American Mineralogist, Volume 61, pages 431434, 1976 Margarite pseudomorphsafter chiastolitein the Rangeleyarea' Maine CHenI-rsV. Gulnorrt Departmentof Geologyand Geophysics,Uniuersity of Wisconsin Madison. Wisconsin 53706 eNo JosN T. CHSNTY Department of Geology,Amherst College Amherst. Massachusetts 01002 Abstract ln recent years margarite has been recognizedas an important rock-forming mineral in severalwidely distributed localities.This paper reports an occurrenceof margarite pseu- domorphs after chiastolite in metamorphosedblack shalesof the Rangeley area, Maine. Analyses are presentedfor coexisting margarite, muscovite, and chlorite. As with most reported analysis of margarite, a considerableamount of the paragonite end-member is Dresentin solid solution (-28.4 mole 7o). Introduction ular calc-silicateto peliticschist (Moench, 1971).It is an unusually distinctive unit inasmuch as it com- Until fairly recentlymargarite has beenconsidered monly contains up to l0 modal percentof pyrrhotite as a mineral of specialcompositional environments. modal percentof graphite. More recently it has been found as a rock-forming and several rocks of northwestern Maine have mineral in numerous localities: in Europe, Sagon The stratified to various grades during the (1967, 1970),Frey and Niggli (1972), Hdck (1974), been metamorphosed Acadian event in Lower Devonian time, e.g., Guid- and Ackermand and Morteani (1973);in the Scottish otti (1970),Boone (1973).ln the vicinity of Small's Dalradian, Chinner (197a); in Vermont, Lanphere Falls the metamorphic history involves poly- and Albee (1974) and A. B. Thompson (personal metamorphism. In some specimens chiastolite is communication); and in British Columbia, Jones replacedby coarsemargarite, and stauro- (197l). Moreover, experimentaldata is now becom- completely lite from adjacent stratigraphic units is commonly ing available on margarite and its phase relations; replaced by aggregatesof coarsechlorite and mus- e.g., Velde (1971),Chatterjee (1974), and Storre and most recent metamorphism took place Nitsch (1974). In addition, some theoretical treat- covite. The garnel-zoneconditions. ments have also been made of margarite phaserela- under tions; e.9., Frey and Orville (1914) and Thompson /r974). PetrograPhY This paper describesan occurrenceof margarite in The specimenin which margarite has been identi- northwesternMaine where margariteoccurs as pseu- fied is fairly typical of the pelitic varieties of the domorphs of chiastoliticandalusite with eventhe fine Small's Falls Formation in the middle grades of detailsof the graphitic crossespreserved. metamorphism.It is a well-foliatedphyllite or schist which is dark grey to black on fresh surfacesand has Geologic setting some minor crinkling. Lenticular blebs of pyrrhotite The margarite-bearingrocks occur in the Small's are readily seenin hand specimen,and in somecases Falls Formation in a road cut at a locality on High- tiny chlorite platesare also visible. way 4 known as Small's Falls (about 8 miles east- The most interestingmineralogical feature of the southeastof the town of Rangeley).The Small's Falls pelitesin the vicinity of Small's Falls is the develop- Formation contains lithologies ranging from gran- ment of chiastolitemegacrysts with dimensionsup to 431 432 C- V. GUIDOTTI AND J. T. CHENEY A. B. Ftc. l. Photomicrographof margaritepseudomorphous after chiastolite,all at l0X. (A)$ portion of a largepseudomorph showing detaileddistribution of graphite.(B) Sameview in crossednicols. TAsr-s 1. Analyses+ of coexisting margarite, muscovite, and 3 cm in width and l5 cm in length. Thesemegacrysts chlorite from specimenRa-a86'-66, Rangeley area, Maine. - tend to lie within the foliation planes. In the speci- Margarite Muscovite Chlorite mens studied thus far the aluminum silicatehas been Sio2 12 )1 47.60 28.0r+ completely converted to margarite. The margarite A1203 50.35 23.50 FeO .32 9.07 occurs as coarsesheaves which make up virtually all Mgo 2q 1.08 of the pseudomorph. Only minor quartz and the Tio2 .04 .7L .05 graphite of the original chiastoliteare presentas ad- MnO .01 .02 .66 ditional minerals in the pseudomorphs x2o 000 9.16 000 (Fig. l). Na2O 22? l.16 000 Visual modal estimatesof the groundmassof the CaO 10.00 000 000 specimen provide the following data: quartz 50Vo, Hzo*' t+.86 r+.t+5 13.I0 muscovite 33Vo,chlorite 5vo, graphite 3Eo,pyrrhotite Fomulas Baseal on 22 Oxvsen (Micas) and 28 Oxvgen (Chlorite.) 8Vo,and rutile l%o.Muscovite and chlorite occur as IV Si 4.248 6.233 5. r+59 fine-grained laths which define the foliation. No AI 3.752 )".7 67 2. 54I plagioclaseor paragonitewas Total Al 7.820 5. +89 5.391 detectedeither by thin- section investigationor by electron microprobe. VI A1 r+.068 3.722 2.850 Be- Fe .01r+ .035 1.477 causegraphite is abundant, fine-grained,and widely Mg .056 .2r2 7 .396 dispersed,it causesa darkening or blurring of the Ti .003 .070 .007 thin sections-a feature which makes petrography Mn .00I .002 .109 and modal estimatesdifficult. The rutile occurs as x +.Ir+2 4. 