Jadeite stability measured in the presence of silicate liquids in the system NaAlSiO4-SiO2-H2O

I&W~~IIIMet coemochimlcadota, 1988, VOL32, pp. 999 to 1012. Pergamon Prew Printed In Northern Imhd

Jadeite stability measured in the presence of silicate liquids in the system NaAlSiO,-SiO,--I&O A. L. BOETTCHER Department of Geochemistry and Mineralogy, The Pennsylvania State University University Park, Pennsylvania 16802 and P. J. WYLLIE Department of Geophysical Sciences,University of Chicago, Chicago, Illinois 60637 (Received6 Janwlry 1968; accepted in revieed form 16 April 1968.) Al&x&-The reactions nepheline + high albite + jadeite and high albite P jade&e + quartz were measured in the presence of high-pressure vapors and silicate melts in the system NaAlSiO,-Si08-q0 for the temperature range 6OO-800°C. Kinetics were such that complete ma&ion was attained in most IW-B. The results of these determinations together with previous studies in this system and recent thermochemical data collectively place narrow restrictions on the positions of the above reactions over a wide temperature range. An entropy change of 3.6 eu accompanying the transformation of low to high albite is in good agreement with these resulta. Some data are also provided for the reaction analcite + high albite +z jadeite + vapor. It is concluded that jade&e can form in a variety of geologic environmentsat pressures lower than previously determined. INTRODUCTION

studies have been directed tow&rds estimation of the depth at, which the common silicate miner&la known &t the surface of the e&rth undergo phase tr&nsitions into high-pressure polymorphs. These studies have important geophysical and petrologic&l applications. The formation of jadeite from combinations of albite, nepheline and analcite h&s applications to the b&s&It-eclogife tr&nsition, magma generation in the mantle, and to the conditions of formation of met&morphic rocks in the glaucophase schist facies. Unfortunately, reaction rates rtmong these phases are sluggish, and previous experiment&l studies h&ve suffered from this problem. Equilibrium conditions were not achieved, and the positions of the transitions were located by establishing the direction of re&ction, and reversing the direction if possible. We have recently extended hydrothermal melting curves for synthetic minerals &nd miner&l assemblages in the system N&AlSiO,-SiO,-H,O to 35 Kb pressure (BOETTCHER and WYLLIE, 1907a,b), into the st&bility field of jadeite. This provided the opportunity to loc&t,ethe positions of the rertctions: MANY experimental

N&AlSi,O, + N&AlSiO, + SN&AlSi,O, albite j &deite nepheline

(1)

N&AlSi,O, +GN&AlSi,O, + SiO, albite quartz jedeite

(2)

999

1000

A. L. BOETTCRER and P. J. WYLLIE

in the presence of a silicate liquid phase at temperatures down to about 600°C. The liquid, containing dissolved H,O, increased the rate of reaction and permitted achievement of complete reaction in most runs, with the noteable exception of runs for reaction (1) at the lower temperatures of about 620°C. The synthetic albite in our runs was in high structural state. The compositions of the crystalline phases are expressed as pure substances, although undetermined degrees of crystalline

solution occur.

PREVIOUSWORK The stability relationships of jadeite have been discussed in many papers, but there have been few successful experimental studies. A comprehensive review would occupy more space than justified, and this description of previous work is therefore limited to the experimental studies. Early attempts to synthesize jadeite, most of which were at pressures below 4 Kb, were unsuccessful (YODER, 1950). According to ROBERTSONet al. (1957), L. Coes first synthesized jadeite in 1948, although publication of the results was delayed until 1955 (COES, 1955). About this time, GRIMS et al. (1955) presented orally the first experimental study of jadeite stability based on the conditions of breakdown of analcite, and a reaction curve: analcite + jadeite + water

(3)

was published the following year, without run data (GRIGGSand KENNEDY,1956). These investigations were followed by detailed reports of reactions (1) in the temperature interval 600°C to 1200°C by ROBERTSONet al. (1957), and of reaction (2) in the temperature interval 600°C to 1OOO’Cby BIRCH and LECOMTE(1960). Glass, synthetic nepheline, and natural low albite, jadeite and analcite were used as starting materials. KENNEDY (1961) published preliminary diagrams for reactions (1) and (2) , taking into account mutual solubility relationships among the minerals albite, nepheline and jadeite, and also for reaction (3): analcite % jade + water. A brief experimental investigation of the kinetics of reaction (1) was undertaken by DACHILLEand ROY (1962). BELL and ROSEBOOM(1965) presented a phase diagram for the melting relationships in the system NaAlSi,-SiO,, with two invariant points terminating reactions (1) and (2), but these points depend upon extrapolations from the subsolidus data of ROBERTSONet al. (1957) and BIRCH and LECOMTE(1960). NEWTONand SMITH(1967) extended the previous experimental determination of reaction (2) down into the range 500°C to 6OO”C,using natural low albite, and they considered the effect of the high-low albite transition on the reaction. In addition, they determined the effect of crystalline solutions of NaFeSisO, and CaMgSi,O, on jadeite stability in reaction (2). HLABSEand KLEPPA (1968) have recently redetermined the enthalpy changes associated with reactions (1) and (2) at 691°C, by oxide-melt solution calorimetry. Using also the calorimetric results for the enthalpy change associated with the low albite to high albite transformation (HOLMand KLEPPA, 1968), they calculated the positions of two curves for each reaction, one for high albite and one for low albite.

