American Mineralogist, Volume 79, pages862-377, 1994 Composition and temperaturedependence of Fe-Mg ordering in cummingtonite-gruneriteas determinedby X-ray diffraction Mlnc Hrnscnulnxr* Bnnmnn W. Evlxs, Hnxroxc Y,q'xc Department of Geological SciencesAJ-20, University of Washinglon, Seattle,Washington 98195, U.S.A. Ansrru,cr The crystal structuresof 24 heat-treatedand six unheatednatural ferromagresianclino- amphiboles have been refined from single-crystalX-ray intensity data. Relative to Mg, Fe2+ is strongly concentrated in the M4 site and weakly depleted in the M2 site in all crystalsexamined, but the degreeof fractionation is lesspronounced in the samplesequil- ibrated at high temperature. Fe shows a weak preferencefor M3 relative to Ml in heat- treated cummingtonite-grunerite, but no significant preferenceis observed in unheated crystals.Site preferencesexpressed in terms of an ideal ordering energy, -Ri"ln K,r, show no temperature dependenceover the range of temperaturesof heat treatment, 600-750 .C. The ideal orderingenergy between Ml and M4, -R7"ln K'o, equal to 18.2 + 0.3 (lo) kJ, is independent of composition except for very magnesiancompositions, for which the degree of ordering decreases.The ideal ordering energy between the Ml and M2 sites, -Ri"ln K,r, decreasesfrom -2 kJ at pure FerSirOrr(OH) to -6 kJ at pure Mg'Si'Orr(OH)r. With the assumption that Ca is restricted to the M4 site and that Mn is strongly ordered into the M4 site, Ml23 vs. M4 site preferencesdetermined by Mtissbauer spectroscopy are consistent with our X-ray diffraction results. The projection of Miissbauer results to the Fe-Mg join is more critically dependentthan X-ray refinementson the assignmentof Ca and Mn onto the appropriate sites. Our projection of the Mdssbauer data of natural cummingtonite and grunerite collected by Hafner and Ghose (197l) indicates stronger ordering between M4 and Ml23 than previously interpreted. At room temperature, heat-treatedcummingtonite with Fe/(Fe + Mg) as g.reatas 0.38 has P2r/m symmetry. Average bond lengths for the Ml, M2, and M3 octahedracorrelate well with mean ionic radius, and, for equivalent site occupancies,the mean bond length of the M3 octahedron is approximately 0.05 A smaller than those of the Ml and M2 octahedra. When corrected for impurities, variations in cell volume with Fe/(Fe + Mg) are linear, giving extrapolated molar volumes of 26.33 + 0.01 (lo) J/bar for magnesio- cummingtonite and 27.84 + 0.01 J/bar for grunerite. The cell volume of ferromagnesian clinoamphibole is indistinguishablefrom that of orthoamphibole of the samecomposition, indicating that cummingtonite molar volumes may also be appropriate for end-member anthophyllite and ferroanthophyllite. The derived molar volume of anthophyllite is l0lo smallerthan that givenby the databases of Berman(1988) and Holland and Powell(1990). There are no detectablevariations in cell volume of cummingtonite attributable to changes in ordering state. Ixrnooucrror the development of quantitative descriptions of the en- amphibole solutions. One of the constraints Becauseof their common occurrenceand composition- ergetics of required for rigorous thermodynamic and kinetic models al complexity, amphibolesare potentially among the most is a detailed description of intra- powerful petrogeneticindicators. Given appropriate un- of amphibole behavior crystalline ordering as a function of temperature, pres- derstanding of their chemical content, amphiboles may sure, and amphibole composition. In this study, we use be applied as thermobarometers,fluid fugacity monitors, X-ray diffraction to determine site occu- and, through documentation of intracrystalline site oc- single-crystal panciesas a function of composition and temperaturefor cupancies, indicators of cooling rates. Unfortunately, natural clinoamphiboles close to the magnesiocumming- compositional and structural complexities have hindered tonite-grunerite binary, (Mg,Fe),Si'O'r(OH),. These de- terminations provide much of the information necessary * Present address:Division of Geological and Planetary Sci- properties ences170-25, California Institute ofTechnology, Pasadena,Cal- to model the mixing of ferromagnesianclino- ifornia9l125, U.S.A. amphibole. Site-occupancy determinations along this 0003-004x/94l09 I 0-0862$02.00 862 HIRSCHMANN ET AL.: Fe-Mg ORDER IN FERROMAGNESIAN AMPHIBOLES 863 Tmu 1. Sample localities,heat treatments, and references Fe No. of crystals heat- treated at r(oc)- (Fe + Mg) Specimen no. Rock type (%l Location Reterences KL14A cum-tr-ol rock 11 45a Feather River. Califomia 1, U. Wash.collection w82-009 metas. cum rock 22 57d 30a Watkins Fiord, Greenland 2, U. Wash.collection USNM118125 metas. cum rock 35-38 11a 1c 8a" Baltimore Co., Maryland 3,4,5,6 DH7-482 iron