MENZERITE-(Y), A NEW SPECIES,{(Y, REE)(Ca, Fe2+) 2}[(Mg, Fe2+)(Fe3+, Al)](Si3) O12, FROM A FELSIC...

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The Canadian MineralogistVol.48,pp.000(2010)DOI:10.3749/canmin.48.5.000

MENZERITE-(Y), A NEW SPECIES, {(Y,REE)(Ca,Fe2+)2}[(Mg,Fe2+)(Fe3+,Al)](Si3)O12, FROM A FELSIC GRANULITE, PARRY SOUND, ONTARIO, AND A NEW GARNET END-MEMBER, {Y2Ca}[Mg2](Si3)O12

EdwardS. GREW§,JeffreyH.MARSHandMartinG.YATES

Department of Earth Sciences, University of Maine, 5790 Bryand Global Sciences Center, Orono, Maine 04469, USA

BiljanaLAZICandThomasARMBRUSTER

Mineralogical Crystallography Institute of Geological Sciences, University of Bern, Freiestrasse 3, CH–3012 Bern, Switzerland

AndrewLOCOCK

Department of Earth and Atmospheric Sciences, University of Alberta, 1–26 Earth Sciences Building, Edmonton, Alberta T6G 2E3, Canada

SamuelW.BELL

Department of Geology, Amherst College, Amherst, Massachusetts 01002, USA

M.DarbyDYAR

Department of Astronomy, Mount Holyoke College, Kendade Hall, 50 College Street, South Hadley, Massachusetts 01075, USA

Heinz-JürgenBERNHARDT

Zentrale Elektronen-Mikrosonde, Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, D–44801 Bochum, Germany

OlafMEDENBACH

Institut für Geologie, Mineralogie und Geophysik – Mineralogie, Ruhr-Universität Bochum, D–44780 Bochum, Germany

Abstract

Menzerite-(Y),anewmineralspecies,formsreddishbrowncores,n =1.844(20),upto70mmacross,rimmedsuccessivelybyeuhedralalmandinecontainingupto2.7wt%Y2O3andbyK-feldsparinafelsicgranuliteonBonnetIslandintheinteriorParrySounddomain,GrenvilleOrogenicProvince,Canada.ItisnamedafterGeorgMenzer(1897–1989),theGermancrystallographerwhosolvedthecrystalstructureofgarnet.Single-crystalX-ray-diffractionresultsyieldedspacegroupIa3d,a=11.9947(6)Å.Anelectron-microprobeanalysisofthegrainrichestinY(16.93wt%Y2O3)gavethefollowingformula,normalizedtoeightcationsand12oxygenatoms:{Y0.83Gd0.01Dy0.05Ho0.02Er0.07Tm0.01Yb0.06Lu0.02Ca1.37Fe2+0.49Mn0.07}[Mg0.55Fe2+0.42Fe3+0.58Al0.35V0.01Sc0.01Ti0.08](Si2.82Al0.18)O12,or{(Y,REE)(Ca,Fe2+)2}[(Mg,Fe2+)(Fe3+,Al)](Si3)O12.Synchrotronmicro-XANESdatagaveFe3+/SFe=0.56(10)versus0.39(2)calculatedfromstoichiometry.ThescatteringpowerrefinedattheoctahedralYsite,17.68epfu, indicatesthatarelativelylightelementcontributestoitsoccupancy.Magnesium,asdeterminedbyelectron-microprobeanalyses,wouldbe a proper candidate. In addition, considering the complexoccupancyof this site, the averageY–Obondlengthof2.0244(16)ÅisinaccordwithapartialoccupancybyMg.ThedominanceofdivalentcationswithMg>Fe2+andtheabsenceofSiattheoctahedralYsite(insquarebrackets)istheprimarycriterionfordistinguishingmenzerite-(Y)fromother

§ E-mail address:[email protected]

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728 thecanadianmineralogist

silicategarnetspecies;themenzerite-(Y)end-memberis{Y2Ca}[Mg2](Si3)O12.Thecontactsofmenzerite-(Y)withalmandineare generally sharp and, in places, cuspate. It is interpreted to have equilibratedwith ferrosilite, augite, quartz, oligoclase,allanite-(Ce),magnetite, ilmeniteandfluorapatite, in theabsenceofalmandine,on theprogradepathat7–8.5kbarandT�700–800°C,andsubsequentlydissolvedincongruentlyinananatecticmelttoformalmandine,mostlikely,atP � 8.5–9.5 kbarand T �800–850°C.

