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The Reaction Albite = Jadeite * Quartz Determined Experimentally in The American Mineralogkt, Volume 65, pages 129-134, 1980 The reaction albite = jadeite * quartz determinedexperimentally in the range600-1200.c TrMorHy J. B. Hou-eNo Departmentof the GeophysicalSciences, University of Chicago Chicago,Illinois 60637 Abstract The reaction NaAlSirOu+ SiO2 :NaAlSirOs jadeite quaftz high albite has been determinedby direct experimentin the temperaturerange 6fi)-1200"C, and the P- T line can be describedby the equation P: 0.35+ 0.0265?("C) + 0.50kbar The brackets,when comparedwith the gas-apparatusdeterminations of this reactionby Hays and Bell (1973)and by Birch and LeComte (1960),show clearly that pressurecorrections are vanishingly small when the NaCl pressurecell is used with the piston-+ylinder device if pis- ton-out methodsare used.The slope(dP/df : 26.5bar oC-t), io conjunction with tabulated entropiesfor quartz andjadeite, leadsto a value for the entropy ofsynthetic high albite. The disorderingentropy of albite is (14.2+2.4J K-' mol-'), and the enthalpy is (12950=3320J mol-t). The disorderingenthalpy is in excellentagreement with recentcalorimetric measure- ments on Amelia albite and its heat-treatedmodification. The best valuesfrom phase equi- libria for the 298 K, I bar, thermochemicaldata for synthetichigh albite are AJI3I".rt*: -3922170+ 4300J mol-' S&r., o- :221.6 + 2.5J K-t mol-' The lack of curvatureof the reaction in the P-T plane imFlies that high albite doesnot un- dergo ordering down to 600'C, at least within the duration time of the experiments.This is also conflrmed by the X-ray patternsof albites from runs at 600"C. Introduction which high_temperature calorimetry (Holm and The stability of the end-member feldspars is of Kleppa, 1968) predicts a slope of 27.6 bar deg-', as fundamental importance to petrology, yet consid- shown later. erable uncertainty exists as to the P-Tlocation of the The importance of this reaction to petrology can- albite to jadeite * quartz curve at high temperatures. not be overstated, as it forms the basis for a number In the temperature range 500-600"C the experi- of geobarometric inferences on the stability of mental location is precise (Newton and Smith, 1967; plagioclase-bearing assemblagesat depth (Boettcher, Johannes et al., l97l; Hays and Bell, 1973), but at l97l\. higher temperatures the slope and position of the This study defines closely the P-Z stabitty of high curve are only loosely constrained by the reversal in- albite relative to jadeite and quartz, using the precise tervals determined by Birch and LeComte (1960) and pressure control aforded by the low-friction NaCl Bell and Roseboom (1969). Boettcher and Wyllie pressure cell with the piston-cylinder apparatus. The (1968) published brackets at 600oC and 800"C which precision made possible by this technique allows ac- demand a lower dP/d'T slope (19.5 bar deg-') than curate determination of the standard thermodynamic expected for the breakdown ofdisordered albite, for properties ofhigh albite. 0003-004X/80/0102-0129$02.00 r29 130 HOLI-AND: REACTION ALBITE : TADEITE + QUARTZ Startingmaterials and experimentaltechnique pressure,ensures initiation of the run under condi- tions of retreating piston. Occasionallyit was neces- High albite was synthesized hydrothermally at sary to bleed off a little oil pressureduring the final 7 50"C, 2 kbaL from a mixture of natural vein quartz stagesof heating to prevent pressureovershoot. and NaAlSi,O. glass(prepared by P. A. M. Ander- Chromel-alumel thermocouples, in AtSiMag son) in a l: I molar ratio. The X-ray pattern is that of sleeves,with the junctions protected by a layer of high albite,with A2d(CuKa) : 1.90ofor (l3l)-(l3l) alundum cement, were used to measure tem- (Goldsmith and Laves, 1954).Natural white jadeite peratures.No pressurecorrection was applied to the from a vein in serpentinite,donated to R. C. Newton thermocouple readings. The reversal mixture was by R. G. Coleman,was used;it containsas principal made up of the crystallinestarting materialshigh al- impurities 0.13 weight percentCaO, 0.12weight per- bite, jadeite, and quartz in reactingproportions. The cent MgO, and 0.45weight percentFerO, (Coleman, mixture was ground repeatedly under acetone to 1961). thoroughly homogenizethe reactants,and was then An end-loaded piston-cylinder apparatuswith a baked at 350oC overnight to burn off any organic 0.75" diameter cylindrical pressurechamber and residue from the acetone. Following Boettcher and NaCl pressuremedium was used for all runs. The Wyllie (1968)in the successfuluse of interstitial melt chamber contained a smooth,but not polished,steel as a flux, the chargewas moistenedby breathing on liner and was lubricated by dry MoS, powder. Pis- the powder before sealingin platinum capsules.The ton-out type runs were made,g5ing pistonswith