Embed Size (px)
Citation preview
American Mineralogist, Volume 70, pages 924-933, 1985
The hydrothermal mblting of low and high albite
JulnN R. Gor,osurrn.q,No D^Lvto M. JnNxwsl
Department of the Geophysical SciencesUniuersity of Chicago
5734 South Ellis AuenueChicago, I llinois 606 3 7
Abstrsct
The hydrothermal (water saturated) melting curveS of both high and low albite have beenexperimentally investigated to determine the effect of AfSi ordering on melting relations. Themelting behavior of low albite had not previotrsly been examined. At pressures above 4 kbarand temperatures below 760"C, the melting curves of low and high albite diverge steadily withincreasing pressure; at l4kbar the temirerature difference is about 30"C, with the curve forlow albite at higher temperatures than that of high albite. At pressures greater than 14 kbarboth curves display a marked decrease in their dP/d'I slopes, and at the higher pressuresthere is a profound charige in the texture ofthe quenched melt product, from a coherent glassto ri finely ,divided or granular amorphous material. It is suggested that this behavior indi-cates a fundamerital change in the fature of the zilbite melt, and that at pressures above 14kbar the melt is highly hydrated and depolymerized. At pressures below 4 kbar and temper-atures above 760'C, the melting curves of high and low albite converge and remain essentiallyindistinguishabie up to the highest temperatufes investigated (1000"C, 0.25 kbar). The influ-eirce of the ordering state of albite on its melting behavior is particularly evident for lowalbite in the t€mperature range'140-76O"C where there is a time (2-7 day) dependence on theextent of melting. The results of this study cast a new light on the properties of hydrousalbitic melts under lower crustal dnd upper mairtle conditions and ort the interplay betweenstructural state and temperature of hydrothermal melting.
IntroductionThe hydrothbrmal melting of albite is a subject of con-
siderable interest to geologists, for it serves as a good start-ing point for uriderbtanding the effect of water cin the for-mation of granitic melts. Experiinental studies by Goran-son (1938), Yoder (1955), Tuttle and Bowen (1958), Luth etal. (196\, Boettcher and Wyllie (1969), Morse (1970), Burn-ham and Davis (1974), ahd Bohien et al. (1982) have alldemonstrated a very strong decrease in the temperature ofvapor-saturated melting with ihcrease in piessure and haveall produced curves that ar€ in approximate agreement.Each of these studies, however, has dealt only with a sub-stantially Al/Si disorderbd form of albite (high aibite) andnot with the geologically mord relevant Al/Si ordered form(low albite). Giveii the large differences in the meltin! tem-peratures of stable and metastable forms of certain sub-stances, such as quartz (<1500"C) relative to cristobalite(1713'C) (Sosman, 1927\, it was felt desirable to comparethe hydrothermal melting of low and high albite. Fr.tr-thermore, it was thought that additional information onthe high-low albite relatlons might be obtained, sup-
1 Current address: Department of Geological Sciences, StateUniversity of Nbw York-Binghamton, Binghamton, New York13901.
000H04x/8 s/09 1H924$02.00
plementing that reported by Goldsmith and Jenkins (1985)in the preceding article.
Figure 1 is a diagrammatic representation of the poly-baric hydrothermal melting behavior of two modificationsof a substance such as albite in which the liquid takes H2Oin solution but the solid does not. For both modificationsthe melting temperatures are lowered with increasingP(HrO). However, the dPldT slopes of the two meltingcurves differ at a given pressure depending on the differ-ences in the Gibbs free energies of the two solids (assumingthe liquids to be equivalent). The curves cross at the pres-sure and temperature (f) where the two modifications arein equilibrium. At temperatures lower than the transitiontemperature f high albite is metastable with respect to lowalbite, and the rnelting curve of high albite (dashed curve)lies at lower,temperatures than that of low albite (solidcurve). At temperatures above { the opposite situationexists. To simplify the above discussion a first-order trans-formation was assuhed instead of the "non-first" ordertransformation determined for albite by Goldsmith andJenkins (1985). A non-first order transformation wouldproduce a range of pressures and temperatures over whichthe two curves would coincide before diverging.
In this study the melting relations of natural low albitehave been investigated as a function of P(H2O)(: P,",")
924
GOLDSM ITHAND JENKINS: MELTING OF LOW AND HIGH ALBITE
,q,
Hiqh Ab
ttro
1 Low Ab
T ---
Fig. 1. Diagrammatic representation of polybaric hydrother-mal melting relations of two modifications of a substance in whichthe liquid takes H2O in solution, whereas the solid does not. Afirst-order transformation is represented, and the intersection ofthe curves marks the equilibrium temperature of the transition ({)at that pressure. Metastable extensions are shown as dashedcurves. If this were the situation for albite, the low albite meltingcurve would be the solid line below ?], and the dashed line athigher temperatures; the high albite melting curve solid at temper-atures greater than f, and dashed at lower temperatures.
and of time. In addition, the curve for high albite wasconcurrently re-investigated.
Experimental methods
Appar atus and techniques
The experimental apparatus and techniques (piston-cylinder ap-paratus and internally heated argon pressure vessels) are essen-tially the same as those described in the preceding article (Gold-smith and Jenkins, 1985) with a few exceptions. In the hy-drothermal runs made at relatively high temperatures and lowpressures (less than - 1 kbar), it was necessary to limit the amountof water in the sealed capsule to avoid pulling or bursting. Theamount of water needed was determined from steam tables aftercalculation of the capsule volume, including its contents, and thewater was carefully added with a microliter syringe. lrakage waschecked by weighing the capsule before and after the run.