1001

Jadeite stability measured in the presence of silicate liquids

EXPERIMENTAL METHODS Experimentalmethodsare reportedin detail elsewhere(BOETTCHIER and WYLLIR, 196%). Briefly, all runs were performed in a piston-cylinder apparatus (BOYD and ENULAND, 1960), with oil-based Molykote-G as lubricant between the half-inch diameter tala pressure assembly end a hardened steel liner within the carbide pressure vessel. All runs were brought first to final pressurewith the piston advancing into the vessel, and then the temperature was raised. The pressureloss due to friction was estimated by calibration against the BiI-BiII transition at 25.4 Kb and 25”C, and the melting curve of LiCl at 12-5 Kb and 850°C; a linear, temperaturesensitive calibration curve was drawn between the friction losses of - 13 ‘4 at 25°C and -7 ‘4 at 850°C. Temperature measurement was calibrated at 4 Kb against two univariant melting reactions in the system CaO-CO,-He0 at 630°C and 655°C (BOETTCXIER and WYLUE, 1967~; 1968e). No correction was made for the pressure coefficient on output emf of the chromelalumel thermocouples. About 5 mg of powdered sample and the desired amount of distilled water were encapsulated in welded, thin-walled platinum tubing. Starting materials were gels with compositions between albite and nepheline, prepared by Table 1. Experiments on jadeite stability Results

Compositionwt. % Solids

H,O

Temp ‘C

Press kb

RlDl Products X-ray

Time hr.

Run

Ne

Ab

Jd

298

W3

4%

-

33

688

11.0

24

Ab, (bl),

276 328 420 417 416 411 412 421 337 329 330 343

4% -

6% -

-

400 40g 40g 40g -

600 6Og 6Og 6Og -

1oog 1000 1000 1oog 1OOg 1OOg -

38 30 6 3 6 6 6 4 6 12

696 620 620 620 636 626 617 620 800 800

10.0 12.0 11.0 13.0 12.0 11.0 12.0 12.0 16.0 16.0

21 23 26 24 23 26 22 24 14 28

Ab, Ab Ab, Ab, Ab, Ab, Ab, Ab, Ab,

1000

11 8

800 800

14.0 13.8

16 24

Ab, Ne Ab, Ne, Jd

Phase sasemblage at PandTofnmby X-ray and optics

Reaction (1): high albite + nepheline + jade&

Reaction (2): high &its 260 287 239 269 241 292 263 279 280 242 267 264 288 317 327 333 348 334

(second -

,

t-d

1oog 1oog 1oog 1oog 1oog stage) 1oOo 1oog 1oog 1OOg 1oog 1oog 1oog 1oog 1oog 1oog 1oog 1oog 1008

_

-

36 38 43 32 39

-

43 34 36 38 40 37 36 37 1 11 11 8 12

atee@

600 600 600 600 600 600 610 620 620 626 626 660 640 637 800 800 800 800 800

19.0 18.0 17.0 16.0 17.0 16.0 17.0 19.0 17.0 18.0 17.0 19.0 19.0 18.0 22.0 21.1 20.0 19.0 21.3 19.0

(Jd)

An1 Anl, (Ne), Jd Jd Ne, (Jd) Anl, (Ne) Anl, Ne Anl, Ne (Ne), Jd -

Ab + (An1 + Jd) + L +V Ab+Anl+V Ab+L+V Ab, Anl, (NE), Jd, (L) Ab, Jd, L Ab, Ne, (Jd), L Ab, hl, (Ne), (L) Ab, Anl, (No), (Jd), (L) Ab, Anl, Ne, (Jd?), L Ab+Ne+ Jd+ L L Ab+Ne+L Ab+Ne+Jd+L

+ jadeite + quartz: 12 24 9.3 47 10 24 26.3 10.6 46 9.3 9.3 11 10 12.3 26 11.6 19 12.6 12 48

Jd, Qz Jd, Qz

Ab, Jd Ab Ab+ Jd$ Ab, Jd Ab, Jd

Jd, Qa

Jd-tQz+v Jd + Qx-I- v Ab+ Jdf (Qz)+V

Ab + (Jd?) + V (Qz) + V aa in 239 Ab+ Jd+V Ab+ Jd+V

Jd + Qz + v

Ab, Jd

Ab+

Ab, Jd

Ab+Jd+L+V

Jd, Qz

J& 82 Jd, Qz Jd, Qz Jd, Be J& (Qz)

Ab Ab, Jd Jd+ Q=+ Ab, Jd

Jd+V

Jd+ Qz+ V

Jd+Qz+L+V Jd + Qz + v Jd+ Qe+v Jd+ Qz+ L Jd+ Qx+ L Ab + (Jd)+ L L

Ab+Jd+L aa in 327 Ab+ Jd+L

Abbreviations taeed: g = gleea, o = crystalline starting materiel, Jd = jade&e, Ab = high albite, Ne E nepheline, Qe = quertz, An1 = enelaite, L = liquid, V = vapor. ( ) = phase present in trace amounts. Symbols for minerals in column “oompwiticm” refer only to oomposition.

