AmericanMineralogist, Volume62, pages 304-308, 1977

Fluorine-hydroxylexchange in syntheticmuscovite and its applicationto - assemblages

J. L. MuNoz Uniuersitvof Colorado,Boulder, Colorado 80309

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().5. GeotogicalSuruey, Reston, Virginia 22092

Abstract

Hycirothermalfluorine buffer techniques are usedto measurethe equilibriumconstant for the exchangereaction: OH-muscovite * HF 3 F-muscovite+ HrO. At a total pressureof two kbar,log ,K : 2lOO/T- 0.1I (T in kelvins).These data, when combined with similardata biotiteand musco- for biotite,fermit evaluationof F= OH equilibriumbetween coexisting vite. The partitioning,which is independentof temperatureand fluid composition,may be portrayedgraphically, and provides a simpietest of F* OH exchangeequilibria in natural micapairs.

These Introduction spectrophotometricmethod (Shapiro,1967)' analysesare converted to F/OH ratios,assuming F * Fluorine is lessabundant in dioctahedralmicas OH 2.0 for muscovite of stoichiometry than in biotite;nonetheless, its presencecan be used KAlasiroro(F,OH)r. An equilibriumconstant is cal- to reconstructfluid compositions(in terms of the culatedfor eachexperiment: OH-muscovite t HF= ratio if temperaturesof muscoviteequilibra- fu,ol nr) F-muscovite+ HrO tion with fluid are known.Experimental calibrations for muscovitehave beencompleted using techniques log K : log(fu,o/fat)rruicr* log (ar/aotr)muacovite' identicalwith thosepreviously described for biotite The fugacity ratio is obtainedfrom thermodynamic (Munozand Ludington,1974). data for the AFSQ bufferphases (Munoz and Lud- 1974),revised using more recentvalues for ExPerimentalProcedure ! ington, anorthite(Thompson, 1974). Mole fractionsare used equilibratingsynthetic Eachexperiment consists of as an approximationto activitiesfor the fluorineand with a fluorine muscovitewith a fluid in contact hydroxylsites in muscovite. 1969).The buffer assemblage(Munoz and Eugster, Reversibilityof F = OH exchangewas demon- assemblage anorthite--sillimanite-quartz-gas stratedin two experiments(Runs MSE-5 and MSE-8 at constant (AFSQ), which fixesthe ratiofr,o/fw in Table l), in which the startingmaterial was a accordingto the equilib- temperatureand pressure fluorine-bearingmuscovite obtained by reacting rium OH-muscovitewith AFSQ at a highertemperature' of fluorine CaAl,Si,O,+ 2HF=CaFz + AlrSiOs+ SiO,+ HzO, Both experimentsshowed a reduction whenequilibrated with AFSQ at a lowertemperature = Ko,rr". f""o/frlr, (Fig.l). experiments,because it was usedfor all muscovite ExPerimentalresults generateshydrogen fluoride fugacities sufficiently that muscovite high to produceeasily measurable fluorine levels in Table I and FiguresI and2 show equilibratedwith muscovite.At theend of eachexperiment, the musco- containsless fluorine than biotite of ther- vite is separatedfrom the buffer,checked by both X- AFSQ at similartemperatures. Reevaluation phases(Ludington and ray diffraction and optical examinationfor purity, modynamic data for buffer that and is analyzedfor fluorine by meansof an indirect Munoz. 1975) has led to the conclusion 304 MUNOZ AND LUD]NGTONSYNTHETIC MUSCOVITE

no experimentallysignificant difference exists in the slopes of the previously determined annite 7o,o (KFe,AlSirO,rH,), siderophyllite (KFe2,Al, Si, uO,2H2),and (KM&AlSi.O,rHr) equi- librium constants,when the data are plotted in terms of log K us. l/T (Fig. 2). Similarly, the data for muscoviteclosely approximate a straight-lineof this same u slope. Although these lines are probably not 600 perfectly straight, no curvature is indicated by the t data. Any curvature that exists (reflecting changing ut reaction enthalpieswith temperature) is masked by I the stated uncertainties(ca. X 0.2 log units) asso_ 0! ciatedwith eachline. Consideringboth the reciprocal Li temperaturescale and proximity of data points, ex_ trapolation somewhat above the temperature range of the experimentsis probably justified, but extrapo_ lation to lower temperaturesis lesscertain. Becausethe slopesof all four exchangereac_ trons are so similar, the implication is that the en_ o 2 .a thalpy for all four reactionsis nearly identical. Ap_ parently the differences in mica structure and octahedral cation occupancieshave little effect on Fig. l. Fluorine contents of synthetic muscovite (mus), reaction enthalpy of the F = OH exchange.The side- rophyllite (sid), annite (ann), and phlogopite(phg) in equilibrium differencesin the interceptson the log K axisbetween with AFSQ at two kbars. Data for annite, siderophyllite,and the various and muscovite therefore must phlogopite are calculatedfrom exchangeequations in Ludington indicate that the entropies of the exchangereaction and Munoz (1975);experimental data is shown only for muscovite. differ substantially between phlogopite and musco- Arrowheads show direction from which equilibrium was ap- vite. Thus, there is a possibility The solid arrowheads highlight the two reversal experi- of F, OH ordering 1."";,:f"O differencesbetween the differentmicas. Such a possi- bifity is mentioned by Hazen and Burnham (1973) as gation is noticeably greater for annite than for a possibleexplanation for the elongationof the ther- phlogopite, suggestingthat annite is more ordered. mal ellipsoids for the OH site parallel to c* in the Further evidencefor this possibilityis given by Gug- phlogopite and annite structures.Indeed. the elon_ genheim and Bailey (1975), who postulate a low-