In experiments in the P(HTOFT regions where the melting be-havior of high albite and low albite could be directly compared,samples of both polymorphs were run adjacent to each other. Asmany as four capsules could be accommodated in the gas vessels,and one or two, rarely three, in the solid media devices.
Analytical techniques
All of the hydrothermal runs were examined under the petro-graphic microscope. A JEOL JSM-35 scanning electron mrcro-scope was used to examine selected run products. Relative propor-tions of crystals and glass in runs that were partially rrelted arebased on best estimates, by eye, under the microscope. Whenever
possible, the estimate was also based on the intensities of theX-ray diffraction pattern relative to scans of crystalline albite.Measurements of the Al/Si ordering were made on some runproducts by measuring the L20l3l-l3l (or simply A131) from theX-ray diffraction pattern as described by Goldsmith and Jenkins0985).
Starting materials
Three natural low albites [LoAb(a), LoAb(b), LoAb(c)] andthree synthetic high albites [HiAb(a), HiAb(b), HiAb(c)] were usedin this study and are described in detail by Goldsmith and Jenkins(1985). To evaluate the effect of any slight but possibly significantcompositional difference between the low albite and synthetic highalbite used, two samples of LoAb(a) were converted to high albiteby heating at 1200'C and 20 kbar for six days (4131 : 1.96, 1.98),to be used for comparison experiments, and are designated inTable I as MAb-15 and MAb-20.
Experimental results
Criteria for hydrothermal melting
Glass and crystals of albite are observed to coexist inquenched hydrothermal experiments. Tuttle and Bowen(1958) reported that albite melts over a temperature rangeof 10"C or more, and ascribed the melting interval to sev-eral possibilities: (1) the liquid and/or the vapor had com-positions off the binary join albite-water, (2) equilibriumwas not reached in their runs, or (3) there was solid solu-tion of water in albite. Tuttle and Bowen (1958) noted thatthe beginning of melting was sharp, and that approxi-mately 90% glass was formed in a temperature interval of10"c.
The question of equilibrium is of interest. Many of thesamples run for a matter of hours, or in some cases. evendays, are glassy'slugs", with clear to bubbly outer layers,and a core ofcrystals and lesser glass. It would appear thatmelting which may begin throughout the compressed pow-dered charge is suppressed in the water-depleted interior ofthe sample by a layer of glass through which water mustdiffuse. This eflect was observed by Burnham and Jahns(1962), who, in measuring the solubility of water in silicatemelts, state, "It is extremely important that each melt becompositionally uniform, and in particular that the intro-duced water be uniformly distributed through it; otherwisecrystals invariably form in local domains that are undersat-urated with respect to water, even though the melt as awhole might be oversaturated." A number of the runs inthis study that show crystalline material encased in glassare not equilibrated from this point of view. However, sincethe "solidus", or beginning of melting is used in this work,no effort was made to determine the equilibrium liquidus.
The suggestion by Tuttle and Bowen (1958) that thecomposition of the liquid or vapor might not lie on thebinary join NaAlSi.Or-HrO is reasonable. We see nocompelling argument for congruent solubility of multicomponent phases. Morey (1957) presents data on albite at500'C and 400, 1000, and 2000 bars P(HrO) indicating thatthe material transported by steam at 400 bars containsalkali oxide and silica in excess of the feldspar ratio, and
926
the remaining material contains an excess of alumina. At2000 bars, however, the ratio is closer to congruency. Arather small deviation from congruent solubility might wellaccount for the 5-15"C melting interval observed in albite.
High albite
Hydrothermal melting of (high) albite has been studiedby Goranson (1938), Yoder (1958), Tuttle and Bowen(1958), Luth et al. (196r'-), Boettcher and Wyllie (1969),Morse (1970), Burnham and Davis (1974\, and Bohlen et al.(1982). To ensure direct comparison with low albite, themelting curve of high albite from 16 kbar to less than Ikbar was re-investigated. The data are in Table 1 and plot-ted in Figure 2.
GOLDSMITH AND JENKINS: MELTING OF LOW AND HIGH ALBITE
A number of experiments were made to evaluate theeffect of time on the location of the solidus, but run dura-tion from 1 to 350 hours produced the same results. Wealso found no significant difference in the melting of thesynthetic high albite and the high albite prepared by disor-dering LoAb(a). Our curve is in rather good agreement(within - 10'C) with Tuttle and Bowen (1958), particularlyat the lower pressures, in excellent agreement with Burn-ham and Jahns (1962) and with Boettcher and Wyllie(1969) at 10 kbar, but is significantly lower (-20"C) thanLuth et al. (1964) at 8 and 10 kbar, and is significantlyhigher than Bohlen et al. (1982) in the range 5-10 kbar.The latter reference discusses (p. a5a) the comparative dataofprevious investigators. Bohlen et al. (1982) point out that
Table 1. Hydrothermal melting data
Starti ngnateri a l
I t m e t
hrsT ' C P ,
KOarRun #MAb.
