of enthalpy and shear relaxation are equivalent. The data obtained on supercooled liquids were compared to high-temperature predictions from the models of (Lange, R.A., Carmichael, I.S.E., 1987. Densities of Na 2O–K 2O–CaO–MgO–FeO–Fe 2O 3–Al 2O 3–TiO 2–SiO 2 liquids: New measurements and derived partial molar properties. Geochim. Cosmochim. Acta 51, 2931–2946; Courtial, P., Dingwell, D.B., 1995. Nonlinear composition dependence of molar volume of melts in the CaO–Al 2O 3–SiO 2 system. Geochim. Cosmochim. Acta 59 (18), 3685–3695; Lange, R.A., 1997. A revised model for the density and thermal expansivity of K 2O–Na 2O–CaO–MgO–Al 2O 3–SiO 2 liquids from 700 to 1900 K: extension to crustal magmatic temperatures. Contrib. Mineral. Petrol. 130, 1–11). The best linear fit combines the supercooled liquid data presented in this study and the high temperature data calculated using the Courtial and Dingwell (1995) model. This dilatometric/calorimetric method of determining supercooled liquid molar thermal expansivity greatly increases the temperature range accessible for thermal expansion. It represents a substantial increase in precision and understanding of the thermodynamics of calcium aluminosilicate melts. This enhanced precision demonstrates clearly the temperature independence of the melt expansions in the An–Wo–Geh system. This contrasts strongly with observations for neighboring system such as anorthite–diopside and raises the question of the compositional/structural origins of temperature dependence of thermal expansivity in multicomponent silicate melts.

Characterization of Ion Distributions Near the Surface of Sodium‐Containing and Sodium‐Depleted Calcium Aluminosilicate Melts

The distribution of cation and anion components of sodium calcium aluminosilicate was studied by classical molecular dynamics simulations in a high‐temperature melt in the bulk and at the vacuum–melt interface. A significant redistribution of the sodium and non‐bridging oxygen (NBO) ions was observed. Subsequently, a sodium depleted calcium aluminosilicate melt was simulated to determine the sensitivity of the redistribution of ions near the vacuum–melt interface to the presence of sodium ions. It is found that the thermodynamic equilibrium condition near the melt surface favors the enrichment of NBO ions that is closely associated with enrichment of the sodium ions.

Coordination properties and structural units distribution of Q T i in calcium aluminosilicate melts from MD simulation

The distribution of Al(j) and the structural units distribution of QTi in calcium aluminosilicate melts were studied by means of molecular dynamics simulation. The results show that provided there exists lower-field strength cation relative to Al3+, such as alkaline and alkaline earth metals, Al will be four-coordinated but not six-coordinated. Meanwhile, if there exist a large number of higher-field strength cations such as Si4+ and little lower-field strength cation, six-coordinated aluminum will be formed. The relation of structural units distribution of QTi with chemical composition shift was also extracted, showing that as Ca2+ exists, the distributions of QSii, QAli or QTi have the similar changing trend with the variation of component. Because of high-temperature effect, the Al-tetrahedral units in melts are greatly active and unstable and there exist dynamic transforming equilibria of Al(3) ⇋ Al(4) and Al(5) ⇋ Al(4). The three-coordinated oxygen and charge-compensated bridging oxygen are proposed to explain phenomena of the negative charge redundancy of AlO4 and location of network modifier with charge-compensated function in aluminosilicate melts.

Bonding preferences of non-bridging O atoms: Evidence from17O MAS and 3QMAS NMR on calcium aluminate and low-silica Ca-aluminosilicate glasses

The fraction of O atoms as non-bridging O atoms (NBO) can be well approximated based on composition alone in many silicate glasses, but the NBO preference for specific network forming cations is much less well known. Using oxygen-17 ( 1 7 O) NMR on low-silica calcium aluminosilicate (CAS) glasses this study shows that Al-NBO (155 ppm) can be readily distinguished from Si-NBO (110-120 ppm), and that there is a strong preference for the latter. This study also presents a consistent equilibrium constant formulation that indicates that for thermodynamic modeling of most CAS melts with Si > Al, Al-NBO are of minor importance, although they could be significant in some models of diffusion and viscosity. Al-27 one pulse NMR and analyses of spinning side bands show that AlO 5 and AlO 6 species are below detection limits (<0.5% in the low-silica (SiO 2 < 20 mol%) glasses of this sludy (NBO/T = 0.6 to 0.8). In addition, 1 7 O MAS NMR does not detect any obvious (<2%?) Al 2 O triclusters; hence calculations of NBO assignments can be assigned unambigunously.

