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Viscosity and structural changes of albite (NaAlSi3O8) melt at high pressures
Earth and Planetary Science Letters, 41 (1978) 87-90 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
87
[51
VISCOSITY AND STRUCTURAL CHANGES OF ALBITE (NaAISi308) MELT AT HIGH PRESSURES I. KUSHIRO Geological Institute, University of Tokyo, Tokyo 113 (Japan)
Received January 27, 1978 Revised version received May 27, 1978
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 X l0 s P at 1 atm to 1.8 X 104 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 (NaA1Si206) 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 = 2 jadeite 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.
1. Introduction Recently, it was shown that the viscosity o f melt o f jadeite composition (NaA1Si206) decreases b y a factor of 12 from 1 atm to 24 kbar at 1350°C [1]. Such viscosity decrease was explained by a structural change of the melt possibly due to the shift of A1 from 4-fold to 6-fold coordination with increasing pressure similar to the crystalline phases (nepheline + albite = 2 jadeite). Density of the quenched melt (glass) increases and the wave length o f A1 Ka and A1 K~ in the glass shifts from that in crystalline albite to that in crystalline jadeite with increasing pressure of quenching, supporting the above structural change o f the melt [ 1,2]. If melt of jadeite composition undergoes a structural change similar to that of the crystalline phase at high pressures, melts of other compositions would also undergo similar pressure-induced structural changes [3]. Albite (NaA1Si3Os) is close in composition to jadeite, and albite melt would most probably show the structural changes similar to those of jadeite melt.
The structure and physical properties of albite melt at low pressures have been studied experimentally and theoretically in detail [4,5]. In addition, most magmas contain significant amounts o f the albite component, and the behaviour of a melt of albite composition at high pressures is useful for understanding the nature o f deep-seated magmas. For these reasons, the viscosity of melt o f albite composition and the density of the quenched melt have been determined over a pressure range from 5 to 20 kbar at 1400°C.
2. Experimental methods Viscosity o f albite melt has been measured in a solid-media, piston-cylinder-type apparatus with the falling-sphere method which is the same as that applied for measuring the viscosity o f a melt o f jadeite composition [ 1 ]. Graphite capsules 5 mm in inside diameter and 10 m m in inside length were used for most measurements and a sealed Pt capsule o f comparable
88 inner size was used for one measurement. A graphite heater with 3° tapered inner wall, which reduces the temperature gradient inside the capsule to about 10° [ 1], was used for all the measurements. Pressure media are fired magnesia inside the graphite heater and Pyrex glass surrounded by a talc sleeve outside the heater. Starting material was glass naade at 1 atm by melting natural albite containing small amounts of K20 (0.08 wt.%) and CaO (0.17 wt.%) supplied by T. Iiyama. Platinum spheres with diameters ranging from 0.61 to 1.03 mm were placed on top of the albite glass before each run, and the distance to which the spheres sank was measured after the run. All the runs were made at 1400°C which is well above the liquidus of albite in the pressure range between 5 and 20 kbar. Under each set of temperature-pressure condition two runs with different run durations were made to determine the velocity of sinking. Lines connecting two points in the time-distance diagram pass near tire origin, indicating that Pt spheres begin to sink as soon as the temperature reached 1400°C and they sank with constant velocities as in the case of the jadeite melt [ 1]. When crystalline albite was used as starting material, it takes a longer time to melt [6,7] and such time-distance relations cannot be obtained. Viscosity of the melt was calculated from the velocity and density of the sinking Pt spheres using Stokes' equation. Density of the quenched melt was used instead of that of the melt in the calculation. The difference (<0.3 g/cm 3) is small compared to the density of Pt (21.37) and does not produce significant error in viscosity. Correction for the wall effect has been made using the Faxen correction [8]. Density of the glass obtained by quenching the melt at different pressures was measured by a torsion balance.