'I .5
l 5
I . 5L J
2Il1 . 522
IIl 51 . 51 . 5l 5
t 51 . 5
771 . 51 . 5I
240II1
72
6306306006 1 0
600630650660670680
600640660670
600630640670680
E50670670680685685
690700680680690690
700700700705705720
722685685700700
L o A b ( a )LoAb( a)L o A b ( a )L o A b ( a )
H i A b ( b )LoAb( a )LoAb ( a)LoI\b ( a )LoI\b ( a )LoAb(a)
H i A b ( a )H i A b ( a )LoAb ( a)LoAb (a )
H i A b ( a )H i A b ( a )H i A b ( a )LoAb(a )LoAb( a )
H i A b ( a )H i A b ( a )L o A b ( a )L o A b ( a )L o A b ( a )L o A b ( c )
1 5 41 5 31 6 51 6 6
1 6 41 5 51 3 71 3 31 0 8r 0 5
162I ] l146147
1 6 3't60
1 5 6't49
148
256b6a
30l04aI 04b
9t 632a32b26a26b
8 . 0 5I888
8II888+
88II7 . 9 06 . 0 5
65 . 9 86 . 0 06
1 9t 818t18 !
1 7t 71 71 71 71 7
l 5't6'16
t 6'15
l 5l 5l 5l 5
l 4l 4t 4l 4t 4t 4
l 4l 4l tl tl ll t
ill ll till lt l
1 . 5
t 5l . c
I J
l . J
I3 . 2 5
1 . 5
A b , q u e n c h f l u i d , s o m e j a d e i t e
A b , q u e n c h f l u i dA b , p u r e j a d e i t eA b , q u e n c h f l u i d , r a r e j a d e i t e
q u e n c h f l u i d , A ba l l A bA b , s o m e q u e n c h f l u i dA b , q u e n c h f l u i dq u e n c h f l u i d , s o m e A bq u e n c h f l u i d , t r a c e A b
A b , q u e n c h f l u i dquench f luid, sore l \ba l l A bq u e n c h f l u i d , A b
a l I A bA b , s p a r s e q u e n c h f l u i dA b , q u e n c h f l u i da l l A bA b , q u e n c h f l u i d
xl s , v. snal I anount gl ass I .85g l a s s + m i n o r x l sa l l x l sa l l x l sa l l x l sa l l x l s
x l s , s m a l l a m o u n t g l a s s 1 . 0 8g l a s s , s o m e r e n a i n i n g x l sa l l x l sa l l x l sx l s + g l a s s ( { 5 0 - 5 0 )x l s + s n a l l a n o u n t g l a s s
a l I g l a s sa l l g l a s sx l s , r a r e g l a s s 1 . 1 2a l l g l a s sx l s , v . s m a l I a m o u n t 9 l a s sa l I g l a s s
x l s , v . s m a l I a m o u n t g l a s sa l l x l sa l l x l sa l l x l s l . 0 Ba l l x l s I . 8 7
x l s , v . s m a l l a n o u n t g l a s s I 9 0a l l x 1 s
, l . 0 9
x l s , s m a ' l l a m o u n t g ' l a s s ' 1 . 9 0
a l l x l s I 1 5a l I g l a s sx l s + < 2 5 % g l a s s
' ] . 0 6
x l s , v . s m a l l a m o u n t g l a s s l . l la l I g l a s sg l a s s + < l / 2 x l sa l I g l a s sg l a s s + <
' l l 2 x l s
a l l x l s l . l 4 - 1 l 7
v r a r e g l a s sa l I x l sa l l x l sa l l g l a s s
36a36b5728b28a34
86a2a2 63
55
234a4b
7A2256
5a5b
241 0
1 0 375a
494 7
t 0 l44
LoAb( a )L o A b ( a )H i A b ( a )r'lAb-20H i A b ( a )Mb-20
M b - 2 0H i A b ( a )LoAb(a )MAb-20L o A b ( a )LoAb( a )
LoAb( a )LoAb( a )H i A b ( a )L o A b ( a )H ' i A b ( a )
H i A b ( a )LoAb( a )H i A b ( a )LoAb( a )H i A b ( a )L o A b ( a )
LoAb (a )H i A b ( a )LoAb( a )LoAb(a )H i A b ( a )L o A b ( a )
H i A b ( a )H i A b ( a )H i A b ( a )H i A b ( a )
1 . 5lI
1 6 81 . 5
72
Il1 . 5I2 . 3
431
1 . 5l 521 . 5
7 1 07157 1 57 1 5720725
7 3 0730740
719728
730731735740
GOLDSMITH AND JENKINS: MELTING OF LOW AND HIGH ALBITE
Table l. {cont.)