A. L. BOETTCHER and P. J. WYLLIE

1002

the method of LUTH and INUAMELZS(1965). These were crystallized hydrothermally, or COIIverted to gIasses by repeated fusions, with intermediate crushing, to ensure homogeneizatiori. Checking the composition of the albite glass with the electron microprobe analyzer revealed no detectable loss of alkalies during the fusions. Glass was used for most runs in Table 1. The phases present at the end of a run were identified by optical and X-ray powder diffraction methods. Liquids quenched to form glass, and the vapor phase deposited material during the quench that was distinguishable optically from the glass and the crystalline phases. EXPERIMENTAL

RESULTS

Results of definitive runs are listed in Table 1 and plotted in Fig. 1. In most runs reaction went to completion; run durations varied between 9 and 48 hours. Crystalline materials were used to confirm that the products obtained from reaction of glass starting materials were equilibrium products. Only five of these runs relate specifically to jadeite stability (Table l), but from these and reversals in the system NaAISiOhSiO,-H,O we are satisfied that the products do represent equilibrium phase assemblages. As usual in synthetic systems, high albite formed at all temperatures instead of albite with structural state appropriate for its temperature of formation, and in this sense we have located metastable equilibrium reactions in the lower temperature range. The results of our experiments attest to the fluxing action of H,O-rich vapors 3

30

500

I

600

I

700

TEMPERATURE

1

I

800

/

900

“C

Fig. 1. Results of experiments on jadeite stability in the system NaAlSiO~SiOz-HzO. Runs are indioated by re&auglm, the size of which indioetea uneert6inty in temptxatureand preaum measumment. The openoira~anlnvarientpoint Ab = albite, (IJ, is from BOETN~HIPEand WYLLIE (1967a, b). Abbreviations: Ne = nepheline, Jd = jadeite, Qz = q&.

Jadeite stability meaaumd in the presenceof silicate liquids

1003

and silicate liquids at high pressures. In most of our runs, complete reaction was achieved and tests indicate that equilibrium also was attained, whereas in previous experimental studies most runs did not achieve complete reaction. Dry experiments have produced very small crystals (ROBERTSONet al., 1967; BECH and LECOMTE,1960), but jadeite crystals in our runs occurred as euhedral prisms often exceeding O-1 mm in length at pressures of about 20 Kb; with increasing pressure the crystals became smaller, being less than 0.01 mm in subsolidus runs at 36 Kb. Similarly, the quartz crystals in our runs formed large anhedral to euhedral crystals easily distinguished from albite using the petrographic microscope. Figure 1 shows the positions for reactions (1) and (2) determined in this study. The detailed ternary phase relationships involving these univariant reactions in the system NaAlSiO,-SiO,-Hz0 have been worked out, but for present purposes we need only note that a melting curve marking the coexistence of the phases albite, jadeite, liquid, and vapor extends downward from the invariant point I, in Fig. 1 (BOETTCHERand WYLLIE, 1967a,b). This curve will be discussed again in connection with the experimental location of reaction (2). Location of invariant points such as I, provides experimental brackets for reactions (1) and (2). Location of the point of intersection of the family of univariant curves around an invariant point provides a good fix on the point. Location of I, and other invariant points in this way produces points on the reaction curves (1) and (2) that are determined independently of the reactions themselves; that is, the positions of reactions (1) and (2) are located without actually examining the reactions, forward and reverse, among albite, nepheline, jadeite and quartz. Extending to higher temperatures from the solidus in this system are curves for the assemblages : albite + nepheline + jadeite + liquid

(1A)

albite + quartz + jadeite + liquid

(2A)

and

which are geometrically coincident, in PT projection, with reactions (1) and (2). Therefore, mixtures with compositions between NaAlSi,O, and NaAlSiO,, in the presence of less Hz0 than required to saturate the liquid under these conditions, can be used to locate reactions (1) and (2) in the presence of a liquid phase. The curves. were bracketed in this way at 800°C. Reaction (1) : high albite + nepheline

6 jadeite

Reaction (1) was bracketed and reversed in the presence of an undersaturated liquid phase between 14 Kb and 15 Kb at 800°C and bracketed between 10 Kb and 11 Kb at 620%. Run 330 at 14 Kb and 800°C yielded albite + nepheline + liquid, and in run 337 at 15 Kb, jadeite was formed in addition (at least 00% of the crystals). Run 337 could represent either incomplete reactions of early-formed metastable albite and nepheline in the jadeite field, or, it could represent a point on the reaction boundary. Reversal of the reaction without seeding is demonstrated by run 343, where a starting mixture of crystalline jadeite + vaporyieldedabundant albite and nepheline (at least 50% reduction in the jadeite), in the presence of the undersaturated silicate liquid.

1004

A. 3;.

UOETTCHER

and I?. J.