Keywords:garnet,menzerite,yttrium,rare-earthelements,newmineralspecies,crystalstructure,GrenvilleProvince,granulitefacies,ParrySound,Ontario.

Sommaire

Lamenzerite-(Y),nouvelleespèceminérale, seprésenteennoyauxbruns rougeâtres,n=1.844 (20), jusqu’à70mmdediamètre,entouréssuccessivementparl’almandinidiomorphecontenantjusqu’à2.7%Y2O3(poids)etparlefeldspathpotassiquedansunegranulitefelsiquesurl’îleBonnet,faisantpartiedudomaineinternedeParrySound,provinceorogéniqueduGrenville,auCanada.LenomrappelleGeorgMenzer(1897–1989),cristallographeallemandquiaétélepremieràrésoudrelastructurecristallinedugrenat.NosrésultatsobtenuspardiffractionXsurmonocristalindiquentlegroupespatialIa3d,a=11.9947(6)Å.UneanalysedugrainleplusricheenY(16.93%Y2O3)avecunemicrosondeélectroniquemèneàlaformulesuivante,normaliséeselonhuitcationset12atomesd’oxygène:{Y0.83Gd0.01Dy0.05Ho0.02Er0.07Tm0.01Yb0.06Lu0.02Ca1.37Fe2+0.49Mn0.07}[Mg0.55Fe2+0.42Fe3+0.58Al0.35V0.01Sc0.01Ti0.08](Si2.82Al0.18)O12 ou, de façon idéale, {(Y,REE)(Ca,Fe2+)2}[(Mg,Fe2+)(Fe3+,Al)](Si3)O12. Lesdonnéesmicro-XANESobtenues avec un synchrotron ont donnéFe3+/SFe= 0.56(10)versus 0.39(2), valeur calculée parstoechiométrie.LepouvoirdedispersionaffinéausiteoctaédriqueY,17.68epfu, indiquequ’unélémentrelativementlégerlogeàcesite.Lemagnésium,telquedéterminéparanalyseavecunemicrosondeélectronique,seraituncandidatapproprié.Deplus,comptetenudelacomplexitédesoccupantsdecesite,lalongueurdelaliaisonY–Omoyenne,2.0244(16)Å,concordeavecuntauxd’occupationpartielparleMg.Laprédominancedescationsbivalents,avecMg>Fe2+,etl’absenceduSiausiteoctaédriqueY(entrecrochetscarrés),seraientlesprincipauxcritèrespourdistinguerlamenzerite-(Y)desautresespècesdegrenatsilicaté;lepôlemenzerite-(Y)possèdeunecomposition{Y2Ca}[Mg2](Si3)O12.Lescontactsentrelamenzerite-(Y)etl’almandinsontfrancs,engénéral,etcuspidésparendroits.Cegrenatauraitétééquilibréavecferrosilite,augite,quartz,oligoclase,allanite-(Ce),magnétite,ilméniteetfluorapatite,enl’absencedel’almandin,aucoursdutracéprogradeà7–8.5kbaretàenviron700–800°C,etparlasuiteauraitétédissoutdemanièreincongruentedansunbainfonduanatectiquepourformerl’almandin,trèsprobablementàunepressionvoisinede8.5–9.5 kbaretàenviron800–850°C.

(TraduitparlaRédaction)

Mots-clés:grenat,menzerite,yttrium,terresrares,nouvelleespèceminérale,structurecristalline,ProvincedeGrenville,facièsgranulite,ParrySound,Ontario.

Here,wedescribeagarnet-groupmineralcontainingupto17wt%Y2O3asthenewspeciesmenzerite-(Y)and report its crystal structure,which provides thecritical information on site occupancies needed todistinguishmenzerite-(Y) fromother garnet species.ThemineralandnamewereapprovedbytheCommis-siononNewMinerals,Nomenclature andClassifica-tion of the InternationalMineralogicalAssociation(IMA2009–050).ItisnamedafterthecrystallographerGeorgMenzer(1897–1989),ProfessorattheLudwig-Maximilians-UniversitätMünchen,whowas thefirsttosolvethestructureofgarnet,andinfactthefirsttosolvethestructureofasilicatemineral(Menzer1925,1926, 1928, Laves 1962). The Levinsonmodifier,-(Y),atteststothedominanceofYovertherare-earthelementspresent.