927
1 350
740741
743149755756765765
738743748748748751
t o A b ( a )H i A b ( a )
L o A b ( a )L o A b ( a )LoAb( a )L o A b I a )L o A b ( a )L o A b ( a )
H i A b ( a )H i A b ( a )L o A b ( a )L o A b ( c )L o A b ( b )H i A b ( a )
L o A b ( a )L o A b ( c )L o A b ( a )H i A b ( a )L o A b ( a )l .4Ab-t 5
L o A b ( a )l l A b - t 5L o A b ( a )L o A b ( a )L o A b ( a )L o A b ( a )H i A b ( a )LoAb( a )H i A b ( a )L o A b ( a )H i A b ( a )
H i A b ( a )LoAb( a )l '1Ab-20H i A b ( a )H i A b ( a )
LoAb (a )H i A b ( a )LoAb( a )H j A b ( a )LoAb( a )L o A b ( a )
LoAb( a)LoAb( a )H i A b ( a )H i A b { a )i{Ab-20toAb( a )
LoAb( a )H i A b ( a )LoAb( a )t4Ab-l 5L o A b ( a )H i A b ( a )
LoAb( a )H i A b ( a )LoAb( a )H i A b ( a )l lAb-20LoAb( a )
L o A b ( a )H i A b ( a )LoAb( a )L o A b ( a )M b - 2 0H i A b ( a )
L o A b ( a )l tAb-20H i A b ( a )LoAb( a )
LoAb( a )r" tAb-20H i A b ( a )L o A b ( a )t4Ab-20LoAb( a )
6 1 6 86 t 0 I 5
x l s + 1 5 0 % g l a s s ' ] 1 0
a l 1 g l a s s
a l l x l s t l lx l s + g l a s s ( i 5 0 - 5 0 ) x l s b o t h l o a n d h i ?x l s + r a r e g l a s sx l s + s o m e g l a s sg l a s s + { l 0 Z x l sg l a s s + a 2 0 U x l s
a l l x l sa l l x l sa l l x l sa l l x l sa l l x l sx l s , t r a c e g l a s s
x ] s , s m a l l a m o u n t g l a s s x l s b o t h l o a n d h ix l s + < 5 0 " l g l a s s x l s b o t h l o a n d h ia l l x l s I 1 2a l I g l a s sa l l x l s I 1 3g l a s s + s p a r s e x l s
a l l x l s ! l . l 7x l s + g l a s s ( ! 2 0 % ? )g l a s s + < 5 0 % x l s l 1 3a l 1 x l s I 1 3 , s o m e h i A b ?a l l x l s l o a n d h j A b ( r I 1 5 + r I 8 )
x l s + a l o i ; g l a s s l o a n d h i A bx l s + . 5 0 % g l a s s 1 8 4a l l x l s l o a n d h j A b ( a I l 0 + I 8 5 )a l l x l s 1 8 5a l l x l s l o A b ( l l 0 ) + s o m e h i A b ( l 8 2 )a l l x l s 1 . 8 5
a l l x l sx l s , H i A b , v s m a l l a m o u n t g l a s sx l s , H i A b , v s n a l l a m o u n t g l a s sx l s , H i A b , s m a l I d m o u n t g l a s sg l a s s , s o m e x l s l - 8 6
s m a l l a m o u n t g l a s s H i A b ( l 8 8 ) + L o A b ( l 0 6 )g l a s s + s o n e x l sg l a s s + s p a r s e x l sa l l x l s I 0 5a l I g l a s sg l a s s + x l s ( a 5 0 - 5 0 ) I 1 2 , s o m e H i A b ?
a l l x l s H i A b , I 8 2x l s + < 5 0 % g l a s s 1 . 0 7g l a s s , s o m e x l sx l s + . , 2 0 % g l a s sx l s + ! 2 0 % g l a s sx l s + . 2 0 " 1 g l a s s L o + s o m e H i A b
x l s + s m a l l a m o u n t g l a s s I 9 lx l s + s n a l l a m o u n t g l a s sa l I g l a s sa l I g l a s sa l l x l s L o + s o m e h i g h i s h A bx l s + g l a s s
a l l x l s L o ( l 0 5 ) + s o m e h i g h ( l 8 3 ) A ba l l x l s I 9 0a l l x l s \ l / 2 H i A b , 1 B zg l a s s + s o m e x l sg l a s s + s o m e x l sg l a s s + s o m e x l s s o m e H i A b
a l l g l a s sa l l g l a s sa l l x l s e u h e d r o n s o f H i A b
- l . 9 5
g l a s s + s o m e x l sg l a s s + s o m e x l sg l a s s + s o m e x l s
x l s + g l a s s L o + H i A b ( 2 0 )x l s + g l a s s 2 0 2x l s + g l a s s 1 9 7a l l x l s s o m e H i A b ?
g l a s s + s o m e x l sg l a s s + s o m e x l sg l a s s + s o m e x l sx l s , v . s m a l l a m o u n t g l a s s L o A b + s o m e H i A b ( l 9 3 )x l s , v . s m a l l a m o u n t g l a s s 2 . 0x l s , t r a c e g l a s s s o m e h i A b ?
154754758758761761
7617617647481607687687617617751 7 5
7797a5785785790
7907907997998008 1 0
800841841850Bs0850
849849890890890890
890890890900900900
937931896948948948
949949949952
I 000'1000
I 000I 000I 000I 000
B l a92a4 l4 53 94 0
9 91 0 09 6 a9 6 b9 6 c
1A2
9 5 a9 5 b
7 a7 b
t 7 b
2 ) a2 t bt 86 lB 7 a
B2a8 2 b7 4 a7 4 b6 3 a6 4 b
6 06 9 a6 9 b69c5 9
6 6 a6 6 b52a52b
5 B
5 48 a8 b
5 3 a5 3 b5 3 c
5 l a5 t bl 9 ar 9 b1 2 a12b
0 B 1 6 90 0 1 6 6
t 50 8 1 . 5
t 5l 0 I 5
4 . 9 7 25 r 0 24 97 t674 . 9 7 1 6 74 . 9 7 1 6 75 . 0 0 2
5 00 1615 . 0 0 I 6 75 . 0 0 I5 0 0 l5 0 0 4 55 0 0 4 5
5 0 3 B5 0 3 B5 0 0 1 3 64 30 2094 . 0 0 3 3 14 02 tB44 02 1843 60 3543 .60 3542 95 1642 . 9 5 I 6 4
3 . 0 4 I 6 33 . 0 4 I 6 33 . 0 5 1 73 0 5 1 73 . 0 4 1 3 83 . 0 7 r . 5
2 9 5 t 53 05 3323 .05 33?3.05 3323 0 0 r 5
l 3 a1 3 bt 4
2 7 b27c
37b37c33
2 . 5 0 1 3 02 . 0 0 r . 82 0 0 t B1 6 3 1 51 . 6 3 r . 5r . 6 3 1 5
1 . 4 7 1 4 41 4 1 1 4 41 3 8 4t . 3 B 41 . 0 0 21 0 0 2
1 0 0 21 0 0 2I 0 0 t 7 51 . 0 0 21 . 0 0 21 0 0 2
1 0 0 21 0 0 24 . 7 7 r 5 00 7 0 I0 7 0 I0 7 0 I
l l al l b4 B3 5 a3 5 b3 5 c
3 8 a3Bb38c4 6 a4 6 b4 3
0 . 5 50 - 5 50 5 50 5 0
0 . 3 00 3 00 3 00 . 2 60 2 60 2 5
lII
L i s t e d i n o r d e r o f d e c r e a s j n q p r e s s u r e .N o . r e f e r s t o A l 3 l o f x l s i n - r u n p r o d u c t .D e c i m a l v a l u e s i n d i c a t e g a s v e s s e l r u n s .