TABLE 1 Critical run data and viscosity of melt of albite (NaAISi308) composition at 1400°C Pressure (kbar)
Radius of Pt sphere (cm)
Velocity of sinking (cm/s)
Viscosity ** (poise)
5 8.5 12 12 t5 * 15 20
0.049 0.038 0.052 0.039 0.046 0.042 0.042
7.22 6.67 1.22 9.44 1.94 1.72 2.61
8.1 6.1 5.2 4.4 2.4 2.8 1.8
x x x x x x ×
10 -5 10 -5 t0 .4 10 .5 10 .4 10 -4 10 .4
x × x x x x ×
104 104 104 l04 104 104 104
* Made in a sealed Pt capsule. ** Wall effect has been corrected (see text). Maximum error is calculated as ±15% on the basis of error in measuring diameter of spheres, distance of sinking and density of
melt. 1.13 X l0 s P at 1 atm to 1.8 X 104 P at 20 kbar. As shown in Fig. 1, the rate of decrease in viscosity is not constant, but is larger between 12 and 15 kbar
105
o m
10 4
NaAI SizO6 i'dE LT ~ (1350°C)
3. Results The measurements of viscosity of albite melt were made at 5, 8, 12, 15 and 20 kbar at 1400°C. The run durations ranged from 10 to 30 minutes. The results of critical runs are given in Table 1 and are plotted in Fig. 1 as a function of pressure. The viscosity of albite melt at 1 atm is quoted from Riebling [9]. The viscosity decreases with increasing pressure from
103
I
5
--
I
10
I
15 Pressure, kb
I
20
;
2
Fig. 1. Change in viscosity (in poise) of melt of albite (NaAISi308) composition with pressure at 1400°C. Solid circle at 15 kbar indicates the viscosity determined with a sealed Pt capsule. Other experiments were made using graphite capsules. Change in viscosity of NaA1Si206 composition is from Kushiro [1].
89
than in other pressure ranges examined. To confirm the viscosity gap between 12 and 15 kbar, two runs were made at both 12 and 15 kbar. One run at 15 kbar was made with a sealed Pt capsule under condition identical to that with an unsealed graphite capsule to check if there is an effect on the viscosity of H20 possibly released from the breakdown of outer talc sleeve. The charge was dried carefully before sealing. The result with a sealed Pt capsule was in good agreement with that obtained with graphite capsule. The polished sections of charges from the runs at 5, 12, 15 and 20 kbar are shown in Fig. 2, which clearly demonstrates the decrease of viscosity with increasing pressure at constant temperature. For comparison the viscosity change of melt of jadeite composition is shown in Fig. 1. The rate of viscosity decrease of this melt is larger than that of albite melt;however, the shapes of both the curves are very similar. It is interesting to note that the rapid viscosity decrease in jadeite melt is observed between 7.5 and 10 kbar, which is about 5 kbar lower than that for albite melt. Density of the glass quenched from 1400°C at 10, 15,20 and 25 kbar, shown in Fig. 3, increases with increasing pressure of quenching from 2.38 g/cm 3 at 1 atm to 2.525 g/cm 3 at 25 kbar. The rate of density increase is not constant but is relatively large between 15 and 20 kbar. For comparison the density change of glass of jadeite composition as a function
260 r
NaAISI20 ~
J
NoAISi308
2.50 O v
g
~3 2,z,0
•
/ /
,,
z
2.3£
--
,
10
I
210
I
310
Pressure~ kb
Fig. 3. Change in density (g/cm 3) of quenched melt of NaAISi308 composition as a function of pressure of quenching. That of NaAlSi206 composition is from Kushiro [11
of pressure of quenching is shown in Fig. 3. The shapes of both the curves are again very similar, but the pressure at which the density increases rapidly is higher for albite glass than for glass of jadeite composition by about 6 kbar.
4. Discussion
NaAISTaO8 melt (1400°C)
P, kb
5
t2
15
15
20
a,mm
0.49
0.39
0.46
0.42
0.42
Fig. 2. Polished charges showing the distance of sinking of Pt spheres in melt of NaA1Si308 composition as a function of pressure at constant temperature (1400°C). Run duration was 30 minutes. P is pressure of the run (kbar) and a is radius of Pt sphere (mm). The charge in the middle is in a sealed Pt capsule.
