comparison of the high-temperature and high-pressure ... · comparison of the high-temperature and...

American Mineralogist, Volume 7 5, pages 318-366, 1990

Comparison of the high-temperature and high-pressure stability limits ofsynthetic and natural tremolite

D.lvro M. JnDcKrNsDepartment of Geological Sciences and Environmental Studies, State University of New York at Binghamton,

Binghamton, New York 13901, U.S.A.

Apnrr, K. Cr-^lnBDepartment of Geology, Rutgers University, New Brunswick, New Jersey 08903, U.S.A.

Ansrnacr

Experiments were performed on two natural tremolite samples and compared withunpublished data of D. M. Jenkins, T.J.B. Holland, and A. K. Clare on synthetic tremoliteto determine what, if any, differences exist between the upper-thermal and upper-pressurestability limits of synthetic and natural tremolite. The upper-thermal stability of naturaltremolite, which was investigated with experimental reversals of the reaction tremolite :

2 diopside + 3 enstatite + quartz + H,O in the range of 1.5-7 kbar, is about 40 + 20'Chigher than that of synthetic tremolite. Similarly, the upper-pressure stability of naturaltremolite, investigated with the reaction tremolite : 2 diopside + talc, is about I kbarhigher than that of synthetic tremolite. A thermodynamic analysis of the results for thesetwo reactions indicates that one natural tremolite (TREM 12) has a Gibbs free energy thatis 2.9 kJ/mol more negative than that of synthetic tremolite, whereas the other naturaltremolite (TREM 8) is 0.4 kJ/mol more negative on a constant-entropy basis. Both valuesare much less than the 9.2-kJlmol diference observed by Skippen and McKinstry (1985).Of the factors that can be quantified with some degree of accuracy at present, i.e., HrOfugacity, grain size, and composition, it appears that the compositional variation, in par-ticular the F content, is sufficient to account for the higher stability and lower Gibbs freeenergy of natural tremolite. One need not invoke the presence of a high density of structuraldefects in synthetic tremolite to explain the observed differences. At least for hydrothermalproc€sses, synthetic calcic amphiboles model closely the behavior of natural calcic am-phiboles if differences in their compositions are considered.

INrnooucrroN grain coarsening and defect annealing, is faster than theSynthetic crystalline phases are often used in experi- rate of reaction among heterogeneous minerals. Direct

mental studies of phase equilibria so that the experimen- experimental investigation of a given univariant reactiontalist can exercise control over their composition and involving first a synthetic and then a natural mineral reac-polymorphism. In most cases it is assumed that the results tant should give a more realistic indication of the differ-obtained with synthetic phases can be applied to nature ences between synthetic and natural substances becauseif corrections are made for any compositional differences. all factors (e.g., recrystallization rate, reaction rate, surfaceRelatively little work has been done, however, to dem- energy, defect densities, etc.) are compared implicitly inonstrate whether differences in grain size or amount of this process.structural defects between synthetic and natural phases Amphiboles may be especially susceptible to the influ-could have a significant effect on phase equilibria. For enceofstructuraldefectsbecauseplanar,dislocation,andexample, calorimetric studies of quartz (Hemingway and chain-width defects are fairly abundant in natural, par-Robie, 1977), tremolite (Nitkiewicz et al., 1984), and sil- ticularly asbestiform, anthophyllite (Veblen, 1980), calciclimanite (Salje, 1986) have shown that the enthalpy or amphiboles (Dorling and Zussman,1987), and syntheticheat capacity of a finely ground mineral can difer by l- Mg-Mn amphiboles (Maresch and Czank, 1988). Skippen2o/o from that of its coarse-grained equivalent. The cal- and McKinstry ( 1985) reported an increase of about I 33culated effect ofthese heat-capacity differences on the po- 'C at I kbar in the location of the univariant boundarysition ofa univariant boundary in P-T space can be quite for the reactionlarge and would suggest that fine_grained synthet:^t_

CarMg,SirOrr(OH), + MgrSiOoterials, possibly with high defect densities, may Sive e1-

*'^"";;;;; ro^rtr*e

roneous results. Experimentally observed diferences rnthepositionofaunivariantboundarymayinfactbemuch + 2CaMgSirOu + 5MgSiO, + HrO /t\less if the rate of mineral recrystallization, involving both diopside enstatite fluid \'/

0003404x/90/0304-o358$02.00 358

JENKINS AND CLARE: STABILITY OF SYNTHETIC AND NATURAL TREMOLITE

TABLE 1, Synthetic and natural phases used for investigating Reactions 2 and 3

359

Starting mixtures"

Re-AGtion

1 4 1 6 2 0 3

Synthesis conditions

T P F o r m tfC) (kbar) (h)CompositionPhase (code no.)

Orthopyroxene (OPX 6-1)Clinopyroxene (CPX 4-1)Clinopyroxene (CPX 4-2)DioDSideTalcQuartzTremolite'(TREM 8)Tremolite* (TREM 12)

(Mgo $Cao o,)MgSi106(Caoe5Mg1 05)MgSirO6(CaoesMgr o5)MgSiro6CaMgSi"O6M$SioO,o(OH)"sio,(lqorNaoojxcal e?M94$Fqi?Fe86Alo@)Si,eoO*(OH, s5Fooe)(Ko 04Nao,6XCa, BsMgo

".F{j,Alo o,)Si, s6Oz(OH,Fo r,)

930925930

1 250700790

6.3 oxides5.9 oxides5.2 oxides1 atm glass

17.5 oxides2.0 oxide

X X X OX X O O0 0 x 00 0 0 x0 0 0 xX X X O0 0 0 xX X X O

5495

19148261 6

'Natural tremolite samples described in Jenkins (1987).'- X : phase present in mixture, O : phase absent. The mixtures labeled 14, 16, and 20 refer to the starting mixtures for the experiments with the

prefix 14, 16, or 20 in Table 2.

when natural tremolite was substituted for synthetictremolite and with the HrO fugacity of the fluid con-trolled by the oxygen-buffering assemblage Ni + NiO.This observation casts considerable doubt on the rele-vance of experimental research involving synthetic calcicamphiboles to the phase equilibria of calcic amphibolesin nature.