Theholotypematerial(sampleno.GB50,partAandthinsectionGB50A1collectedin2008)wasdepositedin theCanadianMuseumofNature, P.O.Box 3443,StationD,Ottawa,Ontario,CanadaK1P6P4,ascata-loguenumberCMNMC86088;asecondsample(GB94,partI,billet4,collectedin2009)fromthetypelocalityisdepositedintheNationalMuseumofNaturalHistory,

Introduction

Yttriumandtheheavyrare-earthelements(HREE)werefirstrevealedtobeimportantconstituentsofgarnetbyJaffe(1951)andYoder&Keith(1951).Currently,YandtheHREEreceiveconsiderableattentionnotonlyassensitiveindicatorsofgarnetgrowthandresorptionhistory (e.g., Lanzirotti 1995, Pyle& Spear 1999),but alsobecause suchhistoriesmaybe related to thegrowth ofminerals such as zircon, thus providing ameansofdating the stagesofmetamorphicevolution(e.g., Rubatto&Hermann 2007,Harley&Kelley2007).Theyttriumcontentsofcommonmetamorphicand plutonic garnets typically range from severaltensofppmtoseveral thousandppm(e.g.,Lanzirotti1995,Pyle&Spear1999),andcommonlyexceedtheyttriumcontents of associated rock-formingmineralsby an order ofmagnitude ormore (Bea 1996). In afewpegmatiticoccurrencesofgarnet,Ycontentsreach24800–26500 ppm (3.15–3.36wt%Y2O3, Enami et al. 1995,Kasowski&Hogarth 1968), correspondingto0.14–0.15Yperformulaunit(onthebasisofeightcationsand12oxygenatoms).

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menzerite-(y),anewgarnetend-member 729

SmithsonianInstitution,Washington,D.C.,ascataloguenumberNMNH174896.SectionsfromsamplesGB50andGB94wereusedinthepresentstudy.

Occurrence

Menzerite-(Y) occurs sparingly as an accessorymineral included in euhedral almandine (Fig. 1) infelsic granulite on the north shore ofBonnet IslandinGeorgianBay, offshore from the town of ParrySound,Ontario,Canada(UTMcoordinates0567531W5003719N;NAD83datum, zone17).Bonnet Islandis an exposureofwell-layeredgranulite-facies supra-crustal rocks in the interior Parry SoundDomain,Grenville Province, and is part of an allochthonousterranethatisnearthehigheststructurallevelofastackof thrust nappes assembled at around1120–1090Ma(Davidsonet al.1982,Culshawet al.1997).

AnalyticalMethods

Optical properties

The optical properties of menzerite-(Y) weremeasured grain #1 in sectionGB50A1 at theRuhr-UniversitätBochum.Reflectancemeasurementsweremadeinairandoil(DIN58884,indexofrefractionofoil:1.518at23°C)withlightofwavelength580nm,andtheindexofrefractionofmenzerite-(Y)wascalculatedusingBeer’sLaw.ThestandardusedwasSiC.

X-ray diffraction

Asingle-crystalX-ray-diffractionstudywascarriedoutattheUniversityofBernusinganAPEXIISMARTdiffractometer.Weusedcommercialsoftwareforinten-sityretrievalfrom3DCCD-framedataassuppliedbytheproduceroftheinstrument.Thissoftwaredoesnotallowpeakdeconvolutionorothersophisticatedprofiletreatmentprocedures.

A composite grain (#2 in sectionGB50A1, Fig.1)was extracted from the thin section using a drillmounted on amicroscope (Medenbach 1986).Thesamplewasfoundtoconsistofmenzerite-(Y)flankedon two sides by almandine,which by visual inspec-tion constituted about 50% of the composite grain(Fig. 2, inset).We tried themicro-milling techniqueasawaytoremovetheenclosingalmandine,becausetheimpactofthismechanicaltreatmentonthecrystalstructurewouldbelessseverethanothertechniquesofseparationlikelaserablationorelectronbombardment.Unfortunately, applying themicro-milling techniqueturnedouttobemoredifficultthananticipatedbecausethe three-dimensional intergrowthwasmore complexthaninitiallybelievedfromtwo-dimensionalpictures.

