Albite Melt Research Articles

Solution of H2O in diopside melts: a thermodynamic model

Structural similarities between dry diopside melt and superhydrous albite melt (X w >0.5) — both lack three-dimensional silicate units — suggest that thermodynamic relations may be similar. A model based on that assumption successfully predicts diopside melting relations and H2O solubilities. For the model, the three partial differential equations describing solution of H2O in albite melt for X w >0.5 have been integrated for diopside melt from X w =0 to X w at least as large as 0.76, with two exceptions: an alternative partial differential equation for Henrian solution of H2O in dilute melts was applied for X w <0.20, and an alternative differential equation for the pressure dependence of a w at pressures below 2 kbar was developed. The latter alternative equation yields relatively small ¯Vw's at low pressures rather than the large ¯Vw's calculated from the equation from the albite system. Available experimental solubility data are not precise enough to offer a choice between the small-¯Vw and large-¯Vw equations. Integration of all the partial differential equations was constrained solely by the P and T of a single experimentally-determined point on the H2O-saturated solidus. Solubilities calculated by a Henrian-analogue solution model (a di=X di 2 ) from the experimental H2O saturated solidus lie outside experimental solubility constraints for dilute melts. On the other hand, a Henrian model (a di=Xdi) successfully predicts solubilities in dilute melts. The formulation of the Henrian model and magnitudes of model molar entropies of solution are consistent with the hypothesis that H2O dissolves in diopside melt as an essentially undissociated species with little ordering on melt structural sites. That species could in turn be consistently, if not uniquely, interpreted to be molecular H2O or a hydroxylation (OH−) complex formed from nonbridging oxygens.

Some Thermodynamic and Experimental Constraints on the Melting of Albite at Atmospheric and High Pressure

Recent redetermination of the melting curve of albite, NaAlSi3O8, has raised questions about possible nonequilibrium states in viscous silicate liquids near their liquidus temperatures and about the thermodynamic description of albite melt at high P and T. Solid solubility of excess nepheline in albite, and of excess silica in anorthite and celsian (on the order of 1-4 wt %) has been reported in the literature. In analogy to similar cases in semiconductors, it is proposed that this homogeneity range of the feldspar phase (from approximately \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage{wasysym} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document}$${\mathrm{X_{NaAlO_{2}}}}= 0.245$$\end{document} to \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \u...

The structures of anorthite and albite melts.

The structures of anorthite and albite melts have been investigated with the X-ray diffraction method. The scattered intensities were measured at 1600°C and 1200°C on the anorthite and albite melts, respectively. The radial distribution function reveals that the basic structural units of the two melts are the TO4 (T=Si, Al) tetrahedra. From least-squares fitting of the observed S·i(S) values to those of model structures, it was found that the framework of the tetrahedra in the anorthite melt has close similarity to that in the anorthite structure at high temperature within the short of r≤5A, based mainly on the four-membered rings, and the framework in the albite melt to the low-tridymite structure where the six-membered rings predominate.

Viscosity and structural changes of albite (NaAlSi 3O 8) melt at high pressures

Viscosity of anhydrous albite melt, determined by the falling-sphere method in the solid-media, piston-cylinder apparatus, decreases with increasing pressure from 1.13 × 10 5 P at 1 atm to 1.8 × 10 4 P at 20 kbar at 1400°C. The rate of decrease in viscosity is larger between 12 and 15 kbar than in other pressure ranges examined. The density of the quenched albite melt increases with increasing pressure of quenching from 2.38 g/cm 3 at 1 atm to 2.53 g/cm 3 at 20 kbar. The rate of increase in density is largest at pressures between 15 and 20 kbar. The melting curve of albite shows an inflexion at about 16 kbar. These observations strongly suggest that structural changes of albite melt would take place effectively at pressures near 15 kbar. Melt of jadeite (NaAlSi 2O 6) composition shows very similar changes in viscosity and density and a melting curve inflexion at pressures near 10 kbar. Difference in pressure for the suggested effective structural changes of albite and jadeite melts is 5–6 kbar, which is nearly the same as that between the subsolidus reaction curves nepheline + albite= 2jadeite and albite=jadeite + quartz. The structural changes of the melts are, however, continuous and begin to take place at pressures lower than those of the crystalline phases.

MELTING OF ALBITE AT HIGH PRESSURES UNDER CONDITIONS

The melting of albite has been investigated in the presence of water in the pressure range up to 30 kb. Fixed amounts of water have been contained in the melts so as to satisfy the condition that the melt is saturated with water or PH2O=Pt up to a certain value of pressure, but at higher pressures the melt is undersaturated or PH2O<Pt. The results show that the melting relation is largely influenced by the presence of only a very small amount of water and strongly support the partial melting hypothesis for the cause of the low-velocity layer. It is suggested that the temperature in the earth's mantle is quite near the melting curve of the mantle when the melting gradient based on the theory of the lattice defects is adopted.