GOLDSMITH AND JENKINS: MELTING OF LOW AND HIGH ALBITE
lemperolure , oC
Fig. 2. The hydrothermal melting of synthetic high albite (A : 1.90-1.92). Open triangles represent no melting solid trianglescomplete melting, and partial melting is indicated by partially filled symbols. Larger triangles represent runs made in piston-cylinder
devices, smaller triangles those done in internally heated gas vessels.
the value of 758'C+3"C at 5.0 kbar obtained by Morse(1970) in cold-seal apparatus and calibrated internallyheated gas-pressure apparatus (754'C if a pressure correc-tion of + 4"C to the thermocouple is removed) is dissonantwith their data and with the values of Tuttle and Bowenextrapolated beyond 4 kbar. Our value at 5 kbar is veryclose to 750"C.
The reasons for fairly large variation in the location ofthe solidus are not clear. Aside from errors in pressure andtemperature measurement or control, or composition of thealbite, differences might be related to detection of the firsttrace of melt, and perhaps, as will be seen in the followingsection, the structural state of the starting material.
Low albite
The data on the hydrothermal melting of low albite asthe starting material are also presented in Table 1, and areplotted in Figure 3. We are not aware of any other deter-minations on hydrothermal melting of low albite. In Figure4 the curves determined from both low and high albite arepresented.
Several things are apparent from Figures 3 and 4 andfrom the data in Table 2:
1. The melting curve determined from low albite, unlikethe high albite curve, has a pronounced inflection, which is
the result of an observed time-dependent melting behaviorat temperatures above 745'C.2 Albite that is unmelted inshort runs may melt to a significant extent in longer runs.For example, at 758" and 5 kbar, run 7a was all crystallinein one hour, and at 760' it was almost half melted in 136hours. This behavior was not observed at 11 kbar (-700")and 8 kbar (-720'), nor, as previously mentioned, was itobserved in runs on high albite. The control in the gasvessels is good enough to obviate variations in temperatureor pressure as contributing to the effect of time on degreeof melting.
2. At temperatures below and pressures above the regionof inflection, the low albite curve3 lies at signiflcantly
2 The existence of an inflection is based on a pair of carefullycontrolled seven day gas-vessel experiments that bracket the curve
at 6 kbar (MAb-91 and 92). A single run (MAb-73) done in piston-
cylinder apparatus, is in conflict with this pair. The pressure and
temperature measurement and control in the solid media device is
not as good as in the internally heated gas vessel, especially in aseven day experiment at only 6 kbar. This point is therefore notplotted in Fig. 3.
3 The phrase "low albite curve" will be used henceforth in place
of the more accurate but awkward description "curve determinedwith low albite as the starting material."
Alb i te o \ ^
GOLDSMIT,H AND JENKINS: MELTING OF LOW AND HIGH ALBITE
Atbire " \rn VoPor
750 800 850Temperolure , oC
Fig. 3. The hydrothermal melting with low albitb (A = 1.I0) used as starting material. Open circles represent no melting, solid circlescomplete fnelting, and paitial melting is indicated by partially filled symbols. Large circles represent runs made in piston-cylinderdevices, smaller circles those done in internally heated gas vessels. See text for discussion oftime-dependence olmelting behavior.
20
r6
t l
R
4
n
lemperolure,oCFig. 4. Composite diagram of high albite melting curve (frcim Fig. 2) and curve detbrmined from low albite $tarting material (from
Fig. 3)' High-piessure segments are from Fig. 5. Also shown is the high albite jadeite + qtafiz bourrdary after Hcilland (1980). Isoplethsof structural state are plotted as straight linbs, with slope of dPldT:350 bars/K, from Goldsmith and Jenkins (1985). Note change inisopleth spacings at A : 1.85 (dashed line) arid higher values.
o-:<
<u=<t)aJ'tq)
o_
930 GOLDSMITH AND JENKINS: MELTING OF LOW AND HIGH ALBITE
higher temperatures than the high albite curve. The curvesdiverge with decreasing temperature and the difference at14 kbar is approximately 30'C.
3. At temperatures above, aird pressures bdlow theregion of inflection, the low Albite curve becomes coin-cident with the high albite curve.
4. A "crossover", as in Figure 1, where the low dlbitecurve would appear rhetastably at a lower temperbturethan the high albite curve is not observed. No melting oflow albite has been detected in the P-T rebion below thehigh albite curve.
Melting pressures aboue 14 kbar
The higher pressure dita on the hydrothermal melting oflow and high albite are given.in Table I and plotted inFigure 5. At pressures above approximately 14 kbar bothsolidus curves undergo h distinct flattening of the dP/dTslope. This greater pressure ddpendency produces meltingat temperatures as low as 600"C as pressures approach ihebreakdown to jadeite plus quartz. In this region the solidusoflow albite is close to 3 kbar above that ofhigh albite.