Table1summarizestheconditionsunderwhichthesingle-crystaldatawerecollected.Aplotofreciprocal

spaceshows that the twosuperimposedcubic latticesareperfectlyaligned;thereflectionsforalmandineandmenzerite-(Y) are clearly distinguished as a result ofthedifferenceintheirunit-celldimensions(Fig.2).Foreachpairofcorrespondingreflections,thoseatslightlyhigherd-valuesweremanuallyassignedtomenzerite-(Y), and thosewith smallerd-values, to almandine.Eachdiffractionspotisshownsimplyasacirclewithitscenteratthecenterofgravity(intensity)ofanobservedreflection;thecirclesarenotintendedtoshowthetrueshapeofadiffractionspot,butonly its idealpositioninaprojectionofreciprocalspace.Thus,Figure2doesnot indicatewhether the shoulders of correspondingreflections overlap orwhether individual reflectionsfromthetwofragmentsarecompletelyresolved.ThisfigureshowsthatbelowaBraggangleu(MoKaradia-tion)of~17%(correspondingtothe1000reflection),even and odd reflection indices fulfilling I-centeringareobserved.Athigheruvalues,observedreflectionswithoddindicesandh+k+l =2nbecomescarce.Inthegarnetstructure,allcationsattheX,Y,andZsitesareonspecialpositionsassociatedwith the reflectionconditions:hkl:h,k=2n,h+k+l=4n.Onlyoxygenoccupiesageneralposition,permittingintensitieswithodd indices.These extinction rules require us to relyon the low-u angle reflections, even though they aremuchmore affected bymutual overlap than those athighuangles.Theorientationmatricesweredeterminedfromthereflectionsassignedeithertomenzerite-(Y)oralmandine basedonpreliminary unit-cell dimensionsa equal to 12.00Å and11.66Å, respectively.Theseparameterswereused for integrationof the reflectionintensities. In order to reduce the effect ofmutualoverlapofthereflections,weusedafixednarrowbox-sizewithx=y=0.5°andz=1°forintegration.Theboxsizewaschosenbytrialanderror.Anarrowbox-widthhasthedisadvantagethatstrongreflectionssufferfromsubstantialtruncationoftheflanks.Ifthebox-sizerefinementoptionintheintensity-integrationsoftwarewereenabledoriftheselecteddimensionsoftheboxwere too large, the integrated intensitywould includecontributionsfrombothreflections,andthefinalcell-dimensionrefinementwouldconvergetobeanaverageofmenzerite-(Y)andalmandine.Insummary,becauseof partial overlap of reflections frommenzerite-(Y)and almandine at lowu angles and truncation of thestrongestreflections,thediffractiondataareofmediocrequalityandtheparametersRintandRsareverydifferent(Table1).Thisprecludesanyquantitativeanalysisofthedisplacementparameters.

For the refinement ofmenzerite-(Y),weused thescatteringfactorsofYandCafor theXsite, thoseofAlandFefortheYsite,andtheZsitewasconstrainedtobe fullyoccupiedbySi inorder to reducecorrela-tionsbetweenthescalefactorandsiteoccupancies.Forthe refinement of almandine,we used the scatteringfactorsofFeandCafortheXsite,thoseofAlandFe

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Fig. 1. Photomicrograph of grains ofmenzerite-(Y) used for single-crystal (#2) andopticalstudy(#1).Plane-polarizedlight.

Fig. 2. Projection alonga*of reciprocal space showing separation ofmenzerite-(Y)reflections[unfilledcircles,a=11.9947(6)Å]fromalmandinereflections[grayfilledcircles,a = 11.6598(8)Å]. Inset showsgrain #2, sectionGB50A1 fromwhich thepatternwasobtained;spacingbetweenthebarsis0.01mm.

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fortheYsite,andtheZsitewasconstrainedtobefullyoccupiedbySi.Assignmentofthese“dummyatoms”allowed determination of the average site-scatteringvalues,which can be compared to the results of theelectron-microprobeanalysis(Table2).Thefinalrefine-mentswere performedwith anisotropic displacementparametersforallsites.Atableshowingtheseparam-etersisavailablefromtheDepositoryofUnpublishedData on theMACwebsite [documentMenzerite-(Y)CM48_???].Comparisonofthescalefactorsconfirmedthat the rimof almandine and the core ofmenzerite-(Y)contributedinnearlyequalpartstothecompositecrystalintermsofweightpercent.