Changes in viscosity of albite melt and density of its quenched melt with pressure are very similar to those of jadeite melt, suggesting that structural changes similar to those in a jadeite melt take place in an albite melt at high pressures. The effective structural changes in albite melt, however, take place at pressures 5 - 6 kbar higher than those in jadeite melt. The difference in pressure between the univariant curves for the reactions nepheline + albite = 2 jadeite and albite = jadeite + quartz [ 1 0 - 1 2 ] near the solidus temperatures is about 7 kbar, which is close to the dif ference in pressure for the rapid changes in viscosity and density between jadeite and albite melts. The pressure-induced structural changes of jadeite and albite melts would be partly due to the presence of A1 in the melt, because the rate of decrease in viscosity of Al-free melts with increasing pressure is smaller
90 than those of N-rich melts. For example, viscosity of melts of Na2Si307 and K2MgSisO12 compositions decreases by factors of about 2.5 from 5 to 20 kbar compared to 4.5 and 5.5 for albite and jadeite melts respectively. The nature of the structural changes of both jadeite and albite melts with pressure is not known at present. In previous papers [1,2] it was suggested that the structural changes of jadeite melt may be associated with the shift of A1 from 4-fold to 6-fold coordination at high pressures. Such a coordination change of A1 must produce non-bridging oxygens. However, recent studies by Raman spectroscopy do not show a significant increase in number of non-bridging oxygens in glass of jadeite composition with increasing pressure of quenching, although changes in polarization of a certain band are observed [13]. On the other hand, the wavelength of A1 Ks and A1 K~ in the same glass changes significantly with increasing pressure of quenching. No clear explanation can be given at present for these two different observations. If albite melt transforms to a higher-density melt effectively at pressures near 15 kbar, the melting curve of albite may also change its slope in this pressure range, because the volume change for melting becomes smaller beyond the pressure range for effective structural change. The melting curve of albite has been carefully determined by Boyd and England [6]. They drew a smooth melting curve based on their data points; however, the curve is better fitted to the data if it has an inflexion at about 16 kbar. The results of the present study indicate that magmas containing appreciable amounts of the albite component would also undergo structural changes similar to those of albite and jadeite melts at high pressures.
References 1 I. Kushiro, Changes in viscosity and structure of melt of NaA1Si206 composition at high pressures, J. Geophys. Res. 81 (1976) 6347. 2 B. Velde and I. Kushiro, Structure of sodium alumino-silicatc melts quenched at high pressures; infrared and aluminum K-radiation data, Earth Planet. Sci. Lett. 40 (1978) 137. 3 H.S. Waft, Pressure-induced coordination changes in magmatic liquids, Geophys. Res. Lett. 2 (1975) 193. 4 C.W. Burnham and N.F. Davis, The role of H20 in silicate melts, II. Thermodynamic and phase relations in the system NaAISi308-H20 to 10 kilobars, 700° to 1100°C, Am. J. Sci. 274 (1974) 902. 5 C.W. Burnham, Water and magmas; a mixing model, Geochim. Cosmochim. Acta 39 (1975) 1077. 6 F.R. Boyd and J.L. England, Effect of pressure on the melting of diopside, CaMgSi206, and albite, NaA1Si3Os, in the range up to 50 kilobars, J. Geophys. Res. 68 (1963) 311. 7 K.E. Windom and A.L. Boettcher, Melting of albite at high pressures: a redetermination, EOS 58 (1977) 1243 (abstract). 8 H.R. Shaw, Obsidian-H20 viscosities at 1000 and 2000 bars in the temperature range 700 to 900°C, J. Geophys. Res. 68 (1963) 6337. 9 E.F. Riebling, Structure of sodium aluminosilicate melts containing at least 50 mole % SiO2 at 1500°C, J. Chem. Phys. 44 (1966) 2857. 10 E.C. Robertson, F. Birch and G.J.F. MacDonald, Experimental determination of jadeite stability relations to 25,000 bars, Am. J. Sci. 255 (1957) 115. 11 P.M. Bell and E.H. Roseboom, Melting relationships of jadeite and albite to 45 kilobars with comments on melting diagrams of binary systems at high pressures, Mineral. Soc. Am. Spec. PuN. No. 2 (1969)151. 12 F. Birch and P. LeComte, Temperature-pressure plane for albite composition, Am. J. Sci. 256 (1960) 209. 13 S.K. Sharma, D. Virgo and B.O. Mysen, Structure and coordination of A1 in NaA1Si206 glass at high pressures by Raman spectroscopy, Carnegie Inst. Washington, Yearb (1978) in press.