In an attempt to corroborate the findings of Skippenand McKinstry (1985), two additional reactions were in-vestigated using natural tremolite:

CarMgrSirOrr(OH),tremolite

+ 2CaMgSiO. + 3MgSiO, + SiO, + HrOdiopside enstatite !-qvrtz fluid

Q)

CarMgrSirO'r(OH), = 2CaMgSirOu + MgrSioO,o(OH)r.tremolite diopside talc (3)

The results of these experiments are compared withthose for the same reactions using synthetic tremolite(Jenkins, Holland, and Clare, unpublished data). Reac-tions 2 and 3 should reveal the differences between syn-thetic and natural tremolite better than Reaction I be-cause the energetics ofReactions 2 and 3 are dictated bytremolite without the presence of any other reactant phase.

ExpnnrvrnNTAl METHoDS

Apparatus

Experiments performed below 8 kbar were conductedin internally heated gas vessels using Ar as the pressuremedium. Pressures were monitored throughout eachexperiment with both a Bourdon-tube gauge and factory-calibrated manganin cell. The cited uncertainties in pres-sure measurement include both the accuracy of measure-ment (+50 bars) and any fluctuation in pressure duringthe experiment. Temperatures were measured by two In-conel-sheathed Chromel-Alumel thermocouples that werecalibrated against the freezing point of Sn (231.9 .C) andNaCl (800.5 "C). The tips of the thermocouples were po-

sitioned at either end of the capsule and the entire sampleand thermocouple arrangement packed with ceramic woolin a cylindrical copper cup to minimize thermal gradientsabout the sample. Temperature uncertainties cited for theexperiments include both the accruacy of the thermocou-ples (+2 "C) as well as any thermal gradients across thesamples.

Experiments above 8 kbar were performed in piston-cylinder presses at the University of Chicago using 3A-in.-

diameter (1.905-cm-diameter) NaCl pressure media. Cal-ibration of the sample pressure in this pressure mediumwas discussed in detail by Holland (1980) and Jenkins(1981). Sample pressures are believed to be accurate to+300 bars. Temperatures were measured with Chromel-Alumel thermocouples, with the tip situated on top of theradially mounted sample capsule and are believed to beaccurate to +5 "C. Individual experirnents were per-formed by first pressurizing the NaCl assemblage at roomtemperature to several kbar below the desired pressure

and then using the well-calibrated thermal expansion ofthe pressure medium to obtain the final pressure uponheating. This procedure does not strictly conform to eitherthe hot or cold piston-out criteria described by Johanneset al. (1971) but has been shown to give results on thejadeite * quartz: albite calibration reaction that requirevirtually no additional "friction" correction (Holland,1980; Jenkins, l98l).

Starting materials and procedure

The phases and starting mixtures used for investigatingReactions 2 and 3 are in Table l. All of the syntheticphases except for diopside were made hydrothermally inAu or Pt capsules. Diopside was made by annealing aglass of the composition CaMgSirOu at about 1250 T^The natural tremolite samples were described by Jenkins(1987) and were ground to an average grain size ofabout60 pm in the long dimension and l0 pm in the shortertwo dimensions. This is considerably larger than the grainsize of synthetic tremolite, which, when made from ox-ides, has an average length ofabout 13 pm and width of

360 JENKINS AND CLARE: STABILITY OF SYNTHETIC AND NATURAL TREMOLITE

E

I n ai j a v

E

I soE

! a ̂ ^u 4.v

o z nN

Eot s 4 V

NIN

oo

B

40 60 80Weight %

Fig. 1. (A) Variation in the peak-height ratio of the talc 665cm-r/trem 755 cm-, rR absorption bands for several mechanicalmixtures of natural talc, diopside, and tremolite prepared in ac-cord with the stoichiometry of Reaction 3 in the text. (B) Vari-ation in the peak-height ratio of the diop (221)/trem (240) xnppeaks for the same mixtures. Abbreviations: Diop : diopside;Trem : tremolite.

2 pm. The starting mixtures for Reaction 2 consisted ofreactants and products mixed in stoichiometric propor-tions while the starting mixture for Reaction 3 consistedof a 3:l ratio of talc + diopside to tremolite. This ratiowas chosen in order to make the intensities of the diop-side (220) and (221) reflections about the same as thoseof tremolite (240) and (l5l) on an X-ray diffractometerpattern of the starting mixture. Notice in Table I that thepyroxene compositions used for studying Reaction 2 werethose of coexisting orthopyroxene and clinopyroxene atabout 900 oC and 5 kbar in the system CaO-MgO-SiO,(Lindsley et al., l98l). These compositions were chosento minimize any reaction between the pyroxenes that wasnot related to Reaction 2.

Reactions 2 and 3 were investigated by sealing a por-tion of the appropriate starting mixture in platinum cap-sules with l5-30 wto/o HrO. Each capsule was checked forleakage by examining the seams at the conclusion of anexperiment and by weighing the capsule before and aftereach experiment.

Analytical methods

All experimental products were analyzed with a No-relco X-ray diffraclometer using Ni-filtered Cu radiation.