~VPLLIE

Run 420 at 11 Kb and 620°C produced an assemblage albite + analcite -:nepheline + jadeite + liquid from crystalline jadeite, indicating incomplete reaction of the jadeite at a pressure below the curve for reaction (1) under conditions where analcite coexists with albite + nepheline + liquid. An attempt was made to reverse this reaction at 62O”C, but run 421 at 12 Kb produced no indisputable jadeite from crystalline nepheline + albite, indicating the slow rate of reactiou at 620°C when using crystalline reactants. Runs 298, 417, 416, 411 and 412 (Table 1) provide additional brackets for reaction (1) in the presence of a liquid phase, and the assemblages produced in these runs and in runs 328 and 275 are compatible with our interpretation of phase relationships in the ternary system. The lowtemperature region for reaction (1) is thus the only part of the system, in the P-T range under consideration, in which most of our runs failed to react completely in the presence of a liquid phase. Albite, nepheline, and analcite crystallize readily, both stably and metastably, as observed by us and previous workers, but jadeite apparently crystallizes only within its stability field under experimental Consequently, we have placed the curve for reaction (1) lower in conditions. pressure than the runs that produced jadeite, in the region of 620°C. Reaction

(2) : high albite

+Z jadeite

+ quartz

There are two singular points on a melting curve extending below the invariant point I, that divide the curve into three parts. The lower pressure part represents the incongruent melting of jadeite, and the part extending about 2 Kb below I, represents the incongruent melting of albite: high albite

+ vapor + jadeite

+ liquid.

(4)

Thesephaserelationships have beenoutlinedinpreliminary publications (BOETTCHER and WYLLIE, 1967a,b), and they will be described in detail in a comprehensive paper dealing with the liquidus relationships in the system NaAlSiO,-SiO,-H,O. Significant for the present paper is the fact that jadeite, without quartz, is formed from albite + water at pressures below the curve for reaction (2). The vapor phase is enriched in SiO, compared to compositions on the join NaAlS&O,-H,O and in subsolidus runs this causes mixtures of albite and water to yield albite + jadeite + vapor in a small area below the curve for reaction (2). Therefore, when curve (2) is approached from the low pressure side, reaction (2) is signalled not by the first appearance of jadeite, but by the appearance of quartz and the disappearance of albite. Our early fears that runs such as 279 and 242 (containing albite, jadeite, but no quartz) had not reached equilibrium were dispelled when we recognized the existence of reaction (4). If X-ray powder diffraction methods only are used for the location of reaction (2) in the presence of a vapor or liquid phase, erroneous results could be obtained. The two peaks usually used for identification of quartz are at 20.8’ and 26.6“ 20 (Cu&); the 20.8’ peak coincides with one for jadeite, and the 26.6’ peak is easily masked when a large proportion of albite is present. When jadeite peaks appear alongside albite peaks, the assumption could reasonably be made that reaction (2) was under way, and that quartz was present as well, with the quartz

Jadeitestabilitymeasuredin the presenceof silicateliquids

1005

peaks being masked by albite and jadeite peaks. However, as discussed above in connection with reaction (a), jadeite develops at pressures below reaction (2), and reaction (2) is signalled only when quartz forms. Fortunately, the presence of a trace of quartz in these mixtures is easily recognized using the petrographic microscope. Reaction (2) was closely bracketed in the temperature interval 600°C to 650°C by the complete reaction of albite glass in the presence of H,O. Two tests were made to confirm that the synthesis runs represented equilibrium. In the first test, albite crystals were used instead of glass in run 292, and jadeite formed, yielding a product nearly identical with those of neighbouring runs 239 and 279, which started as glass. In the second test, a two-stage run was performed, reversing the reaction achieved in run 292. Run 241 was held under the same conditions as run 239, which was known to produce the phases albite +jadeite + quartz + vapor, and then lowered in pressure to the conditions of run 259 (with “piston-in” method), which had previously yielded albite + jadeite (2) + vapor. The product was albite + jadeite + vapor; the quartz produced in the first stage of the run (according to run 239) had reacted completely, and the jadeite was visibly corroded; microscopic examination also indicated that the ratio of jadeitelalbite was lower in run 241 than in 239. Reversal of the reaction was thus achieved, although the jadeite is slow to react at pressures just below its stability field in subsolidus runs. The position of I, was located experimentally as the point of intersection of three melting curves (BOETTCHERand WYLLIE, 1967a,b), and this is plotted in Fig. 1. It lies on the curve for reaction (2), providing independent confirmation for the synthesis and reversal runs plotted in’the temperature interval 600°C to 65O’C. Reaction (2) was also bracketed at SOO”C,in the presence of an undersaturated liquid phase, between runs at 21 Kb and 20 Kb. The formation of albite + jadeite + liquid from starting materials on the join NaAlS&O,-H,O (runs 333, 346, 242) proves that the liquid composition lies on the SiO, side of the join. The synthesis runs were confirmed as equilibrium runs by the two-stage run 334, in which an inferred assemblage jadeite + quartz + liquid above the reaction curve was converted to albite + jadeite + liquid at a pressure below the curve; all of the quartz reacted, and albite grew without seeding. Reaction (3A): analcite + high albite +. jadeite + vapor The conditions for the formation of jadeite from the breakdown of analcite according to the reaction analcite % jadeite + vapor

(3)

were reported by GRIUQSand KENNEDY (1956) and calculated by FYFE and VALPY (1959). However, it is apparent from the results of BARRERand WHITE (1952), SAHA (1961) and PETERSet aE. (1966), that under hydrothermal conditions analcite exhibits a wide range of crystalline solution. The results from our study of the system NaAlSiO,-SiO~-H,O show that analcite + albite breaks down according to reaction (3A), which extends to lower temperatures with a small negative slope, dP/dT (BOETTCHERand WYLLIE, 1967b). This reaction curve 6

A. L. BOETTCHER and P. J. WYLLIE

1006 could either it could pass with positive The ternary of jadeite

continue to another invariant point on the curve for reaction (Z), or through a pressure maximum and then slope down to lower pressures dP/dT, as calculated for the reaction (3) by FYFE and VALPY (1959). phase relationships require that another reaction for the formation analcite + jadeite

exists approximately

+ nepheline

+ vapor

l-2 Kb higher in pressure than reaction

(3B) (3A).