87
[51
VISCOSITY AND STRUCTURAL CHANGES OF ALBITE (NaAISi308) MELT AT HIGH PRESSURES I. KUSHIRO Geological Institute, University of Tokyo, Tokyo 113 (Japan)
Received January 27, 1978 Revised version received May 27, 1978
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 X l0 s P at 1 atm to 1.8 X 104 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 (NaA1Si206) 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 = 2 jadeite 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.
1. Introduction Recently, it was shown that the viscosity o f melt o f jadeite composition (NaA1Si206) decreases b y a factor of 12 from 1 atm to 24 kbar at 1350°C [1]. Such viscosity decrease was explained by a structural change of the melt possibly due to the shift of A1 from 4-fold to 6-fold coordination with increasing pressure similar to the crystalline phases (nepheline + albite = 2 jadeite). Density of the quenched melt (glass) increases and the wave length o f A1 Ka and A1 K~ in the glass shifts from that in crystalline albite to that in crystalline jadeite with increasing pressure of quenching, supporting the above structural change o f the melt [ 1,2]. If melt of jadeite composition undergoes a structural change similar to that of the crystalline phase at high pressures, melts of other compositions would also undergo similar pressure-induced structural changes [3]. Albite (NaA1Si3Os) is close in composition to jadeite, and albite melt would most probably show the structural changes similar to those of jadeite melt.
The structure and physical properties of albite melt at low pressures have been studied experimentally and theoretically in detail [4,5]. In addition, most magmas contain significant amounts o f the albite component, and the behaviour of a melt of albite composition at high pressures is useful for understanding the nature o f deep-seated magmas. For these reasons, the viscosity of melt o f albite composition and the density of the quenched melt have been determined over a pressure range from 5 to 20 kbar at 1400°C.
2. Experimental methods Viscosity o f albite melt has been measured in a solid-media, piston-cylinder-type apparatus with the falling-sphere method which is the same as that applied for measuring the viscosity o f a melt o f jadeite composition [ 1 ]. Graphite capsules 5 mm in inside diameter and 10 m m in inside length were used for most measurements and a sealed Pt capsule o f comparable
88 inner size was used for one measurement. A graphite heater with 3° tapered inner wall, which reduces the temperature gradient inside the capsule to about 10° [ 1], was used for all the measurements. Pressure media are fired magnesia inside the graphite heater and Pyrex glass surrounded by a talc sleeve outside the heater. Starting material was glass naade at 1 atm by melting natural albite containing small amounts of K20 (0.08 wt.%) and CaO (0.17 wt.%) supplied by T. Iiyama. Platinum spheres with diameters ranging from 0.61 to 1.03 mm were placed on top of the albite glass before each run, and the distance to which the spheres sank was measured after the run. All the runs were made at 1400°C which is well above the liquidus of albite in the pressure range between 5 and 20 kbar. Under each set of temperature-pressure condition two runs with different run durations were made to determine the velocity of sinking. Lines connecting two points in the time-distance diagram pass near tire origin, indicating that Pt spheres begin to sink as soon as the temperature reached 1400°C and they sank with constant velocities as in the case of the jadeite melt [ 1]. When crystalline albite was used as starting material, it takes a longer time to melt [6,7] and such time-distance relations cannot be obtained. Viscosity of the melt was calculated from the velocity and density of the sinking Pt spheres using Stokes' equation. Density of the quenched melt was used instead of that of the melt in the calculation. The difference (<0.3 g/cm 3) is small compared to the density of Pt (21.37) and does not produce significant error in viscosity. Correction for the wall effect has been made using the Faxen correction [8]. Density of the glass obtained by quenching the melt at different pressures was measured by a torsion balance.