In addition, the quenched melt-product has a texturequite unlike that of the lower pressure melts. It is not acoherent glass, bi.it a soft powder, amorphous to X-rays.
Figure 6 is an SEM photograph of MAb-105, low albiterun at 17 kbar, 680'C. The "powder" is made up of glob-
ules, mostly 2-5 microns in diameter. Small amounts of
unmelted albite are present in this experiment, and one ofthe more obvious residual albite grains is apparent inFigure 6. Analysis of the globules and grains using a KevexEDS system show them to have the albite stoichiometry.No attempt was ntacie to determine the water content of
this high-pressure melt product. Under the petrographic
microscope it appears to be a somewhat brownish, iso-tropic "cioudy" material. It is less well defined than normalglass; the small particles are not clearly resolved. The melt-
ing interval may be larger at higher than at lower pres-
sures.Figure 5 also contains the curve for the reaction high
albite : jadeite + qnartz determined by Holland (1980).
Newton and Smith (1967) and Hlabse and Kleppa (1968)
considered the reaction low albite : jadeite * quartz, ofin-terest in conjunction with Figure 5, but no reversed dataare available. None of the runs plotted in Figure 5 wereseeded with jadeite, and do not represent reversals. How-ever, the three runs on low albite at pressures > 18 kbarshow small amounts of jadeite, and a highly tentativebreakdown curve for low albite is indicated. The observed
20
IB
O(\l
o--
o
><
t6
l4
t2
r0
B
6 600 650 oc 700 750Fig. 5. The hydrothermal melting curves of high albitc and low albite with data points at the higher pressures. The high albite
jadeite + quartz boundary is after Holland (1980); the dashed line is a tentative boundary for the reaction low albite ?jadeite + quartz
{see textl.
Melt + Vopor
Ab + Vopor
GOLDSMITH AND JENKINS: MELTING OF LOW AND HIGH ALBITE 931
Fig. 6. SEM photograph of the product quenched from lowalbite starting material run at 17 kbar, 680" (MAb-105). The hori-zontal bar represents 10 pm. The large object is a residual crystalof albite.
change in properties of the hydrous liquids takes place atpressures not far below those at which 4-coordinated Al inalbite changes to 6-coordination injadeite.
Discussion
Implications of the melting curvesIf a chemical and structural state equilibrium were es-
tablished during hydrothermal treatment, the metastablelimbs would not have been realized, and a single meltingcurve would have been produced. In a first-order transi-tion, as iilustrated in Figure 1, a single curve with a breakin slope at the transition would have resulted. If the transi-tion had taken place over a limited temperature interval,the break would have been smoothed into an inflection.
The establishment of structural state equilibrium by hy-drothermal treatment in the pressure and temperatureregion of the transition has been shown in previous studiesto be very diffrcult. The separation of the two curves in thelower temperature higher pressure region (Figs. 4 and 5)clearly indicates that the degree of Al/Si order is essentiallyunchanged, even in the longer runs. It can, however, beconcluded independently of the data of Goldsmith and Jen-kins (1985) that at the higher temperatures where the
curves coalesce, high albite is stable. A number of the runson low albite in this region show at least some high albitein the diffraction patterns, with or without glass being pres-ent (e.g. Nos. MAb-95a,82a,63a, 54).
The melting of high albite is presumed to be a relativelystraightforward conversion of an Al/Si disordered crys-talline network to a more random disordered (even if)highly polmerized melt. Water helps depolymerize theliquid, but on a short-term basis has rather little eflect onthe structural state of the solid. But in what way does lowalbite melt? Does it disorder completely to high albite, andmelt aS the disordered modiflcation, or can it be induced togo directly or abruptly to the disordered liquid? It is possi-ble that runs which produced low albite coexisting with aglass at temperatures above the region of inflection (i.e.,MAb-18) are examples of the partial cotrversion of lowalbite to high albite with subsequent melting of the highalbite. Unfortunately we do not see how to reconstruct thehistory of the crystals that have melted. It is, however,reasonable that low albite which does not melt in a matterof hours, but that does melt in significantly longer timesunder the same P(HrOfT conditions, is slowly disor-dering, and is therefore at a temperature above the equilib-rium structural state ofthe starting material.
The absence of a detectable metastable extension of thelow albite melting curve beneath the high albite curve maybe a consequence of the low albite melting process. If disor-dering is a precursor to melting, the metastable extensionwould not be realized. The evidence points to this con-clusion. We are not, however, prepared to consider thebroader aspects of melting, which involves comparing thebehavior of compounds that may retain most of their AfSiorder up to the melting point, such as anorthite (Goldsmithand Laves, 1955; Laves and Goldsmith, 1955), to the "poly-morphic" behavior of albite.
Structural state and melting equilibria
Figure 4 illustrates the fact that hydrothermal meltingbehavior is related to the structural state of the albite andthat each structural state, in the absence of change duringthe run, will have its own hydrothermal melting curve. It isalso likely that dry melting at one atmosphere, if rapidenough to avoid further disordering, would show meltingtemperatures that varied with structural state of the start-ing material. The investigation iiivolving direct reversal ofdegree of order (Goldsmith and Jenkins, 1985) can be used,to better understand the extent to which structural stateequilibrium or disequilibrium may effect the curves.