The reaction direction for Reaction 2 could be readilydetermined from the relative peak heights of the X-rayreflections mentioned above. This was not the case forReaction 3. Probably as a consequence ofthe much lowertemperatures and smaller energetics (i.e., AS and A.I{),the extent of reaction for Reaction 3 was too small todiscern via X-ray criteria within a few kbar of the uni-variant boundary. Greater sensitivity was achieved byexamining the infrared (n) spectra of the experimentalproducts. Samples were prepared for rn spectroscopy bygrinding about 0.5 mg of the experimental products with360 mg of KBr and pressing this mixture under vacuuminto a l3-mm-diameter pellet. The rR spectra were col-lected in the linear absorption mode of either a Perkin-Elmer model 180 or 2838 dual-beam spectrophotometerusing a blank KBr pellet for reference. The spectra wereobtained in the "lattice-stretching" range of 80G550 cm-'with a resolution of about I cm-'. Peak heights were mea-sured from baselines drawn tangent to the background oneither side of the peak. Figure lA shows the variation inthe peak-height ratios for the 665 cm-' to 755 cm-r vi-brations of talc and tremolite, respectively, for five me-chanical mixtures of natural talc, diopside, and tremoliteprepared in accord with the stoichiometry of Reaction 3.Figure lB shows the variation in the ratio of the diopside(221) and, tremolite (240) X-ray diffraction (xno) peakheights for the same five mechanical mixtures. The sizeof each symbol represents the precision (based on fivemeasurements) in measuring a particular peak-height ra-tio. One can see from Figure I that the n peak-heightratio is generally more sensitive than the xnp ratio atdiscerning the proportion of phases present in a givenmixture. For example, the m. technique can indicate towithin 2.5 wto/o the true proportion of tremolite in a mix-ture consisting of 50 wto/o tremolite, whereas the xnptechnique can only distinguish within 15 wto/o.

Rrsur,rs AND THERMoDyNAMTc ANALysrs

Tremolite-enstatitediopsidequartz-HrO equilibrium

The results of experiments performed in the range of1.5-7 kbar on Reaction 2 using natural tremolite (TREM12 in Table l) are in Table 2. Notice that most of theexperiments were performed without any oxygen-buffer-ing assemblage (group A), whereas the last four experi-ments were performed with an oxygen buffer (group B).The results in group A for natural tremolite are shownby the rectangular symbols in Figure 2 along with theunpublished results of Jenkins, Holland, and Clare forsynthetic tremolite shown by the l-beam or bracket sym-bols. The symbols shown on Figure 2 include the maxi-mum range of uncertainty in pressure and temperaturemeasurement for each experiment, i.e., they have been"expanded" to include experimental uncertainties. Onecan readily see that natural tremolite has a higher thermalstability (minimum of about l0 "C) than synthetic tremo-lite at any given pressure. This observation is in accordwith the earlier study of Skippen and McKinstry (1985).

t o

40

rooTrem

20ol_o

Tolc +2 Diop

JENKINS AND CLARE: STABILITY OF SYNTHETIC AND NATURAL TREMOLITE 361

TABLE 2. Experimental results on Reaction 2 using natural tre-molite (TREM 12)

Experi'mentno. T P tOREM) fc) (kba0 (h) Commentst

G?oup A: Without an oxygen butter16-1 853(3) 1.52(5) 280 strong trem growth16-13 868(3) 1.50(5) 112 trem growth16-5 881(3) 1.48(5) 305 no apparent reaction16-15 890(3) 1.50(5) 190 no apparent reaction16-7 90q3) 1 .56(5) 305 slight pyroxene growth16-10 916(3) 1.50(5) 384 pyroxene growth14-1 905(3) 2.9(1) 50 strong trem growrh14-6 917(3) 3.15(5) 289 trem growth14-5 925(4) 2.98(7) 70 no apparent reaction14-4 934{3) 3.04(7) 125 no apparent reaction14-2 948(2) 3.05(5) 106 slight pyroxene growth14-3 971(4) 3.00(5) 145 pyroxene grovvth14-9 930(5) 5.05(10) 53 strong trem growth14-12 940(3) 5.02(5) 306 trem growth14-13 948(4) 4.98(5) 697 no apparent reaction14-11 960(3) 5.10(5) 137 no apparent reaction'16-17 965(3) 5.25(5) 12O slight pyroxene growth14-14 974(4) 5.08(5) 382 pyroxene growth14-10 979(3) 5.10(5) t71 pyroxene growth14-16 928(4) 7.1(2) 98 strong trem growth16€ 940(6) 7.14(101 26 trem growth16-9 953(4) 7.0(251 48 no apparent reaction16-2 970(3) 7.1(15) 94 no apparent reaction16-12 983(4) 7.15(15) 43 pyroxene growth

Group B: Wlth a Ni + NiO oxygen buffer17-11* 865(6) 3.00(5) 58 trem growth2O-1 923(171 3.01(5) 30 trem growth20-2 939(7) 3.02(5) 29 no apparent reaction2O-5 950(5) 3.02(5) 20 slight pyroxene growth

ooI

o=oo

Note.' Experiments are arranged in order of increasing pressure. GroupA experiments were not buffered, whereas group B were conducted in thepresence of a Ni + NiO oxygen bufier. Uncertainty in the last digit is shownin Darentheses.

" Abbreviations as in Fig. 1..'This experiment involved synthetic tremolite made from the oxides,

reacted for a period of 188 h at 740(5)€ and 2.0(1) kbar and then groundand reacted again lot 21 4 h at 806(5) rc and 2.0(1 ) kbar. All other phaseswere the same as in mixtures 14 and 16 (Table 1).

However, the amount of increase in the thermal stabilityof natural tremolite (about 40 "C) is much less than the133 "C reported by Skippen and McKinsrry (1985).