TABLE 1 Critical run data and viscosity of melt of albite (NaAISi308) composition at 1400°C Pressure (kbar)
Radius of Pt sphere (cm)
Velocity of sinking (cm/s)
Viscosity ** (poise)
5 8.5 12 12 t5 * 15 20
0.049 0.038 0.052 0.039 0.046 0.042 0.042
7.22 6.67 1.22 9.44 1.94 1.72 2.61
8.1 6.1 5.2 4.4 2.4 2.8 1.8
x x x x x x ×
10 -5 10 -5 t0 .4 10 .5 10 .4 10 -4 10 .4
x × x x x x ×
104 104 104 l04 104 104 104
* Made in a sealed Pt capsule. ** Wall effect has been corrected (see text). Maximum error is calculated as ±15% on the basis of error in measuring diameter of spheres, distance of sinking and density of
melt. 1.13 X l0 s P at 1 atm to 1.8 X 104 P at 20 kbar. As shown in Fig. 1, the rate of decrease in viscosity is not constant, but is larger between 12 and 15 kbar
105
o m
10 4
NaAI SizO6 i'dE LT ~ (1350°C)
3. Results The measurements of viscosity of albite melt were made at 5, 8, 12, 15 and 20 kbar at 1400°C. The run durations ranged from 10 to 30 minutes. The results of critical runs are given in Table 1 and are plotted in Fig. 1 as a function of pressure. The viscosity of albite melt at 1 atm is quoted from Riebling [9]. The viscosity decreases with increasing pressure from
103
I
5
--
I
10
I
15 Pressure, kb
I
20
;
2
Fig. 1. Change in viscosity (in poise) of melt of albite (NaAISi308) composition with pressure at 1400°C. Solid circle at 15 kbar indicates the viscosity determined with a sealed Pt capsule. Other experiments were made using graphite capsules. Change in viscosity of NaA1Si206 composition is from Kushiro [1].
89
than in other pressure ranges examined. To confirm the viscosity gap between 12 and 15 kbar, two runs were made at both 12 and 15 kbar. One run at 15 kbar was made with a sealed Pt capsule under condition identical to that with an unsealed graphite capsule to check if there is an effect on the viscosity of H20 possibly released from the breakdown of outer talc sleeve. The charge was dried carefully before sealing. The result with a sealed Pt capsule was in good agreement with that obtained with graphite capsule. The polished sections of charges from the runs at 5, 12, 15 and 20 kbar are shown in Fig. 2, which clearly demonstrates the decrease of viscosity with increasing pressure at constant temperature. For comparison the viscosity change of melt of jadeite composition is shown in Fig. 1. The rate of viscosity decrease of this melt is larger than that of albite melt;however, the shapes of both the curves are very similar. It is interesting to note that the rapid viscosity decrease in jadeite melt is observed between 7.5 and 10 kbar, which is about 5 kbar lower than that for albite melt. Density of the glass quenched from 1400°C at 10, 15,20 and 25 kbar, shown in Fig. 3, increases with increasing pressure of quenching from 2.38 g/cm 3 at 1 atm to 2.525 g/cm 3 at 25 kbar. The rate of density increase is not constant but is relatively large between 15 and 20 kbar. For comparison the density change of glass of jadeite composition as a function
260 r
NaAISI20 ~
J
NoAISi308
2.50 O v
g
~3 2,z,0
•
/ /
,,
z
2.3£
--
,
10
I
210
I
310
Pressure~ kb
Fig. 3. Change in density (g/cm 3) of quenched melt of NaAISi308 composition as a function of pressure of quenching. That of NaAlSi206 composition is from Kushiro [11
of pressure of quenching is shown in Fig. 3. The shapes of both the curves are again very similar, but the pressure at which the density increases rapidly is higher for albite glass than for glass of jadeite composition by about 6 kbar.
4. Discussion
NaAISTaO8 melt (1400°C)
P, kb
5
t2
15
15
20
a,mm
0.49
0.39
0.46
0.42
0.42
Fig. 2. Polished charges showing the distance of sinking of Pt spheres in melt of NaA1Si308 composition as a function of pressure at constant temperature (1400°C). Run duration was 30 minutes. P is pressure of the run (kbar) and a is radius of Pt sphere (mm). The charge in the middle is in a sealed Pt capsule.
