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Fluorine-Hydroxyl Exchange in Synthetic Muscovite and Its

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Fluorine-Hydroxyl Exchange in Synthetic Muscovite and Its AmericanMineralogist, Volume62, pages 304-308, 1977 Fluorine-hydroxylexchange in syntheticmuscovite and its applicationto muscovite-biotite assemblages J. L. MuNoz Uniuersitvof Colorado,Boulder, Colorado 80309 eNn Srsvr LuPtNcroN ().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-fluorite-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 phlogopite(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 mica 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 biotites 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 kaolinite and t<oil, at two kilobars and 500oc. The starting naterial was the 1M polynorph of muscovite; all exchange runs at 59ooc. and above yielded entlrely the 2M1 polliorph; fower tenperature runs resulted in nixtures of polynorphs. Analytical uncertainties in fluorine analys6s are always less than + 102 (relative)' Least squares fitting of log K vs. reciprocal temperature yields the following log K = 2L00/T - 0,II (T in kelvins). "q..tlor,-- 306 MUNOZ AND LUDINGTON: SYNTHETIC MUSCOVITE 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' Mineral 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 margarite, 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 micas. 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 similarity in 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,
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