bev- small amount of interstitial water-undersaturated elled tops cappedwith pyrophylli{s ssalinggaskets as melt was found sufficient to produce 100 percent re- describedin Holland (1979a),by the following proce- action in many runs. Direction of reaction was de- dure. The cold assembly was brought up to a pre- tectedby changesof more than 15 percentin relative determinedpressure of 3.5 kbar or more below the fi- peak intensitieson the diffractometertrace. Runs at nal desiredrun pressureand was then heatedto the 600"C (from Holland,1979a) were conductedunder required temp€rature for the run. The large thermal HrO-excessconditions. expansion of the NaCl assembly during heating, a and final run pre-calibrated function of temperature Results jadeite Table 1. Experimental results on the reaction jadeite + quartz : Kinetics of the albite to * quartz reaction high albite in the presenceof small amounts of melt are such that complete reaction occurred in less than 3 hours 1200oand,24 hours at 800"C. Runs at and above -o^ at Ti-me Resul t* (hours) ll00"C cannot be consideredreversed, as the albite breaks down to jadeite + quartz during the approach (Table l). However, 4 600 16.O 24.O A to the final P-T conditions 6 600 L6.5 8.O JQ growth of albite is extremely rapid (Newton, in Jo- 24 800 2l .o t3.25 resultscertaidy represent 23 800 22.O 23.75 JQ hanneset al.,l97l) and the equilibrium. Table 1 and Figure I display the experi- 28 900 23.5 23.5 A straight-line fit (by eye) 25 900 24.25 22.75 NR mental results. The best 27 900 25.O 2L.50 JQ through the brackets can be described by P (kbar) : 18 1000 26.o 23.25 A 22 1000 27.5 zz, I ) JQ 0.0265TC+0.35+0.50. l3 1100 28-O 3.25 A Discussion L4 I 100 29.O 5.2 A to I 100 30.0 o.2 JQ Pressurecalibration of piston cylinder apparatus 11 I 100 30.o -** JQ The use of the NaCl pressure medium in recent L5 1200 31.O A L6 1200 32.O 5.4 NR years has significantly improved the pressuredeter- L7 1200 33.O 4.25 JQ mination in piston-cylinder experiments (Johannes et al., l97l; Mirwald et al., 1975;Johannes, 1978; A - albite growth; JQ - jadeite + quartz growthi Danckwerth and Newton, 19781,Holland, 1979a).It NR no reaction. is instructive to comparethe presentresults at 600oC ** Run was quenched as soon as desired P,T conditions with the gas-apparatusdetermination by Hays and had been reached. Bell (1973)and in the range 800-1000'C with the de- HOLI,AND: REACTION ALBITE : IADEITE + QUARTZ 13r termination by Birch and Lecomte (1960),also per- formed with gas pressure medium. At 600'C the pressuresobtained in this study, 16.0-16.5kbar, agree well with the Hays and Bell value (16.4+0.5 JADEITE kbar). The bracketsin the range 800-1000"C all tie E. +QUARTZ <.? within the reversal interval of Birch and LeComte ao- and can be regardedas a refinement :< of their results. I Although Birch and LeComteused natural low albite l,! ALBITE in their starting material, the product albite which t/) t/) was nucleated and/or grown in their runs was with- t! out doubt disorderedhigh albite. On the other hand o-1E the lessstable low albite in their runs probably broke down to jadeite + qtrurtz at slightly lower pressures than thoserequired for the breakdownof high albite. 500 700 900 1100 1300 The equilibrium curve for high albite : jadeite + TEMPERATURE- DEGt qtartz should therefore be expected to lie within or Fig. l. P,7 brackets on the jadeite + quartz to albite reaction. at marginally higher pressures than the Birch and Filled squares denote growth of jadeite + qtJaftz, open squares LeComte reactionbrackets. The fact that the present denote growth of albite, and partially-fiIled squares denote no resultsall lie within the Birch and LeComte intervals apparent reaction. indicates that the pressuresdetermined with the NaCl cell, as used here, are not overestimates, and extrapolatingthe c-quartz heat contentdata in Robie that subtractive friction corrections yxli5hingly ur" et al. (1978),the one-barenthalpy for the reactionja- small. Indeed, any correction due to friction would deite + quartz: albite, involving a-qtartz and high be much less at these higher temperatures (800- albite, becomes11405+2670 I at 964 K. With mean 1200'C) than at 600oC, where it was shown to be thermal expansionsand compressibilities from Table negligible (this work; Hays Bell, and 1973;Johannes 2 and the equilibrium pressureof 18.7kbar, the slope et al., l97l). Johannes(1978) reported pressure a of mav be calculatedfrom: lessthan 16 kbar for a piston-out reversalat 600oC, in contrastto the 16.25=0.25kbar reportedhere. Al- dP/dr: ,..+ Wv- r^@ryd4fr^v though both studiesused the NaCl pressuremedium, [or, 1," the methodsof arrival at the final pressureand tem- The result,27.6!1.5 bar "C-', is in good agreement peraturediffered. It is apparentthat with this method with the measuredslope, 26.5+1.5 bar oC-'.
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