Becauseofthepossibleoverlapofthemenzeriteandalmandinedata,wecomparethesite-scatteringvaluesfromthesingle-crystal refinement(SREF)andresultsof theelectron-microprobeanalyses (EMPA) inordertodeterminewhetherthesiteoccupanciesobtainedformenzerite-(Y)were affected by almandine and viceversa.ComparisonofthedatainTable2showsthatthesite-scatteringvaluesfortheXsitefromtheSREFandEMPAareingoodagreementforbothmenzerite-(Y)andalmandine,asarethesite-scatteringvaluesfortheYsiteinalmandine,i.e.,thedifferencesare2–4%ofthe

measuredvalues,similartotheesdvaluesoftheSREFsite-scatteringvalues.TheSREFandEMPAsite-scat-teringdiffersby9.5%fortheYsiteinmenzerite-(Y),adiscrepancyattributedtochemicalzoninginmenzerite-(Y)(seebelow)andtothefactthattheSREFisbasedontheentirevolumeoftheextractedcrystal,whereasEMPAisbasedonasmallvolumeatthecenterofthecrystal. In summary,weareconfident thatoverlapofreflectionsissufficientlylowtoavoidendingupwithan average occupancy betweenmenzerite-(Y) andalmandine.Furtherjustificationforourconfidenceisthesubstantialdifferenceof5.2electronsperformulaunitinsitescatteringbetweenmenzerite-(Y)andalmandineattheXsite(Table2).

To validate the reliability in our oxygen coordi-nates,wehave performed a simple test by assumingthatalmandinesurroundingmenzeritehasasimplifiedcomposition of 26.0% grossular, 62.3% almandine,and 11.7%pyrope.These approximate numbers areobtained if the lowY content is considered as gros-sular-like, and theMn2+ content, as almandine-like.Oxygencoordinatesforthissolid-solutionmemberarecalculatedaslinearcombinationsofthecorrespondingoxygencoordinatesoftheendmembers(RTdata)for

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grossular(Geiger&Armbruster1997),almandineandpyrope(Armbrusteret al.1992).ThisapproachyieldedapredictedOcoordinateofx=0.0350,y=0.0482,z=0.6525,whichagreeswithin3.5esdorlesswiththerefinedcoordinates(Table3).Consideringthesimplicityofthemodel,theagreementisexcellent.

X-ray absorption spectroscopy

Giventheambiguitiesindeterminingsiteoccupan-cies and the valences ofV,Cr,Mn, and Fe, and inmeasuring theH content in a complex garnet solid-solution,itwouldhavebeendesirabletosupplementthecrystal-structure refinements and electron-microprobeanalyseswith infrared, visible-light, andMössbauerspectroscopicmeasurements(Geiger2004).However,themenzerite-(Y)grainsaresparse(nomorethan10–12grainsperthinsection),veryfine(mostly<50mm)and,with rare exceptions, enclosed in almandine, leavingX-ray absorption spectroscopy near the absorptionedgeonamicroscopicscale (micro-XANES,Dyaret al.2002,Mottana2004)as theonlypracticalmethod

forthematerialathand.Opticalspectroscopyfromthenear-ultravioletthroughvisibleintotheinfraredofthelargestmenzerite-(Y)grainsis intheoryalsopossiblebecausespotsizesassmallas“ontheorderof20mm”have been achieved (Wildner et al. 2004), but thiswouldrequireverychallengingsamplepreparationandspecializedinstrumentationthatisbeyondthescopeofthepresentstudy.

Thus,XANES spectrawere acquired at beamlineX26A at theNational Synchrotron Light Source atBrookhavenNationalLaboratoryusingan8 3 10mmbeamsize.IncidentX-raysweretunedusingaSi(311)monochromator.TheXANES spectrawere collectedinfluorescencemodeusinganine-elementhigh-purityGesolid-statedetectorarray.Datawereacquiredfromthecentersandedgesoftwograins.Spectrawereedge-step-normalizedandcorrectedforself-absorptionusingtheFLUOalgorithm.Energieswerecalibratedusingamagnetitestandard,wherethepositionofthepre-edgepeakisknownprecisely.Afunctionwasfitintheregionof the pre-edge to approximate the upwardly slopingmain edge, and then subtracted.Lorentzian functions