A thermodynamic analysis of the data shown in Figure2 was performed for the dual purpose of demonstratinginternal consistency and for extracting Gibbs free-energyvalues for natural and synthetic tremolite to quantify theirdifferences. A set of experimental data for a univariantreaction can be tested for internal consistency using theG' plot described by Day and Kumin (1980) and used byJenkins (1983). Stated briefly, the condition that AG ofthe reaction be zero at each P-?" point along a univariantboundary allows the following relationship to be written:

850 900 950 loooTemperoture, "C

Fig.2. Experimental results on the upper thermal stability ofnatural (dashed curve) and synthetic (solid curve) tremolite ac-cording to Reaction 2 in the text. Open rectangular symbols :

tremolite growth; filled symbols : pyroxene growth; half-filledsymbols : no apparent reaction: I-beam symbols : regions inwhich the univariant boundary for synthetic tremolite must lieaccording to the study ofJenkins et al. (1990). All symbols rep-resent maximum uncertainty ranges in pressure and tempera-ture. Calculation of univariant curves discusssed in the text. Ab-breviations: Cpx : clinopyroxene; Opx : orthopyroxene; Qtz :quartz; Trem : tremolite.

rPwhere I AVptu dP

.t pn

= ir^v^,^ + a(ar)(T - To) - A@np/zlp.

The value of G' is calculated at the extreme P and Zrangeof the experiments that bracket the location of the uni-variant curve using heat capacity (Cr), volume (V), ex-pansivities (a), compressibilities (0), and activity data(expressed in terms of the equilibrium constant, K) forthe phases participating in the reaction as well as data onthe fugacity (f) of HrO. Internal consistency is demon-strated ifa straight line can be drawn between the brack-eting G' values on a plot of G' vs. (T - I). The slopeand intercept of this line are the AS.,.o,.o arl,d - Lc.,.,povalues of the reaction, respectively.

Numerical evaluation of G' was performed in the fol-lowing manner. Heat-capacity expressions for all of thephases as well as the volumes, expansivities, and com-pressibilities of the solid phases were those of Powell andHolland (1988). Fugacity data for HrO were read or in-terpolated from the tables of Burnham et al. (1969) atpressures below 1.5 kbar, instead ofbeing calculated fromthe equation for RZ ln/"ro given by Powell and Holland(1985), because the equation deviates strongly from the

G, : - [' (t:^+ a) ar * f 'o

tvy,,o" 7p+ nRTlnf I f + RZln K

: - AGlo,r6 + As%,plf - Zo),

I

l:vlz tNolurol ITrem I

\ ,=-J a'-


--- Nolurol tremolile- Synlhelic fremolite

>/YG Yin ond Greenwood (1983) // i

Opx . Cpx.Qtz . H2O

700 800 900 roooT- 298 K

Fig. 3. Internal consistency analysis ofthe experimental datafor Reaction 2 involving natural (dashed lines) and synthetic(solid lines) tremolite according to the G'technique described inthe text. Short (thick) solid lines labled YG are based on datafrom Yin and Greenwood (1983). Unlabeled short solid lines arefrom Jenkins et al. (1990). Unlabeled short (thick) dashed linesare from this study. All G' values were calculated with the max-imum range ofpressure and temperature uncertainties included.Abbreviations as in Fig. 2.

tabulated fugacity data in the pressure range of I kbarand below. The value of the equilibrium constant (K) isassumed to be unity for several reasons. First, the com-positions of the solid and fluid phases do not vary much,if at all, over the limited pressure and temperature rangeof the experiments for Reaction 2, and any compositionaleffects on the AG, of the reaction are essentially constant.By assuming K is unity, one forces any differences be-tween natural and synthetic tremolite to be expressed bythe differences in their derived values of AGr. Probablesources for these differences (i.e., composition, grain size,etc.) can then be sought for systematically. Second, well-documented activity vs. composition relationships aresimply not available for calcic amphiboles.

Values of G' were determined for the experiments in-volving natural tremolite that bracket the univariantboundary for Reaction 2 flisted in Table 2 (group A)].The extreme range of G' values, calculated by using themaximum pressure and temperature uncertainties, foreach bracket plot as the four thick, short dashed lines(rather than rectangles) on the scale of the diagram inFigure 3. Internal consistency is demonstrated if at leastone straight line can be drawn through each ofthe brack-ets, as is shown by the long, thin dashed line in Figure 3.Also shown in Figure 3 is an analysis of the experimentaldata for Reaction 2 involving synthetic tremolite that wasreported by Yin and Greenwood (1983) and Yin (1988,personal communication) for the pressure range of 0. I to1.4 kbar and by Jenkins, Holland, and Clare (unpublisheddata) for the range of 1.5 to 7 kbar. The G' values for the

Tmle 3, Experimental results on Reaction 3 using natural trem-olite (TREM 8)

Experi-mentno. T P t

(rR) fc) (kba4 (h) Comments*

22 725(sl24 725(5)28 725(5121 750(5)19 750(5)20 750(5)

-- 60(,

trem growthtrem growthtrem growthtrem grovvthtrem growthtalc + diop growth

Note.' Experiments are arranged in order of increasing temperature.Uncertainty in the last digit is shown in parentheses.

- Abbreviations as in Fig. 1.

experiments involving synthetic tremolite were calculat-ed using the same data and method as for the experimentsinvolving natural tremolite so that the two data sets canbe compared directly. Notice that the greater temperatureand pressure range for synthetic tremolite experimentsplaces a much greater restriction on the choice of linesthat can be drawn through the bracketing G' values (thick,short solid lines) in Figure 3. Indeed, it is not possible todraw one line through all of the experimental brackets,indicating a lack of internal consistency between the twosets of experiments for synthetic tremolite. With the ex-ception of the highest temperature bracket of Yin andGreenwood (1983), a straight line can be drawn throughthe remaining 7t/z brackets with slopes ranging between164.59 and 166.60 J/K. The long solid line has the in-termediate slope of 165.60 J/K and the associated inter-cept of -67 .46 kJ (: -AGP.rnr*., ou. of reaction). By forc-ing the reactions involving synthetic and natural tremoliteto have the same A$.rnr*., 0". of 165.60 J/K, one can de-rive the extreme range of AG! values from the interceptsof the two most widely spaced lines that can be drawnwith this slope through all four natural tremolite brackets.The long dashed line in Figure 3 for the natural tremolitereaction data is drawn with an intercept of -70.39 kJwhich is intermediate between the extreme values of-69.24 and -71.54 kJ.