COMPARISON WITB OTHER RESULTS There are only a few papers on jadeite stability with experimental details that can be compared with our results, and these are shown in Fig. 2. HLABSE and KLEPPA (1968) compare the same experimental data with their calculated curves for high and low albite.

Fig. 2. Comparison of the present experimental results on jadeite stability (see Fig. 1) with previous experimental work and recent calculations based on Ab = albite, Ne = nepheline, new thermochemical data. Abbreviations: Jd = jadeite, Q,z = quartz, L = liquid. Reaction

(2):

albite +s jadedte +- quartz

BIRCH and LECOMTH (1960) located this reaction between 600°C and lOOO’C, using dry natural minerals &s starting materials. They bracketed the equilibrium curve by approaching the reaction from both sides; parti4 reaction gave the direction of change in most runs, but they obtained complete reaction and reversal at 1000°C. Their definitive runs and reaction curve are plotted in Fig. 2. The crystalline prodnets were extremely small, and identification depended mainly

Jade& stability measured in the presenceof silicate liquids

1007

upon X-ray powder diffraction patterns. BIRCIEand LECOMTEstated that their curve may be slightly high in pressure because of the problem of recognizing the development of a small amount of jadeite. NEWTONand SMITH (1967), also working with natural minerals, determined brackets by reversing the direction of reaction at 500, 550 and 600°C. They completed runs of considerably longer duration than BIRCH and LECOMTE,and used water as a flux. Their brackets are lower by l-5 to 2 Kb than the extrapolated curve presented by BIRCHand LECOMTE(1960). BIRCH and LECOMTE(1960) and NEWTONand SMITH (1967) used low albite as their starting material, but within the temperature range of these experiments the stable form of albite becomes increasingly disordered. NEWTONand SMITH took into consideration the entropy correction for disordering of albite, and its effect on the position of the stable equilibrium curve. In view of the uncertainties, they noted the evidence that the greater part of the disordering occurred through the temperature interval 600°C to 700°C, and assumed arbitrarily that the entropy of albite increases smoothly by 3.5 cal/mol-deg within this interval. Their equilibrium curve for the reaction, shown in Fig. 2, was drawn to satisfy their three reversed brackets, the runs of BIRCEand LECOMTE(1960) for higher temperatures, the slopes of the curve at 0°C and 500% calculated from heat capacity data, and the change in slope between 500% and 700°C caused by the entropy change for disordering. This change in slope carries their curve to higher pressures than that of BIRCHand LECOMTEat temperatures above 850°C; yet the curve remains between the rather widely spaced runs of BIRCHand LECOMTEnear 1000°C. Figure 2 shows that our results obtained with synthetic high albite in the presence of water vapor or silicate liquid between 600°C and 800% are consistent with the curve of NEWTONand SMITH,although the curve passes 0.5 Kb above the high pressure side of our 800°C bracket. We conclude that reaction (2) is now well established at temperatures below about 800°C in a position 1.5 to 2 Kb lower than the curve of BIRCH and LECOMTE(1960). However, its position above 800°C depends upon assumptions concerning the entropy change caused by the disordering of albite, and the temperature interval within which the disordering occurs. Figure 2 shows the positions of dry melting curves in the system NaAlSiO,-SiO, reported by BELL and ROSEBOOM(1965) after a review of available experimental data. The invariant point 1, was located tentatively by extrapolation of BIRCH and LECOMTE’Sreaction (2) to the melting curve: albite + quartz + liquid.

(6)

Figure 2 shows that a straight line can be drawn for reaction (2) from I,, at 32.5 Kb, through all three sets of experimental data (BIRCHand LECOYTE, 1960; NEWTONand SMITH, 1967; our results). This may be regarded as the lowest possible curve, because disordering of albite makes the reaction curve concave upwards. An upper pressure limit for I, is given by three runs yielding jadeite reported by BELL and ROSEBOOM(1965) at 36 Kb between 1380°C and 1420%. Thus, the true position of I, lies between 32.5 Kb and 36 Kb. Extrapolation of