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werefit to the individualpeaks in thepre-edgeusingthePANpeak-fittingsoftwarewrittenbyR.M.Dimeoat theNISTCenter forNeutron research.The posi-tions (energies) and calculated peak-areaswere usedto create aweighted peak-center for each spectrum.Several other garnet standardswith knownFe3+/SFewerealsomeasured,andacalibrationcurverelatingtheweightedpeak-centerenergiestoFe3+/SFewascreated.Thisexpressionwas thenused todetermine theFe3+/SFeinmenzerite-(Y)andthesurroundingalmandine.Theinterpretationofspectraobtainedonsinglecrystalsofgarnetisnotaffectedbyorientationbecausegarnetis isometric,andthereisnocontributiontotheerrorsfrompolarization effects (Dyaret al. 2002). Furtherinformation on this calibrationwill be reported inDyaret al.(inprep.).Thedatareportedinthepresentpaper are preliminary pending further refinement ofthatcalibration.

Electron-microprobe analysis

Menzerite-(Y)andalmandineinsamplesGB50andGB94wereanalyzedwithaCamecaSX–100electronmicroprobeusingwavelength-dispersivespectroscopy(WDS)attheUniversityofMaine.Aspecialprotocolwasdevelopedformenzerite-(Y),butthefinalmethodwasdevelopedonlyaftergrain#2insectionGB50A1had been extracted for structure refinement.Columnconditions in thefinalmethod for elementsMg toYwere15kV,10nA,and5mmbeamdiameter,andforthe rare-earth elements (REE), 25 kV, 20 nA, ca. 5mm(focused)beamdiameter.Thedatawereprocessedusing the X-Phi correction ofMerlet (1994). For

menzerite-(Y),weused the following standards (andlines): pyrope (MgKa), synthetic {Y2}[Al2](Al3)O12(AlKa), zircon (SiKa), diopside (CaKa), Scmetal(ScKa),rutile(TiKa),Vmetal(VKa),Cr2O3(CrKa),rhodonite(MnKa),almandine(FeKa);synthetic{Y2}[Al2](Al3)O12(YLa),andsyntheticREEP5O14(LaforLa,Ce,Nd,Eu,Gd,Tb,Er,Tm, andYb;Lb forPr,Sm,Dy,HoandLu).NootherelementwithZ35wasdetectedinWDSscans.Grain#2ofmenzerite-(Y)insectionGB50A1,whichwasusedforrefinementofthecrystalstructure,wasalsoanalyzedundertwodifferentsetsofconditions;forelementsMgtoYplusLu,condi-tionswere 15kV, 10nA; for the rare-earth elementsexceptLu,theywere25kV,20nA,bothwitha5mmbeamdiameterandthefollowingstandards(andlines):pyrope(MgKa),almandine(AlKa,SiKa,FeKa),diop-side(CaKa),Scmetal(ScKa),rutile(TiKa),Vmetal(VKa), rhodonite (MnKa), synthetic {Y2}[Al2](Al3)O12 (YLa), and syntheticREEP5O14 (La forLa,Ce,Nd,Gd,Tb,Er,TmandYb;LbforPr,Sm,Eu,Dy,andHo;MaforLu).Analyticaltotalsrangefrom100.57to103.50wt%withtherefinedprotocol,whereasthetotalforgrain#2was97.40wt%.

AlmandineinsectionGB50A1wasanalyzedforallelements at a single set of conditions, 25kV,10nA,and ca. 5mm (focused) beamdiameter.We used asstandards (and lines) diopside (MgKa,CaKa), albite(AlKa),wollastonite (SiKa), rutile (TiKa),Vmetal(VKa),Cr2O3 (CrKa), rhodonite (MnKa), almandine(FeKa); synthetic {Y2}[Al2](Al3)O12 (YLa), andsyntheticREEP5O14 (La for Er andYb;Lb forGdandDy).

Almandine in sectionsGB50C andGB94I4wasanalyzedforallelementsatasinglesetofconditions,15kV,10nA,and5mmbeamdiameter.Weusedstandards(and lines) jadeite (NaKa), diopside (MgKa,CaKa),almandine(AlKa,SiKa,FeKa),sanidine(KKa),rutile(TiKa), rhodonite (MnKa), and synthetic {Y2}[Al2](Al3)O12(YLa).