The experimental results and the thermodynamic anal-ysis for Reaction 2 indicate that natural tremolite (TREM12 in Table l) is approximately 40 "C more stable thansynthetic tremolite which, on a constant-entropy basis,amounts to a difference of about -2.9 + l.l kJ/mol intheir AGrat 298 K and I bar. The univariant curves drawnin Figure 2 were calculated by reversing the thermody-namic analysis procedure described above and solving forP and T for a given AS.,.o,"o and AGlro..o. Both curveswere calculated for I AS?.rrr*., *. of 165.60 J/K and aAGf,rs*., *, of 67.46 kJ (solid curve) or 70.39 kJ (dashedcurve).

Trernolite-talc-diopside equilibrium

The results of experiments performed on Reaction 3 inthe pressure range of 25J8 kbar and involving natural

25.0(3)26.0(3)26.5(3)26.0(3)27.0(3)28.0(3)

487272962724

3 0 L600


tremolite (TREM 8 in Table l) are listed in Table 3 andshown in Figure 4A. As seen in Figure 4A, the univariantboundary has only been bracketed at 750 "C; however,data at 7 25 "C indicate that the boundary must lie above26.5 kbar. For comparison, only those experiments byJenkins, Holland, and Clare (unpublished data) involvingsynthetic tremolite and the same diopside and talc as usedin Figure 4A are shown in Figure 48. Considering thepressure uncertainties associated with the experiments,there could be as little as 0- and as much as 2.6-kbardifference in the upper-pressure stabilities ofnatural andsynthetic tremolite at 7 50 "C. The boundaries shown inFigures 4'A' and 4B are placed at the midpoints of thebrackets and have the same dP/dT slope (22 bars/K),which is based on the larger data set of Jenkins, Holland,and Clare (unpublished data) for synthetic tremolite.

There are obviously insufficient data in Figures 4A and48 to perform a G' analysis as was done in the previoussection. A simple calculation of the difference in the AG.of reaction for the reactions involving natural and syn-thetic tremolite can be made using the expression

AG,(Naturar) _ ̂ "":,,lTR";*l _ AG(synthetic),

where P, and P, are the pressures of the univariantboundaries for natural and synthetic tremolite, respec-tively, at a constant temperature. As in the previous sec-tion, AS and AV of both reactions are assumed to beequal, and no adjustment is made for mineral solid so-lutions (K: l), which forces all differences between syn-thetic and natural tremolite to be expressed by the differ-ences in AG.. Furthermore, AZis assumed to be constantover the small pressure range of the calculation (-3 kbar).Using the volume, expansivity, and compressibility dataof Powell and Holland (1988), one can calculate a AV of-0.364 J/bar at27 kbar and 750 "C. Substituting this AZinto the above expression and noting that P, - P, rangesfrom 0 to 2.6 kbar gives values of 0 to -0.95 kJ for thedifference between AG, for natural and synthetic tremo-lite. A value of -0.36 kJ is obtained for the l-kbar dif-ference indicated in Figure 4.

Souncns oF DrscREpANcy

The results indicate a real difference in the stability ofnatural and synthetic tremolite with the natural tremolitesamples used in this study having the greater P-7 stabil-ity. This difference in stability corresponds to a AG, ofnatural tremolite that is about 0.4-2.9 kJ more negativethan that of synthetic tremolite for a fixed entropy. Thethermodynamic analysis performed by Skippen andMcKinstry (1985) on their experimental data for Reac-tion I yielded a difference in AG,. for natural and synthetictremolite of -9.2 + 1.8 kJ (for fixed entropy), which isat least three times greater than was observed in this study.It is important to consider possible sources of discrep-ancy between this study and that of Skippen and Mc-Kinstry (1985) that could explain these differences and,

720 7Q 760 7A 7Q 760Temperolure,"C

Fig. 4. Experimental results on the high-pressure stability oftremolite according to Reaction 3 in the text. (A) Results fornatural tremolite from Table 3. (B) Results for synthetic tremo-lite, investigated with the same experimental apparatus and talc+ diopside as for natural tremolite, from Jenkins, Holland, andClare (unpublished data).

in the process, identify what factors are most likely con-trolling the difference in stability ofnatural and synthetictremolite.

HrO fugacity

The largest difference in experimental technique be-tween this study and that of Skippen and McKinstry(1985) is the use of H,O fugacity buffers. Skippen andMcKinstry (1985) used both an oxygen buffer (Ni-NiO)and hydrogen buffer (C-CH.) to buffer indirectly the HrOfugacity in their experiments and so eliminated this as avariable in their experiments. A relatively high fugacityof oxygen (as with pure HrO) may affect the relative sta-bilities of natural and synthetic tremolite if the minoramounts ofFe (<0.7 wto/o FeO in both studies), Mn (<0.1wto/o MnO), or Ti (<0.04 wto/o TiOr) in natural tremoliteare oxidized and increase its stability via a magnesiohas-tingsite (Semet and Ernst, l98l), magnesioriebeckite(Ernst, 1960), or oxyhornblende component.