Table l. Experimentaldata for F=OH muscoviteexchanse

Temperature Run Time Final Conposition Run Number ("c. (days) ) Conposition (wt. Z F) F/F+OH 1og K

MSE-7 462 59 OH-ns o.5"1 0.05 MSE-8 2.49 462 59 2.0% F r.5z 0.16 MSE-10 3.03 500 6I OH-ms r.oz 0.11 MSE-12 2.59 590 68 OH-ms I .gii 0.19 2.32 MSE_6 6r2 59 0H-ns r.8,2 0.19 2.22 MSE-5 612 59 2.92 F 2.72 o.22 211 MSE-3 6_40 51 0H-ms 2.0"t o.2L MSE-1 2.13 690 36 OH-ns 2.92 0.31 2.12

AI1 experiments were conducted at a total fluid pressure of two kilobars; oxygen fugacity was inposed by the ste11lte pressure vessel. TemPerature uncertainty is * 50. oH-muscovite (oH-mus) was synthesized in 250 ng. lots from a stolchionetric mixture of and t

T contentsof coexistingmuscovite and biotite may not be usedas a geothermometer.Indeed, simultaneous 7o,0 aoo 900 1000 solutionof the log K equationsfor the pairsmusco- 5 vite=phlogopite,muscovite-annite, and- muscovite- siderbphylliteyield numbers for biotite-muscovite exchangethat are independentof both temperature and fluid composition'For the reaction: OH-biotite * F-muscovite= F-biotite * OH-muscovite,

log 1(bto-m"" : lol(ar/aor)oto * log (aou/ar)-"". Y Combiningthe data for muscovitepresented in this paper(Table 1, Fig. 2) with the reviseddata for the o biotitesreported in Ludington and Munoz (1975) o all the J -.\ allows solution of the above equation for biotiteend-members studied, yielding: \=_ o log 6nn"--". : 1.633,log 6"""--"" : 0.526,

and log /(Eid-mu' : 0.180. These numbers were used to construct Figure 3, which shows the variability in log K for bio- tite-muscoviteexchange as a function of octahedral

14 13 12 11 10 2 1e4/T Fig. 2. Equilibriumconstants for mica F = OH exchange' abbreviationsare as in Fig. I Filled circlesrepresent experimentsin which OH-muscovitewas the startingmaterial' : Open circlesrepresent the two reversalexperiments. Log K 2100/T- 0.1t. f measuredin kelvins. \: Y group for , which I symmetry (Cm/c) space (, T to splittingof the OH siteinto two. r leads o o The data presentedhere cannot be rigorouslyap- J o x..'\ o muscovitescontaining significant soo \.. plied to natural 2 N amountsof octahedralcations other than aluminum. ID For example,it is likelythat both lithium and magne- 0 sium, two of the most common minor cationsin dioctahedralmicas. should increasefluorine parti- 1\ tioning into muscovite.A rough correlationexists .a .6 4 .2 o betweenthe fluorineand lithium contentsof natural dioctahedralmicas (Foster, 1960; Munoz, 1968).In Xpnc the caseof magnesium,it seemsreasonable that the Fig. 3. Muscovite-biotite "equilibrometer," showing the effects samemarked preference for fluorine shownby mag- of differential postcrystallization loss of fluorine from muscovite nesian biotite should also apply to dioctahedral (mus) or biotite (bio). The equitibrium constant (K) refers to the mus and bio' Filled circle . distribution of F and OH between representsa biotite of composition phgroannro(FruOHs)coexisting represent APPIication with a muscovite of composition (FzoOHro)'Open circles 10, 25, and 50 percent fluorine loss from biotite (log K decreases) A practicalresult of the similarityin slopesof or muscovite (log K increases).Dashed lines representthe 0'2 log all the mica exchangereactions is that the fluorine units uncertainty in K discussedin the text. MUNOZ AND LUDINGTON.. SYNTHETIC MUSCOVITE 307

Table2. Siteoccupancies and F;:OH exchangeequilibrium constants for biotite* muscoviteassemblages reported by Miiller(1966)

--bio F occupancy vrr uLguParrLJ Sample No. bio nus 1og K

2 0. 30 0.120 0.052 r, 424 1.948 0.499 4 0.20 0.164 0.058 1. 318 L,942 0 .620 5 0.08 0. 387 0.376 1.055 r .624 0. 200 I7 0.22 0. 386 o.729 r. 525 1.871 0.s65