Figure 4 contains, in addition to the hydrothermal melt-ing curves, isopleths of structural state obtained from thedata in Figure 1 of Goldsmith arid Jenkins (1985) using avalue of dPld'f : 350 bars/K. The A : 1.85 isopleth, thevalue at which the rate of change of the disorder in albitehas slowed and which is at 790 800"C at l7 18 kbar, cutsthe melting curves near 760" and 4.5 kbar, essentially at thecoalescence of the curves determined from high albite andlow albite. The degree of order represented by A131 : 1.85
932 GOLDSMITH AND JENKINS: MELTING OF LOW AND HIGH ALBITE
is also that value at which the rate of change of order withtemperature undergoes a large decrease (Goldsmith andJenkins, 1985, Fig. 1). The coincidence of these two obser-vations justify a definition of high albite as albite withA131 : 1.85. At temperatures greater than 760'C, thesingle curve probably represents thg equilibrium hy-drothermal melting curve for those high albites that arestable at temperatures >760". The data in Table 1 tend tosubstantiate the view that at temperatures >760'the hy-drothermal conversion of low albite to high albite is rapidenough that the melting of a high albite determined theliquidus, even if residual low albite remains. The A131-value of the highly disordered albites in equilibrium withthe melting curve should increase with T' > 760" to values> 1.85 as indicated by the isopleths in Figure 4.
The lower-temperature limb of Figure 4, the high albitecurve, is certainly not in equilibrium with melt, and is arelic of the slowness of the ordering process even under thelower temperature but higher P(HrO) conditions. Thetime-dependency of melting of low albite has been pointedout, particularly in the temperature range 74V76O"C.Little change of behavior with time is observed at lowertemperatures. If equilibrium were presurired, the interpreta-tion could be that the curve at temperatures below ap-proximately 740' rdpresented stable low albite, and ihat inthe 740-760"C region intermediate albites were stable. Theisopleths of A131 show, however, that at all temperaturesdown to approximately 680'C, interniediate albites arestable, and that at the pressures of the melting curves,the range of intermediate albites extends from -680' to-760"C. Certainly at temFeratures <650'C low albite isstable, as is this portion of the low albite cufve. The signifi-cantly more refractory nature of low albite compared tohigh albite at these low temperatrlres may be a conse-quence of the fact that at low temperatures lirw albite mustmelt as an ordered compound. The separation of the curvesis an expression of the energetic difference of the two statesof AllSi order.
It may be that the T regiori of timedependent meltingexists only because the rate of Al/Si hydrothermally in-duced change in structural state is rapid enough to beobserved in the time allotted to the experimental runs, andat tefiiperatures below -740'C it is too slow for even thevery patient investigator. The region of inflection is thusalmost certainly an expression of kinetic, not equilibriumbehavior, for the equilibrium transition takes place overthe entire lower temperature region of the determinedcurves. The equilibrium melting curve, at temperaturesbelow 760'C. lies somewhere between the two curves ofFigure 4.
It was noted that in the region of 5-10 kbar, the curve ofBohlen et al. (1982) is at lower temperatures than our highalbite curve. Bohlen et al. heated their synthetic albite at1100'C for 24 hours before determining the melting curve.No Al3l-value or measure of disorder of the albite wasgiven, but it is possible that their melting curve was deter-mined with material of a higher structural state than thatin Figure 2. It is not likely, however, that the difference
between the burve of Bohlen et al. (1982\ and that of Figure2 could be due entirely to this effect, based on the spreadbetween high and low albite in Figure 4.
Albite liquid aboue 14 kbar
The observed melting behavior of albite at prbssuresgreater than 14 kbar oalls for comment, even if premature.It is well known thai water depolymerizes silicate melts (c.f.Burnham, 1975), and that water content of melts in0reaseswith P(H,O). farge decreases in viscosity in highly poly-merized melts such as albite are the result of the dist'uptionof the melt framework by interaction of HrO with bridgingoxygens to form OH groups and a less polymerized melt.Stcilper (1982) presents evidence for the existence of molec-ular water in solution as well as hydroxyl groups; and alsofor an inciease in the proportion of moleculdr rdater withinciease in H2O concentration in the melt. At pressures>10 kbar (>15 wt.o/o water) Stolper (1982) states thatmost of the dissolved water is present as molecular water.Thus the initial extensive depolymerization produced bythe hydroxyl groups at low water concentration becomesdecreasingly effective as concentration lncreases, and meltviscosity, which dramatically decreases as watei is firstadded, should show less of a decrease with additional in-cremenis of water. This prediction does not explain ourobservations in this study at P(H,O) > 1{ kbar.
The HrO content of the high pressure liquids in thisstudy hhs not been determined. Burnham and Davis (1974)show a value of 16.5 wt.oh water in the saturated albitemelt at 700"C and 10 kbar; at higher pressures and lowbrtemileratures the value must be significantly greaier. Thevalue of 16.5 wt.% translates to 74 mol percent; it wouldappear that at thd higher pressures the "ftelt" is a low-density rather tenuous fluid. The flattening of the solidiimay even indicate an approach to maxima on the three-phase curves, as observed in KrSi2O5 (Morey and Fenner,19l7t.
Cation coordination rhust also be considered, particu-larly as effected by the presence of water. A coordinationchange of Ai from 4 to 6 in aihyilrous aluminosilicate liq-uids; which would induce depolyrilerization, has been pro-posed to explain the observeil decrease in viscosity ofaluminosilieate melts at high ,pressures (Kushiro, 1976;1978). No evidence for octalredral Al has been found in anumber of investigations of Al-containing glasses quenchedat high pressures (Sharma et al., 1979; Sharma and Simons,1981) or even in the more favorable case of gallium-containing glasses (Fleet et al., 1984). There is no assurance,however, that a coordination change would be preservedduring the quench, and in situ observations at high pres-sures and temperatures have not been carried out.