Several experiments were performed in this study usingthe Ni-NiO oxygen buffer to bracket the location of Re-action 2, involving natural tremolite at 3 kbar. The re-sults of these experiments are in Table 2 (group B). Itshould be noted that the experiment durations were pur-posely kept short to minimize expenditure of the bufferresulting from the rapid diffusion of H, through the wallof the outer Au capsule at temperatwes of 900-950 'C

(Chou, 1986). Also notice that the larger, double-capsuleconfiguration used in these experiments resulted in a larg-er thermal gradient across the capsule and therefore largeruncertainty in the temperature measurement. As shownin Table 2, there is essentially no difference between theresults obtained with the buffered and unbufered exper-

E

oL ^ ^

zo

Nolurol Tremolile

Diopside + Tolc

Synlhelic Tremolife(Jenk ins , e l o l , l99O)

'++

Tremolife

Tfol+

Y

Tt-ol

-L


iments at 3 kbar, indicating that the buffer does not sig-nificantly influence the upper-thermal stability of naturaltremolite. An additional experiment was performed witha synthetic tremolite-bearing mixture using the Ni-NiObuffer (17-l I in Table 2) to see if the buffer might lowerthe stability of synthetic tremolite. Definite tremolitegowth occurred at 865 "C and 3 kbar, which agrees withthe univariant boundary for synthetic tremolite shown inFigure 2 and shows that any displacement in this curveto lower temperatures must be less than 20 "C.

Grain size

Differences in the grain size, and attendant diferencesin surface areas, of natural and synthetic tremolite mightaccount for the differences in their AG' in terms of theirsurface free energies. Qualitatively, this reasoning is cor-rect because the synthetic tremolite used in this study andthat of Skippen and McKinstry (1985) has a finer grainsize (greater surface area per mole of tremolite) and agreater surface free energy (always positive) than naturaltremolite. Quantitatively, it appears that surface free en-ergy alone is not sufrcient to account for the observeddifferences. No effort was made in this study to performany sieving of the tremolite in order to restrict the grain-size range of the starting material because of the ex-tremely fine grain size and limited quantity of the syn-thetic tremolite. Instead it was assumed that thoroughgrinding of the natural tremolite would yield a range ofgrain sizes comparable to the range ofgrain sizes obtainedin the synthesis of tremolite. This was not quite the case.A survey was made of the grain sizes of the natwal andsynthetic tremolite used in the starting mixtures with thepetrographic microscope. The largest synthetic tremolitegrains were 30 pm long and 5 pm wide (assumed equalto depth) with the average length being about 13 pm andwidth being 2 pm. The largest natural tremolite grainswere I l01rm long and 30 trrm wide, with the average beingabout 56 pm long and l0 pm wide. The aggregate surfaceatea can be calculated to a first approximation by assum-ing that all tremolite grains have the same dimensions asthe average grain sizes listed above. Knowing the densityof tremolite (2.98 glcm}), one can calculate that synthetictremolite has a surface area of 7220 cm'/g (587 m'?lmol)whereas natural tremolite has a surface area of 1460 cm2/g (l 19 m'?lmol). A reasonable range of surface free ener-gies for surface-fluid interfaces is 0.14.6 J/m'z (Ridleyand Thompson, 1986). Applying the maximum value of0.6 J/m'to the surface areas above yields surface freeeneryies of 0.35 kJ/mol and 0.07 kJ/mol for synthetic andnatural tremolite, respectively. The maximum differenceof 0.28 kJ/mol attributed to surface free energy is notquite enough to account for the observed differences of0.4-2.9 kJlmol.

Composition

The natural and synthetic tremolite used in this studyare similar but not identical in composition (Table l).Seemingly minor differences in their compositions may

have a noticeable effect on their thermal stabilities. Oneelement that could account for an increase in the stabilityof natural tremolite is F. The experimental data on theupper-thermal stability of synthetic F end-member trem-olite (Gilbert et al., 1982) indicate that it is stable up toabout ll90qC at 5 kbar (midrange of this study). FromFigure 2 it can be seen that OH end-member tremolite isstable to 9 l0 'C at 5 kbar. Combining these observationsgives a linear increase of 60 "C per wto/o F in the stabilityof tremolite. The natural tremolite used to investigateReaction 2 (TREM l2) has 0.5 wtTo F, which correspondsto a calculated 30 "C increase in its thermal stability. Thisis very close to the observed increase of 40 'C at 5 kbar.

A comparison of the upper-pressure stabilities of OHend-member tremolite and F end-member tremolite isnot so meaningful, because the high-pressure breakdownof F end-member tremolite does not involve talc (owingto the apparent instability of F end-member talc), butinvolves instead the assemblage orthopyroxene, clino-pyroxene, coesite, and fluorite (Gilbert et al., 1982).Nevertheless, the stability of F end-member tremolite at700 "C is 7 kbar greater than that of OH end-membertremolite, which is in qualitative agreement with the in-creased stability of the F-bearing natural tremolite (TREM8) used for studying Reaction 3.

Composition can also account for the differences in thederived values for AG, of natural and synthetic tremolite.If one adopts a simple ideal-mixing model for the com-ponent CarMgrSirOr,(OH), (: TR) in tremolitic amphi-bole, then the resultant activity expression for the TRcomponent is

4tt'h : (XAXI}..XXM t'r'3)5(Xr'',)8(13fr )r,

where ff is the mole fraction of cation I on site k and tr: vacancy. Notice that the octahedral-cation distributionis averaged over the Ml, M2, and M3 sites (Ml, 2, 3),and the tetrahedral-cation distribution is averaged overthe Tl and T2 sites (Tl, 2). Differences in the derivedvalues of AGrof pure OH end-member tremolite can nowbe related to the deviations that natural and synthetictremolite have from the ideal composition by the rela-tionship

G+ton - G$R : RZ ln c+t'n.