1008

A. L. BOETTCHER and P. J. WYLLIE

the curve of NEWTON and SMITH (1967) to the melting curve (6) would place the invariant point at about 37 Kb, just above the upper pressure limit. HLABSE and KLEPPA (1968) calculated three curves for the reaction (2), one for low albite and two alternatives for high albite assuming that the entropy change for disordering in the temperature interval 500°C to 700°C was either 4.5 eu or 3.5 eu. The calculated curve for low albite, below 6OO”C, is effectively identical with the experimental curve plotted in Fig. 2, and it passes through the bottom edge of our 800°C bracket determined with synthetic high albite. Linear extrapolation of this calculated curve to the melting curve (6) would place a metastable invariant point I, at a pressure of 30.5 Kb. Linear extrapolation of the calculated curve for high albite with 4.5 eu change for disordering would produce an invariant point I, at about 39 Kb, which is well above the experimental limit of 36 Kb. Extrapolation of the calculated curve for albite with 3.5 eu change for disordering would produce an invariant point at 34 Kb pressure; this lies between the lower experimental limit provided by I, shown in Fig. 2, and the higher limit provided by the jadeite-bearing runs at 36 Kb. HLABSE and KLEPPA (1968) noted that this curve fitted the experimental data up to 1000°C better than the 4.5 eu curve, and this conclusion is reinforced by extrapolation of these curves to melting temperatures. The calculated 3.5 eu curve is therefore plotted in Fig. 2 for comparison with the experimentally determined curves. It passes through our 800°C bracket determined with synthetic high albite, but it occurs below the other experimental brackets at lower temperatures. Considering the available data, it appears to us that the curve of NEWTON and SMITH (1967) may be somewhat too high in pressure at temperatures greater than 700°C. A curve with less upward concavity would have a better fit with our 800°C bracket, and extend to an invariant point on the melting curve at a pressure between the experimental limits shown in Figure 2, instead of just above the upper limit. The reaction curve would occupy such a position if the entropy change in albite due to disordering actually occurred through a wider temperature interval than the 500°C to 700°C adopted by NEWTON and SMITH (1967), or if the entropy contribution is lower than 3.5 eu, perhaps because of significant local Si-Al order (J. V. SMITH, personal communication, 1967). In conjunction with the experimental brackets between 500°C and 800°C shown in Fig. 2, a direct experimental determination of I, from study of the melting curves would establish the position of the reaction curve (2) to 1360°C. This would provide more information about the change in slope of the curve and thus about the entropy change and the temperature interval through which disordering occurs in albite, although it would not provide a unique solution for these two unknowns. Reaction (1):

albite + nepheline % jadeite

ROBERTSON et al.(1957) presented a curve for this reaction between 600°C Phase identification and lZOO”C, based on three experimental approaches. usually depended on X-ray powder diffraction patterns. The reaction of synthetic nepheline and the natural minerals low albite and jadeite proved to be too slow to define the univariant reaction in runs of 1 to 2 hours duration, even in the

J&&e

stability measured in the presence of silica& liquids

1009

presence of such catalysts as water, gibbsite, and sodium fluoride. Their curve shown in Fig. 2 was determined by crystallization of glass of jade&e composition, and it does lie between the upper and lower pressure limits provided by partial reactions of the minerals. Up to 50 per cent crystallization of glass was achieved, especially when catalyzed by water. In dry runs, crystallization of jade& from the glass was usually accompanied by metastable albite (high structural state) and nepheline. In the third experimental approach, the reaction products of natural analcite located a curve almost, identical with Ghat determined by the crystallization of glass; just as with the glass, the formation of jadeite was usually accompanied by metastable albite and nepheline which did not transform into jadeite in runs up to 3 hours duration. ROBERTSON et al. (1957) also obtained preliminary information on the dry melting relationships and located approximately the point where reaction curve (1) intersected the solidus : albite + nepheline + liquid.

(7)

This was labelled invariant point I, by BELL and ROSEBOOM(1965), and its relationship to I, as deduced by them is sketched in Fig. 2. Two curves calculated by HLABSEand KLEPPA(1968) for reaction (1) with low albite or high albite are shown in Fig. 2. The curve for high albite with an entropy change of 3.5 eu introduced by disordering passes through I,. The calculated curve for low albite differs only slightly from the 3.5 eu high albite curve. Our experimental brackets for reaction (1) are plotted in Fig. 2, and a straight line drawn from I, through the brackets is presented as the experimentally derived reaction curve. At 600°C it lies about l-5 Kb lower in pressure than the curve of ROBERTSONet al. (1957), and the difference increases to 3 Kb at 200°C. The curve is parallel to the calculated curve for low albite and about 1 Kb above it. Although if has been drawn as a straight line, the equilibrium curve for reaction (1) must be concave upwards as a result of disordering of albite, and possibly a more precise experimental determination of I, would move it, to a somewhat, higher pressure, and permit the portion of the curve above 700°C or 800°C to have a slope equivalent to the calculated slope for the reaction involving high albite. Alternatively, if I, is in its correct position, upward concavity of the curve in the temperature interval 500°C to 7OO’C would raise the curve to somewhat higher pressures below 500°C. The data of YODER and WEIR (1951) indicate that the effects of compressibility and thermal expansion tend to cancel each other in this reaction and should not greatly influence the position of the reaction curve. Pressure error with piston-cylinder

apparatue

The main uncertainty with piston-cylinder apparatus is the magnitude of the pressure loss due to friction within the cylinder (GREEN et al., 1966; BOYD et al., 1967; BOETTCHERand WYLLIE, 1968b). The range of this uncertainty is illustrated by the runs at 800°C for reaction (2). BIRCH and LECOMTE(1960) bracketed the curve with runs at, 22.52 f 0.12 Kb and 20.06 f 0.06 Kb. Our nominal pressures were 22.6 Kb and 21.5 Kb, and these pressures corrected

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A. L. BOETTCHER and P.

J.