46 0.04 0.688 0. 408 0.573 r,592 0.67r 53 0.23 0.168 0.052 1.405 1.948 0.651 59 0.34 0.157 0. 060 1. 365 L.940 0.570

Structural formulas were calculated on the basis of 7 (blo) and 6 (mus) tetrahedral + octahedral cations,

composition in biotite. Inherent in such a construc- from biotitewill lowerlog K, whereasloss from mus- tion and in subsequentapplications is the assumption covitewill havethe oppositeeffect. that all activity coefficientsfor both octahedral and (F,OH) sitesin muscoviteand biotite are unity, and Example that the effectof minor substitutionsin both musco- Completepublished analyses of coexistingbiotites vite and biotite can be ignored. If equilibrium con- and muscovitesare rare. As an exampleof the useof stants are calculated for natural coexisting musco- the equilibrometer,we havechosen a setof analyses vite-biotite pairs, then all such pairs whose given by Miiller (1966)for micasfrom a seriesof compositions reflect F = OH exchangeequilibrium biotite-muscovitegranites. The equilibrium con- should plot within the shaded area on Figure 3. stantsand the data from whichthey were calculated Conversely,gross departures from the shaded area are given in Table 2, and illustratedin Figure 4. are a persuasiveargument against intercrystalline When one considersanalytical uncertainties, the equilibrium. Thus, we refer to the graph in Figure 3 as an "equilibrometer." In order to plot coexisting micas on the equili- brometer, it is necessaryto know both F and OH occupanciesfor muscoviteand biotite and the com- position of the octahedrallayer of the biotite. Unfor- tunately, there is no unique method of proceeding from chemical analyses to ion occupancies; the method that we believeis most satisfactoryis based on the assumption of full octahedral occupancy in V I biotite and muscovite(Ludington and Munoz,1975). o In testingnatural assemblagesby this method, it is o imperative to transfer analytical uncertainties J through to the calculation of log K. Typically, an uncertainty of +10 percent(relative) in fluorine and ++\ hydroxyl occupanciestranslates to approximately0.2 log units of uncertainty in log K. However, when the absolute amount of fluorine in the mica approaches small values (lessthan 0.5 weight percent), or. the rela- 1 6 4 tive error will commonly be significantlylarger. Postcrystallizationleaching of fluorine from micas Xpnc has been suggestedto be a common phenomenon Fig. 4. Muscovite-biotite pairs from European granites (Miil- (Carmichael et al. 1974). Figure 3 shows the effect of ler, 1966)plotted on the equilibrometer.Error barsrepresent * l0 preferentialfluorine losi by biotite or muscovite;loss percent uncertainty in fluorine and hydroxyl site occupancies. 308 MUNOZ AND LUDINGTON SYNTHETIC MUSCOVITE micas appearto be in equilibrium with respectto F= margarite structure in subgroup symmetry. Am Mineral , 60, OH exchange.The result is not unexpected,if the 1023-1029. Hazen, R. M and C. W. Burnham (1973)The crystalstructures of Frac- dynamicsof a cooling intrusion are considered. one-layerphlogopite and annite.Am Mineral, J8, 889-900. tionation of fluorine into the micas is so strong that Ludington, Steve and J L Munoz (1975) Application of an overwhelming proportion of the fluorine in the fluorhydroxyl exchangedata to natural micas(abstr.). Geol Soc system is in the hydrous phases, even during ex- Am , Abstracts with Programs, 7, 1179. solution of an aqueousfluid. Without alater influx of Miiller, Georg (1966) Die Beziehungenzwischen der chemischen Zusammensetzeng,Lichtbrechung, und Dichte einiger koexistie- unrelated fluids, no way to trans- conceivable exists render Biotit, Muskovit, und Chlorit aus granitischenTiefen- port enough fluorine to generaterecognizable dis- gesteinen Beitr Mineral. Petrol , 12, 173-l9l. equilibrium. Munoz, J. L. (1968) Physicalproperties of synthetic lepidolites. Am Mineral , 53, 1490-1512. Acknowledgments - and H P. Eugster(1969) Experimentalcontrol of fluorine reactionsin hydrothermal systemsAm Mineral , 54,943-959. We would like to thank D. K. Nordstrom. who oerformed the - and S. D. Ludington(1974) Fluorrde-hydroxylexchange in fluorine analyses. biotite ,4n J st'i , 274, 396-413. References Shapiro, Leonard (1967) A simple and rapid indirect determina- tion of fluorine in and rocks. U S Geol Suru Prof. Pap 575-D, D233-D235 Carmichael,L S. E., F. J. Turner and J. Verhoogen(1974) Igneous Thompson, A B (1974) Gibbs energy of aluminous minerals. Petrology McGraw-Hill, New York. 739 p. Conrrih Mineral Petrol ,18, 123-136. Foster, Margaret D. (1960) Interpretation of the composition of lithium micas U S Geol Suru , Prof. Pap 354-E, Ell5-8147. Manuscript receiued,May 28, 1976; accepted Guggenheim,Stephen and S. W. Bailey (1975) Refinementof the for publication,September 15, 1976