The composition of synthetic tremolite deviates from theideal by having l0o/o excess Mg in the M4 site (Jenkins,1987) giving d+i*h : 0.81. Using the compositions citedin Table I for the natural tremolites and the site-occu-pancy scheme ofleake (1978), one obtains values of0.70and 0.53 for the 4ltph of TREM 8 and TREM 12, respec-tively. The difference between TREM 12 and synthetictremolite at the mean temperature of 930 'C for the up-per-thermal stability of tremolite at 5 kbar is

Gbrffh(TREM 12) - @tr"(Synthetic): RZ ln d+tDh(TREM 12) - RT ln aii*h(Synthetic),

which is -4.24 kJ/mol. A similar calculation for syn-thetic and natural tremolite TREM 8 at 750 oC, the tem-


perature at which the calculations for Reaction 3 weremade, gives a difference of -0.92 kJlmol. These calcu-lated values are slightly greater than the observed valuesof -2.9 and -0.4 kJ/mol for Reactions 2 and,3, respec-tively. The main point, however, is that rather minordifferences in the compositions of natural and synthetictremolite are more than enough to account for the differ-ences in their thermochemical properties if ideal mixingis assumed.

The exact chemical substitutions that are giving rise tothe reduced activity of the TR component in naturaltremolite and the corresponding increase in its P-Z sta-bility are not known. For example, ideal mixing of tre-molite with either fluortremolite, richterite (Na), pargas-ite (Na,Al), or magnesiohastingsite (Na,Al,Fe3*) willincrease its thermal stability at the rate of 2-3 "C permolo/o of the amphibole component (see review by Gil-bert et al., 1982). If the observed increase in the thermalstability of natural tremolite is attributed to its F content,then it is important to briefly consider the rate of chem-ical reaction vs. the rate of exchange of F- for OH be-tween the crystal and ambient hydrothermal solution. Therate of chemical reaction for the dehydration of tremolitevia Reaction 2 canbe calculated from the procedure ofWalther and Wood (1984). The univariant boundary in-volving synthetic tremolite for Reaction 2 lies at 870 'C

at a pressure of2 kbar. For a 5 "C overstep ofthis bound-ary, the AG. of reaction is -0.51 kJ and the correspond-ing reaction rate, dm/dt, is 2.25 x l0 I' mol/(cmr.s) ofoxygen. The time necessary to completely break down asphere of tremolite with a radius of 50 pm and an oxygen"density" of 0.085 moVcm3 is 220 d. Recent investiga-tion in this laboratory into the kinetics of F-OH interdif-fusion in tremolite by D. Brabander (personal commu-nication) indicates a maximum diffusion rate of 2.5 xl0 13 cm2.s-r at 875 "C. Substituting this value into theequation for diffusion from a source of limited extent(Crank, 1975, p. l5), one can calculate the rate at whichF diftrses from a 5O-pm-radius sphere of tremolite, whoseinitial concentration is 0.5 wto/o F throughout, into theambient aqueous fluid. After 220 d, this sphere wouldhave lost only 23o/o ofits total F. It is clear that the rateof chemical reaction (complete in 220 d) is much fasterthan the rate ofF-OH interchange and that a F-bearingtremolite crystal is effectively impervious to any signifi-cant F-OH exchange between the crystal and fluid on thetime scale of the experiments performed in this study.

CoNclusroNs

The main observation of this study is that natural andsynthetic tremolite do indeed have different upper-ther-mal and upper-pressure stabilities, with natural tremolitehaving the greater stability. The extent to which they dif-fer, however, is not nearly so large as indicated by thestudy of Skippen and McKinstry (1985). Of the factorsthat can be quantified with some degree of accuracy, i.e.,HrO fugacity, grain size, and composition, it appears thatcompositional variation is adequate to account for the

higher stability and associated lower AGr of natural trem-olite. Minor differences in F content, for example, couldaccount for the different thermal and (perhaps) pressurestabilities of natural and synthetic tremolite.

The reasons for the conflicting results between this studyand that of Skippen and McKinstry (1985) are not known.Based on the study ofReaction I by Jenkins (1983), Skip-pen and McKinstry (1985) suggested that tremolite syn-thesized at high pressures (> l5 kbar), might be structur-ally closer to that of natural tremolite. This is notsubstantiated because the synthetic tremolite used in thisstudy was made at 2 kbar (Table 2), the same pressureused by Skippen and McKinstry (1985). It is clear fromtheir own analysis and from the arguments given herethat compositional differences would account for some,but certainly not all, of the approximately 133 "C differ-ence in thermal stability that they observed. One impor-tant observation made in this study that may offer anexplanation is that the (isobaric) experimental bracketsfor synthetic tremolite are much tighter (22'C on theaverage) than those for natural tremolite (41 "C on theaverage). This probably arises from the slightly morecomplex composition of natural tremolite imparting a di-variant nature on a nominally univariant reaction. Theoverall effect is to make it more difficult to interpret atwhat temperature tremolite grows or breaks down.

At this time there is little information available on thetype or amount of structural defects in synthetic and nat-ural tremolitic amphiboles. Even if such information wereavailable, it would not be obvious what influence suchdefects would have on the macroscopic properties of am-phiboles. In view of the effect that composition can have,there is no immediate reason to assume that structuraldefect density is dominating the behavior of amphiboles.At least in hydrothermal processes, synthetic calcic am-phiboles closely model the behavior of natural calcic am-phiboles if differences in composition are taken into con-sideration.

AcrNowr,BocMENTs

This research was supported by NSF grant EAR-8803437 to D.M.J.and NSF grant EAR-81071l0 to Robert C. Newton at the University ofChicago. Financial support for this research was also provided by an NSFSummer Research Fellowship to A.K.C. Thanks go to Andy Davis (Chi-cago) and Jon Stickles (Binghamton) for help with IR spectroscopy and toDan Brabander for the F-OH interdiffusion data. The manuscript wasimproved by the reviews of R. G. Berman and J. V. Chernosky, Jr. Manu-script preparation by Kay Holley, Anne Darling, and Dave Tuttle is geat-ly appreciated.