WYLLIE

according to our calibration curve are 21.0 f O-7 Kb and 20.0 & O-7 Kb. The friction correction thus amounts to about 1.5 Kb at this pressure. Both nominal and corrected pressure values lie within the pressure bracket determined by BIRCH and LECOMTE(1960). GEOLOCUCAL APPLICATIONS The reaction curves in Fig. 2 have applications to three major geological problems: the low-temperature portions provide information about the conditions of formation of rocks of the glaucophane schist facies; the middle portions are relevant to the location of the gabbro-eclogite transition zone; and the hightemperature portions provide information applicable to the behaviour of pyroxenes during magma generation within the upper mantle. The positions of the reactions below 800°C may now be regarded as well established, but some uncertainty remains for the high-temperature part of reaction (2). In the geological environments, the mineral involved is not pure jadeite but a jadeite-bearing pyroxene, and experimental work with whole rock compositions has more direct applications to the location of a gabbro-eclogite transition zone (RIWWOOD and GREEN, 1966) and for the generation of magmas than do the simple reactions shown in Fig. 2 (COHENet al., 1967; ITO and KENNEDY, 1967; GREEN and RINGWOOD,1967). However, the glaucophane schist problem is directly related to the stability conditions of a jadeite-rich pyroxene. Reaction (2) is relevant, rather than reaction (l), in most geological environments. Experimental studies of minerals such as jadeite, kyanite and aragonite have indicated that surprisingly high pressures were required for the formation of many metamorphic rocks, which suggests either that pressures greater than lithostatie pressures are developed during metamorphism, or that sediments undergoing metamorphism have been buried at depths greater than usually proposed. RUTLAND (1965) recently reviewed the problem of tectonic overpressures. He concluded that geological estimates of depth of metamorphism cannot be greatly modified, and therefore, if stable crystallization of these minerals occurs, then some method of producing an overpressure is required. COLEMAN and LEE (1962) discussed the development of tectonic overpressures of up to 3 Kb during the genesis of glaucophane schists. NEWTONand SMITH(1967) revised the position of reaction (2) downwards in pressure, and our work confirms this revision. According to BIRCHand LECOMTE (1960), albite is stable to 12 Kb pressure at 3OO”C, whereas the revised curve in Fig. 2 reduces this pressure to 10 Kb. NEWTONand SMITH (1960) also determined experimentally the conditions of formation of a jadeite-rich pyroxene with composition similar to that studied in the Franciscan schists (MCKEE, 1962; COLEMANand CLARK, 1968). This pyroxene was stabilized less than 0.7 Kb lower than the pure jadeite at 600°C. NEWTONand SMITH (1967) discussed the stratigraphic thickness of sedimentary and volcanic rocks in geosynclinal columns, and they concluded that tectonic overpressures were not required, because a “great discrepancy between the pressure indicated by experimental work and the available overburden pressure does not seen to be in evidence.” HLABSEand KLEPPA (1968) have pointed to what may be a very significant

Jadeib stabilitymeasuredin the presenceof silicateliquids

1011

factor in decreasing the pressure, or depth of burial, required for the formation of jadeitic pyroxenes in glaucophane schists. Their calculated curves for high albite, with 3-6 eu contributed by disordering, agree well with the experimental data at high temperatures. The metastable extension of their curve for reaction (2) to low temperatures is about 2-5 Kb below the experimental curve for low albite. This indicates that a disordered albite preserved metastably in a volcanic rock would yield jadeite at a pressure of about 7.5 Kb at 300°C. The influence of solutions in rocks may also cause the formation of jadeitic pyroxenes at pressures lower than indicated by the univariant reactions. Our results for the incongruent melting of albite in the presence of a vapor phase (reaction 6) show that jadeite, without quartz, is formed from albite in the presence of water within a recognizable pressure interval of about 2 Kb below reaction (2). Because of this incongruent reaction, albite + jadeite + vapor is also stable in a subsolidus area below the curve for reaction (2); this area could conceivably extend to low temperatures as a band below reaction (2). If detailed textural studies of the mutual relationships among plagioclase, jadeitic pyroxene, and quartz should indicate that jadeite does form directly from plagioclase feldspar, without simultaneous formation of quartz, then the action of sblutions yielding jadeite at pressures lower than the reaction curve (2) can be implied. Acknowledgmentost of this work was supportedat,The University of Chicago by National Science Foundation Grant GA-023, using apparatus supplied by Advanced Research Projects Agency Contra& SD-80. Part of the publications costs and some experimental work was supported by National ScienceFoundation Grant GA-1364 at, The Pennsylvania State University. We thank R. C. NEWTON and J. V. SMITEfor helpful discussionand for review of a part of the manuscript. We are grateful to 0. J. KLEPPA of The University of Chicago for allowing us the use of unpublished data. REFERENCES BARREBR. M. and WHITE E. A. D. (1052) Hydrothermal chemistry of silicates. Part II. Synthetic crystalline ahunino-silicates. J. Chem. Soo. London 1501-1571. BELL P. M. and ROSEBOOM E. H., JR., (1065) Phase diagram for the system nepheline-quartz. Year Book 64, Carnetie In8t. Wshington, pp. 130-141. BIRCHF. and LECOMTEP. (1060) Temperature-pressureplane for albite composition. Amer. J. Sci. ~,200-217. BOETTCHER A. L. and WYLLIE P. J. (1068a) The calcite-aragonitetransition measured in the system CaO-CO%--H,O. J. Qeol. in press. BOETTCHER A. L. and WYLLIE P. J. (1068b) The quartz-coesite transition measured in the presence of a silicate liquid and calibration of piston-cylinder apparatus. Co&rib. Mineral. Petrol. in press. BOE~C~ZR A. L. and WYLLIE P. J. (1067a) Hydrothermal melting reactions in the system NaAlSiO,SiO,-H,O at pressuresabove 10 Kbs: jade&e stability. Program Ann. Meeting Geol. Soo. Am. 1067, p. 17. BOETTCHER A. L. and WYLLIE P. J. (1067b) Hydrothermal melting curves in silicate-water systems at pressures greater than IOkilobars. Ndurs 216, 572-573. BOETTCHER A. L. and WYLLIE P. J. (10670) Melting relations in the system CaO-COa-HsO to 40 Kb. Tram. Amer. Cfeophys. Union 48, 250. BOYD F. R. and ENGLANDJ. L. (1060) Apparatus for phase-equilibrium measurements at, pressuresup to 50 kilobars and temperatures up to 1760°C. J. Cfeophye. Res. 65, 741-748. BOYD F. R., BEG P. M., ENQLANDJ. L. and GILBERTM. C. (1067) Pressure measurements in single-stage apparatus. Year Book 65, CarnegieInat. Wadington, pp. 410-414. COESI,., JR. (1066) High pr,essureminerals. Amer. Ceram. Sot. J. 88, 298.