0+l II. 839 tiny amber-colored prisms, some of which display XII K 000 1.530 000 knee-shapedtwins. Tourmaline is present Na .570 .29+ 000 as tiny yel- Ca I. +12 000 000 low prisms. t 1.982 1.824 000 MglEe 5.006 Mineral data MnlFe .07'+ Na,/Na+Ca+K .288 . 16l Table I provides chemical data for the margarite, *ArcLU.s,eBbA elect"on muscovite, and chlorite. Figure 2 is a plot nienoprcbe; Bee Guidotti qlqL. (1925) for of the detailo of run eotditione,' *aValue coexistingmargarite and muscovite white f-ot H2ObU differerce f"on 100* on the mica Y.',tr1i3,"tr"=lfT;',1!|,"".==';!,!,i,[3',i,\;8'855(s)'e=plane of the system ALOr-CaO-NarO-KrO with Egtirctel. atcndqd, ereora @e in puentheeee and to thp SiO, in excessand an HrO-rich fluid assumedpresent. Laet d,ecircl, plrce. "efer As has been found in most studies on marsarite MARGARITE PSEUDOMORPHSAFTER CHIASTOLITE IN THE RANGELEY AREA, MAINE coexistingwithother whitemicas,this margaritecon- tains a considerableamount of the paragonite end- member in solid solution (e.g., see the margar- ite-muscovite pairs from H6ck (1974) and Fox (1974)as plotted on Fig. 2). Table I also presentscell dimensionsof the margarite from the Small's Falls locality, obtained via the program of Appleman and Evans(1973). Discussion This paper presentsdata which establishanother tie line of the white mica plane.It therebycontributes further to understanding the miscibility relations K At2(Al NoAla(Al SraO,j(OH), among the white micas. Of interestis the fact that this margarite,like those Flc. 2. Plot of margarite-muscovite pair from specimen Ra- a86'-66 on the margarite-muscovite-paragonite plane of the sys- in a number of other occurrences [e.g., Chinner tem ALOa-NazO-KzO-CaO (quartz and an HzO-rich fluid as- (1974), Lanphere and Albee (1974)), seemsto have sumed present); also shown are margarite-muscovite pairs from formed in associationwith a pseudomorph-produc- Hdck (1974)(H-t48/70 and H-l3l/70) and Fox (1974)(F-267)' ing event. This suggeststhat in some casesthe pres- enceof margarite may be related to aspectsof poly- point, becausethey have dealt with water-saturated metamorphism rather than being direct primary systemsthat are free of NazO and KrO. formation. Acknowledgments Many (but not all) occurrencesof margarite as a Thiswork has been supported by NSF Grant GA-42834. We are rock-forming mineral seemto involve the presenceof grateful to Drs. N Chatterjee, M. Frey, and A B. Thompson for graphite [e.g.,this report, Sagon(1967,1970), Chin- critical comments on an earlier version of the manuscript and to ner (1974), Frey and Orville (1974), and Frey and Dr. J S. Fox for permissionto make use of some of his unpub- Niggli (1972)1.Graphite may be present in other lisheddata. paragenesesinvolving margarite but the published References reports have not always listed the opaques present AcrsnvlNo, D. lNo G. MonrneNt (1973)Occurrence and break- [e.g., graphite is present in many of the specimens down of paragonite and margarite in the Greiner Schiefer Series (1974), (personal studied by Hcick according to Frey (Zillerthal Alps, Tyrol). Contrib. Mineral. Petrcl. 40' 293-304. communication)]. This is regrettable,because it is ApprrulN, D. E. eNo H. T. EvnNs,JR. (1973)Job9214: Indexing possiblethat in somecases the presenceor absenceof and least-squaresrefinement of powder diffraction data. Natl. margariteas a rock-forming mineral might be a func- Tech. Inf. Serv., U.S. Dep. Commer., Springfield, Virginia tion of fluid compositionduring metamorphism.lFor 22151,Document PB 216 188. Boons, G. M. (1973) Metamorphic stratigraphy,petrology, and example,at a given fluid composition may con- PZ, structural geology of the Little Bigelow Mountain map area, trol not only the stability field of pure margarite but western Maine. Maine Ceol Suru. Bull 24, Dep. of Con- also its reactionswith other phases,and thus deter- servation,136 p. mine whethermargarite can occur in rocks with com- Cunttetrrn, N. D. (1974) Synthesisand upper thermal stability mon pelitic bulk compositions. limit of 2 M-margarite, CaAL[Al,SizO,"/(OH)r], Schweiz. Min- eral. Petrol. Milt. 54,753-767. Obviously many more data are neededon margar- CHINNER,G. A. (1974) Dalradian margarite:a preliminary note. ite-bearing parageneses to fully understand their Geol Mag lll' 75-78. significance. As pointed out by Chatterjee (1974), Eucsrnn, H. P. eNo G. P. Srlppnx (1967)Igeneous and metamor- the phase relations of margarite in the context phic reactionsinvolving gas equilibria. In P. H. Abelson, Ed., Vol. 2, New York: John Wiley and of the tetrahedron defined by the components Researchesin Geochemistry, Sons, p. 492-520. AlrO'-CaO-NarO-KrO (quartz-saturated) will be of Fox, J. S. (1974) Pelrology of some low'uariance meta-pelites from prime interestfor understandingthe petrologicimpli- the Lukmanier Passarea, Switzerland. Ph.D. thesis' Cambridge cations of margarite.However, to date,the available University,220 p. experimentaldata have provided little help on this FREv, M.