Rnrnnrnqcns CITED

Burnham, C.W., Holloway, J.R., and Davis, N.F. (1969) Thermodynamicproperties ofwater to 1000 qC and 10,000 bars. Geological Society ofAmerica Special Paper 132.

Chou, I.M. (1986) Permeability of precious metals to hydrogen at 2 kbtotal pressure and elevated temperatures. American Journal ofScience,286, 638-658.

Crank, J. (1975) The mathematics ofdifftrsion (2nd edition). Oxford Press,Oxford.

Day, H.W., and Kumin, H.J. (1980) Thermodynamic analysis of the alu-


minum silicate triple point. American Journal of Science, 280, 265-287.

Dorling, M., and Zussman, J. (1987) Characteristics of asbestiform andnon-asbestiform calcic amphiboles. Lithos, 20, 469489.

Ernst, W.G. (1960) Stability relations of magnesioriebeckite. Geochimicaet cosmochimicaActa. 19. 10-40

Gilbert, MC, Helz, R.T., Popp, R.K, and Spear, F.S. (1982) Experi-mental studies of amphibole stability. In Mineralogical Society ofAmerica Reviews in Mineralogy, 98,229-253.

Hemingway, B S., and Robie, R.A. (1977) Enthalpies of formation of lowalbite (NaAlSi3O8), gibbsite (AI(OH),), and NaAlO,; Revised values forAI{r* and AGfrr. of some aluminosilicate minerals. U.S. GeologicalSurvey Journal of Research, 5, 413429.

Holland, T.J.B. (1980) The reaction albite : jadeite + quartz determinedexperimentally in the range 600-1200 qC. American Mineralogist, 65,t29-r34.

Holland, T J.B., and Powell, R. (1985) An internally consistent thermo-dynamic dataset with uncertainties and correlations: 2. Data and re-sults. Journal of Metamorphic Geology, 3, 345-370.

Jenkins, D.M. (198 l) Experimental phase relations of hyfuous peridotitesmodelled in the system H,O-CaO-MgO-AlrO,-SiO,. Contributions toMineralogy and Petrology, 7 7, 166-17 6.

- (1983) Stability and composition relations ofcalcic amphiboles inultramafic rocks. Contributions to Mineralogy and Petrology, 83, 375-384.

-(1987) Synthesis and characterization of tremolite in the systemH,O-CaO-MgO-SiO,. American Mineralogist, 7 2, 707-7 15.

Johannes, W., Bell, P.M., Mao, H.K., Boettcher, A.L., Chipman, D.W.,Hays, J.F., Newton, R.C., and Seifert, F. (1971) An interlaboratorycomparison ofpiston-cylinder pressure and calibration using the albitebreakdown reaction. Contributions to Mineralogy and Petrology, 32,24-38

Leake, B.E. (1978) Nomenclature of amphiboles. American Mineralogist,63, r023-t052

Lindsley, D.H., Grover, J.E., and Davidson, P.M. (lgEl) The thermo-dynamics of the M&Si,O6-CaMgSiOu join: A review and an improvedmodel. In R.C. Newton, A. Navrotsky, and B.J. Wood, Eds., Ther-modynamics of minerals and melts, p. 149-175. Springer-Verlag, NewYork.

Maresch, W.V., and Czank, M. (1988) Crystal chemisry, growth kineticsand phase relationships of structurally disordered (Mn'?*,Mg)-amphi-boles. Fortschritte der Mineralogie, 66, 69-121.

Nitkiewicz, A , Kerrick, D.M., and Hemingway, B.S. (1984) The effect ofparticle size on the enthalpy of solution of tremolite; implications forphase equilibria and solution calorimetry. Geological Society of Amer-ica Abstracts with Programs, 16, 610.

Powell, R., and Holland, T J.B. (1985) An internally consistent thermo-dynamic dataset with uncertainties and correlations: l. Methods and aworked example. Journal of Metamorphic Geology, 3, 327-342.

-(1988) An internally consistent data set with uncertainties andcorrelations: 3. Applications to geobarometry, worked examples and acomputer program. Joumal of Metamorphic Geology, 6, 173-204.

Ridley, J., and Thompson, A.B. (1986) The role of mineral kinetics inthe development of metamorphic microtextures. In J.V. Walther andB.J. Wood, Eds., Fluid-rock interactions during metamorphism, p. 154-193. Springer-Verlag, New York.

Salje, E. (1986) Heat capacities and entropies ofandalusite and silliman-ite: The influence offibrolitization on the phase diagram ofthe AISiO,polymorphs. American Mineralogist, 7 l, 1366-137 L

Semet, M.P., and Ernst, W.G. (1981) Experimental stability relations ofthe hornblende magnesio-hastingsite: Summary. Geological Society ofAmerica Bulletin, Part I, 92, 7 l-74.

Skippen, G., and McKinstry, B.W. (1985) Synthetic and natural tremolitein equilibrium with forsterite, enstatite, diopside, and fluid. Contribu-tions to Mineralogy and Petrology , 89 , 256-262 .

Veblen, D.R. (l 980) Anthophyllite asbestos: Microstructures, intergrownsheet silicates, and mechanisms of fiber formation. American Miner-alogist, 65, 1075-1086.

Walther, J.V., and Wood, B.J (1984) Rate and mechanism in progrademetamorphism. Contributions to Mineralogy and Petrology, 88,246-259.

Yin, H.A., and Greenwood, H.J. (1983) Displacement ofequilbria of OH-tremolite and F-tremolite solid solution. I. Determination of the equi-librium P-7" curve of OH-femolite (abs.). EOS,64,347.

Mnxuscnrrr RECETVED M,lv 2, 1989M.q.Nuscmpr AccEprED Noveuner 30, 1989

comparison of the high-temperature and high-pressure ... · comparison of the high-temperature and...

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