Melting of albite at high pressures in the presence of water

The melting of albite has been investigated in the presence of water in the pressure range up to 30 kb, from the point of view that the effect of water on the melting of silicates is important for the investigation of the state of the earth's interior and the earth's thermal history. The melting relations have been determined under the following conditions: (a) that the water pressure is equal to the total pressure or the albite melt is saturated with water, and (b) that albite melt contains a fixed amount of water (10 wt% and 14 wt%) and is saturated with water up to a certain value of pressure but undersaturated in the higher pressures. As oressure increases, the melting point of albite, in the presence of a fixed amount of water, decreases from 1118°C along the water-saturated melting curve up to a certain value of pressure. In the higher pressure region albite begins to melt incongruently, and the temperatures of the beginning of melting decrease with increase of pressure along the water-saturated melting curve and the temperatures of the end of melting increase approaching asymptotically the curve for the dry condition. In the region between the solidus and the liquids, crystal and liquid co-exist. The value of pressure, where the incongruent melting begins, is in good agreement with the results of the solubility of water in albite melt.

Liquid Structures and Glass Formation*

Glasses have the structures of those liquids from which they formed on cooling. The structures of liquids are the key to the understanding of glass formation. Weyl and Marboe assume three extreme cases of liquid structures. (1) Sodium chloride melts under disproportionation of all binding forces, forming abruptly a very fluid melt which consists of ionic clusters separated by fissures (Frenkel-type liquid). (2) Organic compounds of high molecular weight consist of molecules with strong intramolecular forces and weaker intermolecular forces. Depending on the shape and size of these molecules, the liquids undergo orientation when stretched (orientable liquid of the Stewart type). (3) Cristobalite and many silicates (albite) melt because increased thermal motion gradually overcomes the binding forces in the crystal. Melting is a gradual process; the optical properties of the crystal are retained for long times above the melting temperature. The resulting “flawless” liquid of the Bernal type can be easily supercooled and forms a glass. Glass formation does not take place from atomic liquids (argon, most metals) nor from low molecular-weight liquids (benzene, carbon tetrachloride) but from liquids whose structures correspond to (2) and (3) of the idealized structures. The structure of liquids which can form glasses is discussed on the basis of a ternary diagram, the corners of which are representing the three idealized extreme structures. Conventional thermodynamic treatment can not distinguish between the average bond strength and a distribution of bond strengths. The average bond strength might be the same for (1), (2), and (3). Even if one could derive the distributions of all binding forces within a condensed system, one still cannot distinguish between the structures (2) and (3), because the difference lies in the lifetime of the “clusters” (short life) and the “molecules” (long life).

A method for determining the solubility of water in silicate melts

The method described in this paper for determining the solubility of water in silicate melts consists primarily of locating the boundary (or projected boundary) between the field of liquid and the field of liquid+gas in successive isobaric T-X _H2O sections for a given system. It involves the sealing of known amounts of water and silicate starting material in noble-metal capsules, the fusing and homogenization of these charges in pressure vessels, equilibration at designated experimental run temperatures and pressures, and quenching after prescribed periods of time. The quenched products are then examined for evidences of excess water. In determining solubility values from the results of critical runs, corrections are made for aqueous bubbles initially trapped and subsequently preserved within the charges, i.e., for water effectively unavailable for solution in the melt. An alternate method involves locating, also in successive isobaric T-X _H2O sections for a given system, the boundary (or projected boundary) between the field of liquid and the field of liquid + crystals. This method generally yields results that are less satisfactory, owing chiefly to the moderate slopes of this boundary for many systems and to difficulties in obtaining the precise control and measurements of temperature that are required. The solubility of water in melts derived from albite glass and Harding, New Mexico, pegmatite have been determined by the excess-water method as a function of pressure up to 10,000 bars and at temperatures of the H ₂ O-saturated liquidus for each melt. Solubility values for the pegmatite melt are 4.2 wt % at 1000 bars and 655 degrees C., 6.6% at 2000 bars and 635 degrees C., 11.2% at 5000 bars and 650 degrees C., and nearly 20% at 10,000 bars and 675 degrees C. Corresponding values for albite melt are 4.2 wt % (930 degrees C.), 6.4% (850 degrees C.), 9.9% (750 degrees C.), and 16.8% (700 degrees C.). The temperature coefficient of water solubility in Harding pegmatite melt, at a pressure of 1000 bars, is about -0.2 wt % of water per 100 degrees C. rise in temperature through the range 650 degrees to 850 degrees C. The solubility values obtained in these investigations are substantially lower, at pressures up to about 4500 bars, than values reported by Goranson for comparable materials on the basis of his weight-loss-on-ignition method. However, they are much higher, at pressures above 5000 bars, than values derived by the most reasonable extrapolations of Goranson9s results to these higher pressures. The discrepancies are attributed mainly to aqueous bubbles that normally are trapped and preserved in the experimental charges; these bubbles evidently were reckoned as dissolved water in the earlier work on albite and granite melts, which would account for pressure-solubility curves of seemingly incorrect position and shape.