Structural properties of molten silicates fromab initiomolecular-dynamics simulations: Comparison betweenCaO−Al2O3−SiO2andSiO2

We present the results of first-principles molecular-dynamics simulations of molten silicates, based on the density functional formalism. In particular, the structural properties of a calcium aluminosilicate $[\mathrm{CaO}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}\ensuremath{-}{\mathrm{SiO}}_{2}]$ melt are compared to those of a silica melt. The local structures of the two melts are in good agreement with the experimental understanding of these systems. In the calcium aluminosilicate melt, the number of nonbridging oxygens found is in excess of the number obtained from a simple stoichiometric prediction. In addition, the aluminum avoidance principle, which states that links between ${\mathrm{AlO}}_{4}$ tetrahedra are absent or rare, is found to be violated. Defects such as two-fold rings and five-fold coordinated silicon atoms are found in comparable proportions in both liquids. However, in the calcium aluminosilicate melt, a larger proportion of oxygen atoms are three-fold coordinated. In addition, fivefold coordinated aluminum atoms are observed. Finally evidence of creation and anihilation of nonbridging oxygens is observed, with these oxygens being mostly connected to Si tetrahedra.

Rheology of bubble-bearing magmas

The physical effects of air or argon bubbles on the rheology of a calcium aluminosilicate melt have been measured at temperatures ranging from 830° to 960°C, at 1 bar pressure. The melt composition is SiO 2:64, Al 2O 3:23, and CaO:13 (wt%), while bubble volume fractions are: 0, 0.06, 0.13, 0.32, 0.41 and 0.47. Measured Newtonian viscosities range from 10 10 to 10 14 dPa s. Melts with bubble fractions of 0.06 and 0.13 show with increasing temperature ( T) an increasing relative viscosity for T < 850°C. However at T > 850°C, for all bubble fractions the viscosity decreases markedly with temperature. The observed maximum decrease of the relative viscosity is 75% for a bubble fraction of 0.47 at 907°C. At all bubble fractions the viscosity is independent of the applied stress, which ranged from 11 to 677 bars. No clear indications were observed of non-Newtonian rheological behavior. Under our experimental conditions the relative viscosity of the two phase liquid depends primarily on the bubble fraction. Physical and volcanological implications of these measurements are discussed.

Surface Tension of Calcium Aluminosilicate Glass Using Computerized Drop Shape Analysis

Numerical analysis is used in this study to derive surface tension from the shape of a liquid silicate sessile drop. The speed afforded by the drop shape analysis package facilitates analysis of experimentally derived drop profiles. Drop symmetry, contact angles, and local shape variations can be readily determined, thereby permitting the detection of experimental errors. The experimental and analytical technique was first validated by determining the surface tension of high‐purity gallium and aluminum under carefully controlled furnace atmospheres. Results for a calcium aluminosilicate melt are compared to pendant drop and maximum‐bubble pressure measurements reported in the literature. The use of an internal scaling factor and the effect of substrate interactions are discussed.

Entropy and structure of silicate glasses and melts

Low-temperature adiabatic C p measurements have been made on NaAlSi 2O 6, MgSiO 3, Ca 3Al 2Si 3O 12 and Ca 1.5Mg 1.5Al 2Si 3O 12 glasses. Above about 50 K, these and previous data show that the heat capacity is an additive function of composition to within ±1% throughout the investigated glassforming part of the system CaO-MgO-Al 2O 3-SiO 2. In view of the determining role of oxygen coordination polyhedra on the low-temperature entropy, this is interpreted as indicating that Si and Al are tetrahedrally coordinated in all these glasses, in agreement with structural data; whereas Ca and Mg remain octahedrally coordinated. In contrast, heat capacities and entropies are not additive functions of composition for alkali aluminosilicates, indicating increases in the coordination numbers of alkali elements from about six to nine when alumina is introduced. A thermochemical consequence of additivity of vibrational entropies of glasses is that entropies of mixing are essentially configurational for calcium and magnesium aluminosilicate melts. For alkali-bearing liquids, it is probable that vibrational entropies contribute significantly to entropies of mixing. At very low temperatures, the additive nature of the heat capacity with composition is less well followed, likely as a result of specific differences in medium-range order.