formation 38-39 21a Bloom Lake, Quebec 5,7, 8, 9, 11,12, 13, 14 DH10-95 iron formation 46 6b Bloom Lake, Quebec 8,9,11, 12 DH7484 iron formation 49 14a,149 4a,4b Bloom Lake, Ouebec 'l7c 8,9 DH10-85 iron formation 55 Bloom Lake, Quebec 8,9 10-K iron formation 59 18a Labrador City, Labrador 9,10,11 AMNH27022 iron formation s8-60 3b Montenevoso,Alto Adige, ltaly Lab51 0 iron formation 61 2b Wind River Mts., Wyoming 15, U. Wash.collection 7B iron formation 66 19a Bloom Lake, Quebec 8,9,11, 12 AMNH24159 iron formation 66-70 12a 5b,5c 9a*' Mikonui R., Westland, N.Z. 3,4,16 cL-1 iron formation 74 16a Bloom Lake, Quebec 8,9 1-K iron formation 89 15b 221 Labrador City, Labrador 3,4,9,10, 11, 17 : A/ote;USNM UnitedStates National Museum, washington, DC. AMNH : AmericanMuseum of NaturalHistory, New york. References:(1) Stop 14' FieldGuide: Paleozoic-Mesozoic Rocks of the NorthernSiena Nevada, GSA Cordilteran Section, 1977; 1Z; Wiifins Flord,Kangerdtugsiuiq, e. Greenland(S. Hofimancollectionl (3) qyTs and Strens(1966); (a) Bancroftet at. (1967);(5) Ghoseano wLibner(1972): (6)Wat6r andsaristiury (1989);(7) Ghose (1961); (8) Mueller (1960); (9) Hafner and Ghose (1971); (10) Ktein0964);'1it; Xtein and Watdbaum (r9'62);-(rZ) Viswanathan ani Ghose(1965); (13) Fischer (1966); (14) ' Mitchellet al.(1971); (15) BdLnus (1i7oi; (16) iaason (ig5s); (17) Finger (1969). Heatedcrystals are identifiedin Tables2-7 and in the text by thesd numbers,which were a3signboo-urin! neai treatingand crystal set€ction. Unheatedcrystals are identified by theirspecimen numbers. " Crystalslost during polishing. Averaged microprobe analyses of 1c and1 1 a wereused tor the 8a constrainedrefinement and 5b, Sc, and 1 2a were usedtor the 9a constrainedrefinement. compositionally simple join can also provide a founda- magnesian amphibole and, combined with constraints tion for understanding ordering and thermodynamic from heterogeneousphase equilibria, will be employed to mixing properties of more complex amphiboles, such as calibrate simple amphibole mixing properties. anthophyllite, gedrite, and actinolite. Two commonly applied techniquesfor determining site Single-crystal X-ray structure refinements of natural occupanciesin minerals are single-crystalX-ray diffrac- membersof the cummingtoniteseries (Ghose, l96l; Fi- tion and M<issbauerspectroscopy. Both techniqueshave scher, 1966; Finger, 1969; Ghose and Ganguly, l9B2) advantagesand disadvantages,and both have been ap- have shown that Fe2+ strongly prefers the M4 site, Mg plied toward understanding ordering in amphiboles. Di- prefers M2, and the Ml and M3 sites have similar site rect comparison of the two techniquesapplied to ortho- occupancies.Mdssbauer spectra taken at 298 and,77 K pyroxene raises the possibility that there may be do not separatelyresolve M I , M2, and M3, but they con- systematic differences between site occupancies deter- firm the strong preferenceof Fe2+for M4 (Bancroft et al., mined by Miissbauer spectroscopyand those determined 19671,Hafner and Ghose, l97l; Buckley and Wilkins, by X-ray diffraction (Domeneghetti and Steffen, 1992; l97 l; Barabanov and Tomilov, 1973); they have also Skogby et al., 1992). Such systematicdifferences may be shown that ordering between M4 and Ml23 (M123 : compounded in amphiboles, where additional uncertain- combined Ml, M2, and M3) is temperaturedependent ties arise from the role of minor substituentssuch as Ca, (Ghose and Weidner, 1972). The combination of M6ss- Mn, Al, and Fe3*. The present work provides an oppor- bauer and infrared absorption spectroscopyalso shows tunity to investigate possible systematic differencesbe- that Fe2+favors Ml3 over M2 (Bancroft et al., 1967: tween these two techniques for site-occupancydetermi- Buckleyand Wilkins, l97l; Ying et al., 1989). nations in amphiboles.With this in mind, Dyar and Grant In the present work, we confirm the relative site pref- (personal communication) have acquired Mdssbauer erencesdetermined previously and document quantita- spectrafrom many of the samplesexamined in this study. tively the variations in M-site occupanciesof cumming- tonite as a function of composition and temperature. Expnnrurrttlr. pRocEDr.lREs Measuredsite occupanciesindicate that a three-sitemod- el (MI3, Ml" Is{4) is required to describe the configura- Specimenpreparation tional entropy of cummingtonite accurately. Becauseof Samples selected for this investigation (Table l) in- the uneven number of different M sites in the formula clude several used in previous Mdssbauer (Hafner and unit (two Ml, two M2, one M3, two M4), the configu- Ghose, l97l: Ghose and Weidner. 1972\ and other in- rational entropy of cummingtonite is an asymmetric vestigations of the monoclinic ferromagnesian amphi- function of composition and cannot be described