The presence of hydroxJ'l groups in place of oxygerratoms around Al and Si should favor higher coordination.Hydroxyl units are associated with Al, not Si, in alumi-nosilicates, normally with the Al in 6 coordination. TheOH group has a smailer charge than oxygen, and highercoordination is aided by reducing repulsion among the co-ordinating atoms (Goldsmith, 1953).
GOLDSMITH AND JENKINS: MELTING OF LOW AND HIGH ALBITE 933
AcknowledgmentsThis research was supported by NSF grant EAR-8305904. We
are indebted to Robert G. Coleman, D. B. Stewart. and T. J. B.Holland for albite samples LoAb(a), LoAb(b), and LoAb(c), toMark Barton for high albite sample HiA(c), to Leslie Pearson,who keeps the high-pressure laboratory operational, to JohnSuber for skillful fabrication of parts that are doomed to destruc-tion, to Ed Olsen for SEM data and photographs, and to Cass-andra Spooner for patient typing.
References
Boettcher, A. L. and Wyllie, P.J. (1969) Phase relationships in thesystem NaAISi3OB-SiO2-H2O to 35 kilobars pressure. Ameri-can Journal of Science, 267, 87 5-909.
Bohlen, S. R., Boettcher, A. L. and Wdl, V. J. (1982) The sysremalbite-HrO{Or; a model for melting and activities of water athigh pressures. American Mineralogist, 67, 451462.
Burnham, C. W. (1975) Water and magrnas: a mixing model. Ge-ochimica et Cosmochimica Acta, 19, 1077-1084.
Burnham, C. W. and Jahns, R. H. (1962\ A method for determin-ing the solubility of water in silicate melts. American Journal ofScience, 260,721-745.
Fleet, M. E,, Herzberg, C. T., Henderson, G. S., Crozier, E. D.,Osborne, M. D. and Scarfe, C. M. (1984) Coordination of Fe,Ga and Ge in high pressure glasses by Mossbauer, Raman andX-ray absorption spectroscopy, and geological inplications. Ge-ochimica et Cosmochimica Acta,48, 1455-1466.
Goldsmith, J. R. (1953) A "simplexity principle" and irs relation to"ease" of crystallization. Journal of Geology, 61, 439451.
Goldsmith, J. R. and Jenkins, D. M. (1985) The highJow albiterelations revealed by reversal of degree of order at high pres-sures. American Mineralogist, 70, 911-923.
Goldsmith, J. R. and Laves, F. (1955) Cation order in anorthite(CaAlrSirOr) as revealed by gallium and germanium substitu-tions. Zeitschrift fiir Kristallographie, 106, 213-226.
Goranson, R. W. (1938) Silicate-water systems: Phase equilibria inthe NaAlSi.O.-HrO and KAlSi3Os-H2O systems at high tem-peratures and pressures. American Journal of Science, 35-A,7l-91.
Hlabse, T. and Kleppa, O. J. (1968) The thermochemistry of jade-ite. American Mineralogist, 53, 128l-1292.
Kushiro, l. (1976) Changes in viscosity and structural changes ofalbite (NaAlSi.Or) melt at high pressures. Earth and PlanetaryScience Letters, 41, 87 -9O
Laves, F. and Goldsmith, J. R. (1955) The effect of temperatureand composition on the Al-Si distribution in anorthite. Zeit-schrift fiir Kristallographie, 106, 227 11 5.
Luth, W. C., Jahns, R. H. and Tuttle, O. F. (1964) The granitesystem at pressures of 4 to l0 kilobars. Joumal of GeophysicalResearch.69,759-773.
Morey, G. W. (1957) The solubility of solids in gases. EconomicGeology, 52,225-251.
Morey, G. W. and Fenner, C. N. (1917) The ternary systemH2O-K2SiO3-SiOr. Journal of the Arnerican Chemical Society,39, tt73-1229.
Morse, S. A. (1970) Alkali feldspars with water at 5 kb pressure.Journal of Petrology, ll, 221-251.
Newton, R. C. and Smith, J. V. (1967) Investigations concerningthe breakdown of albite at depth in the earth. Journal of Ge-ology,75,268-286.
Sharma, S. K. and Simons, B. (1981) Raman study of crystallinepolymorphs and glasses of spodumene composition quenchedfrom various pressures. American Mineralogfsi. 66, 118-126.
Sharma, S. K., Virgo, D. and Mysen, B. O. (1979) Raman study ofthe coordination of aluminum in jadeite melts as a function ofpressure. American Mineralogist, 64, 779-'187.
Sosman, R. B. (1927) The properties of silica. American ChemicalSociety Monograph Series, p. 89-92. The Chemical CatalogueCompany, New York.
Stolper, E. (1982) The speciation of water in silicate melts. Ge-ochimica et Cosmochimica Acta. 46.2609-2620.
Tuttle, O. F. and Bowen, N. L. (1958) Origin of granite in the lightof experimental studies in the systemNaAlSirOr-KA1Si3O8-SiO2-H20. Geological Society ofAmerica, Memoir 74.
Yoder, H. S., Jr. (1958) Effect of water on the melting of silicates.Carnegie Institution of Washington Year Book,57, 189-191.
(Manuscript receir.ted, October 1, 1984;
accepteilfor publication, May 13, 1985