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COHEN L. H., ITO K. and KENNEDY G. C. (1967) Melting and phase relations in an anhydroiis basalt to 40 kilobars. Amer. J. Sci. 2&j, 475-518. COLEMAN R. G. and CLARIC J . R. (1968) Pyroxenes in the blueschist facies of California. rln~~,r. J. Sci. 268, 43-59. COLEMAN R. G. and LEE D. E. (1962) Metamorphic aragonite in the glaucophane schists of Cazadero, California. Amer. J. Sci. 250, 577-595. DACHILLE F. and ROY R. (1962) Opposed-anvil pressure devices. In Alodern. lTery HQh Pressure Techniques (editor R. H. Wentorf, Jr.), pp. 163-180. Butterworths. FIFE W. S. and VALPY G. W. (1959) The analcime-jadeite phase boundary: some indirect, deductions. Amer. J. Sci. 257, 316-320. GREEN D. H. and RINGWOOD A. E. (1967) An experimental investigation of the gabbro to eclogite transformation and its petrological applications. Geochim. Cosnaochim. Acta 81,767-833. GREEN T. H., RINGWOOD A. E. and MAJOR, A. (1966) Friotion effects in a piston-cylinder apparatus at high pressure and temperature. ,r. Geaphys. Res. 71,3589-3594. GRIGGS D. T. and KENNEDY G. C. (1956) A simple apparatus for high pressures and temperatures.

Amer. J. Sci. 255, 722-735. GRI~GS D. T., FYFE W. S. and KENNEDY G. C. (1955) Jadeite, analcite, and nepheline-albite equilibrium. Bull. Geol. Sot. Amer. 66, 1569. HLABSE T. and KLEPP_4 0. J. (1968) The thermochemistry of jadeite (NaAlSisO,). Submitted to Amer. Mineral. HOLM J. L. and KLEPPA 0. J. (1966) Thermodynamics of the disordering process in albite (NaAlSisO,). Amer. Mineral. in press. ITO K. and -NEEDY G. C. (1967) Melting and phase relations in a natural peridotite t,o 40 kilobars. Amer. J. Sci. 265, 519-538. KENNEDY G. C. (1961) Phase relations of some rocks and minerals at high temperatures and high pressures. In Advances in Geophysics (editors H. E. Landsberg and J. Van Mieghem), pp. 303-322. Academic Press. LUTH W. C. and INUAMELLS C. 0. (1965) Gel preparation of starting materials for hydrothermal experimentation. Amer. Mineral LW, 255-258. MCKEE B. (1962) Widespread occurrence of jadeite, lawsonite, and glaucophane in central California. Amer. J. Sci. 260, 596-610. NEWTON R. C. and SMITH J. V. (1967) Investigations concerning the breakdown of albite at depth in the earth. J. Geol. 75, 268-286. PETERS T., LUTH W. C. and TUTTLE 0. F. (1966) The melting of analcite solid solutions in the system NaAlSiO,-NaAlSisOs-HsO. Amer. Mineral. 51, 736-753. RINGWOOD A. E. and GREEN D. H. (1966) An experimental investigation of the gabbro-eclogite transformation and some geophysical implications. Tectonophyeics 3, 383-427. ROBERTSON E. C., BIRCH F. and MACDONALD G. J. F. (1957) Experimental determination of jadeite stability to 25,000 bars. Amer. J. Sci. 265, 115-137. In Controla of Metamorphism (editors W. RUTLAND R. W. R. (1965) Tectonic overpressures. S. Pitcher and G. W. Flinn), pp. 119-139. John Wiley. SAHA P. (1961) The system NaAlSiO, (nepheline)-NaAlSiaOs (&bite)-H,O. Amer. Mined.

48, 859-884. YODER H. S., JR. (1950) The jadeite problem. Amer. J. Sci. a48, 225-248, 312-334. YODER H. S., JR. and WEIR C. E. (1951) Change of free energy with pressure of the reaction nepheline + albite = 2 jadeite. Amer. J. Sci. a49, 283-294.