Changes in viscosity of albite melt and density of its quenched melt with pressure are very similar to those of jadeite melt, suggesting that structural changes similar to those in a jadeite melt take place in an albite melt at high pressures. The effective structural changes in albite melt, however, take place at pressures 5 - 6 kbar higher than those in jadeite melt. The difference in pressure between the univariant curves for the reactions nepheline + albite = 2 jadeite and albite = jadeite + quartz [ 1 0 - 1 2 ] near the solidus temperatures is about 7 kbar, which is close to the dif ference in pressure for the rapid changes in viscosity and density between jadeite and albite melts. The pressure-induced structural changes of jadeite and albite melts would be partly due to the presence of A1 in the melt, because the rate of decrease in viscosity of Al-free melts with increasing pressure is smaller
90 than those of N-rich melts. For example, viscosity of melts of Na2Si307 and K2MgSisO12 compositions decreases by factors of about 2.5 from 5 to 20 kbar compared to 4.5 and 5.5 for albite and jadeite melts respectively. The nature of the structural changes of both jadeite and albite melts with pressure is not known at present. In previous papers [1,2] it was suggested that the structural changes of jadeite melt may be associated with the shift of A1 from 4-fold to 6-fold coordination at high pressures. Such a coordination change of A1 must produce non-bridging oxygens. However, recent studies by Raman spectroscopy do not show a significant increase in number of non-bridging oxygens in glass of jadeite composition with increasing pressure of quenching, although changes in polarization of a certain band are observed [13]. On the other hand, the wavelength of A1 Ks and A1 K~ in the same glass changes significantly with increasing pressure of quenching. No clear explanation can be given at present for these two different observations. If albite melt transforms to a higher-density melt effectively at pressures near 15 kbar, the melting curve of albite may also change its slope in this pressure range, because the volume change for melting becomes smaller beyond the pressure range for effective structural change. The melting curve of albite has been carefully determined by Boyd and England [6]. They drew a smooth melting curve based on their data points; however, the curve is better fitted to the data if it has an inflexion at about 16 kbar. The results of the present study indicate that magmas containing appreciable amounts of the albite component would also undergo structural changes similar to those of albite and jadeite melts at high pressures.
References 1 I. Kushiro, Changes in viscosity and structure of melt of NaA1Si206 composition at high pressures, J. Geophys. Res. 81 (1976) 6347. 2 B. Velde and I. Kushiro, Structure of sodium alumino-silicatc melts quenched at high pressures; infrared and aluminum K-radiation data, Earth Planet. Sci. Lett. 40 (1978) 137. 3 H.S. Waft, Pressure-induced coordination changes in magmatic liquids, Geophys. Res. Lett. 2 (1975) 193. 4 C.W. Burnham and N.F. Davis, The role of H20 in silicate melts, II. Thermodynamic and phase relations in the system NaAISi308-H20 to 10 kilobars, 700° to 1100°C, Am. J. Sci. 274 (1974) 902. 5 C.W. Burnham, Water and magmas; a mixing model, Geochim. Cosmochim. Acta 39 (1975) 1077. 6 F.R. Boyd and J.L. England, Effect of pressure on the melting of diopside, CaMgSi206, and albite, NaA1Si3Os, in the range up to 50 kilobars, J. Geophys. Res. 68 (1963) 311. 7 K.E. Windom and A.L. Boettcher, Melting of albite at high pressures: a redetermination, EOS 58 (1977) 1243 (abstract). 8 H.R. Shaw, Obsidian-H20 viscosities at 1000 and 2000 bars in the temperature range 700 to 900°C, J. Geophys. Res. 68 (1963) 6337. 9 E.F. Riebling, Structure of sodium aluminosilicate melts containing at least 50 mole % SiO2 at 1500°C, J. Chem. Phys. 44 (1966) 2857. 10 E.C. Robertson, F. Birch and G.J.F. MacDonald, Experimental determination of jadeite stability relations to 25,000 bars, Am. J. Sci. 255 (1957) 115. 11 P.M. Bell and E.H. Roseboom, Melting relationships of jadeite and albite to 45 kilobars with comments on melting diagrams of binary systems at high pressures, Mineral. Soc. Am. Spec. PuN. No. 2 (1969)151. 12 F. Birch and P. LeComte, Temperature-pressure plane for albite composition, Am. J. Sci. 256 (1960) 209. 13 S.K. Sharma, D. Virgo and B.O. Mysen, Structure and coordination of A1 in NaA1Si206 glass at high pressures by Raman spectroscopy, Carnegie Inst. Washington, Yearb (1978) in press.