Menzerite-(Y) in grain 5, sectionGB50A2wasmappedforMg,Al,Ca,Ti,Y,ErandYbwithafocusedbeamat15kVand200nAusingthefollowingcrystals:TAP(MgKa,AlKa),PET(CaKa,TiKa,YLa),LIF(YbLa,ErLa).Titanium,Er andYbweremappedwithtwo spectrometers simultaneously, and the resultingimageswereaddedtooneanother.ProfilesofMg,Ti,Mn,Y,andYbconcentrationsintraversesacrosselevengrainsofmenzerite-(Y)andtheenclosingalmandineinsectionsGB50A1andGB50A2wereobtainedwithafocusedbeamat15kVand100nAusingthefollowingcrystals:TAP(MgKa),PET(TiKa,YLa),LIF(MnKa,YbLa).

MicrostructuresofMenzerite-(Y)andAlmandine

Menzerite-(Y) occurs as an accessory phase in afoliatedfelsicgranulitethathasanindistinctcomposi-

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tionallayeringonamillimeterscale.Themostabundantmineralsassociatedwithmenzerite-(Y)areoligoclase(An25–28), quartz, and ferrosilite (Fs56–61En38–43Wo1–3plus ~1%MnSiO3).K-feldspar, augite (Fs27–29En29–30Wo41–43), calcic amphibole, biotite, almandine,ilmenite andmagnetite are subordinate (Grew et al.2010).Accessoryphasesincludefluorapatite,allanite-(Ce),andzircon,whicharewidespread,whereasmona-zite-(Ce),xenotime-(Y),pyrite,chalcopyrite,sphalerite,hercynite,andMg–Fecarbonateoccurverysparsely.

Althoughmostly isolated from other phases byalmandine,menzerite-(Y) is locally in contactwithK-feldspar (Figs.3,4a)andwith severalof theothermineralsenclosedinalmandine:allanite-(Ce)(Fig.3),ilmenite andfluorapatite (Fig. 5a).No contactswitholigoclase,quartz,ferrosiliteoranyothermineralwerefound.Grains ofmenzerite-(Y) range fromequant toelongate, up to 70mm in the longest dimension. Itscontacts are sharp and commonly cuspate or showembayments(Figs.4a,4c,5a).Inafewcases,severalgrainsofmenzerite-(Y)areenclosedinasinglegrainofalmandine,e.g.,theatollinFigure5a.

Almandinesurroundingmenzerite-(Y)iseuhedral,in placeswith smooth faces and sharp corners (Figs.3,4a,4e,4f),andsurroundedbyamoatofK-feldspar,locallywith quartz (Fig. 5a). In some cases, alman-dine overgrowing inclusion-poor almandine aroundmenzerite-(Y) contains quartz vermicules (Figs. 4b,4d).This almandine–quartz symplectite,which canalsoincludeaugitevermicules,ismorecommonaroundferrosilite,magnetite and ilmenite.Almandine alsoformsasymplectitewithbiotiteindecussateaggregatesandquartzinsampleGB94I(Grewet al.2010).

PhysicalProperties,IndexofRefractionandCompatibilityIndexofMenzerite-(Y)

Most of the physical properties ofmenzerite-(Y)cannotbedeterminedbecauseof the smallgrain-sizeand scarcity.Cleavagewas not evident.Tenacity isbrittle, as shown by a broken grain (Fig. 2 inset).Density,calculatedwiththeempiricalformula,is4.31g/cm3 (grain #5 for composition; grain #2 for cellparameters,sectionGB50A1).Menzerite-(Y)istrans-parentandisotropic.Reflectanceis8.83%(air),0.97%(oil),whichgivesn=1.844(20)at580nmforgrain#1,sectionGB50A1.Thecolorisdeepreddishbrowninthinsectionsofstandardthickness(~0.03mm,Fig.1).

TheGladstone–Dale relation (Mandarino 1981)givesacompatibilityindex1–(KP/KC)=0.003,excel-lent;itwascalculatedusingthecellvolume,indexofrefractionandchemicalcompositionfromgrains#2,#1and#5(sectionGB50A1),respectively.

X-RayDiffractionStudyandCrystalStructureofMenzerite-(Y)

Byandlarge,thesiteassignmentsgiveninTables2and3formenzerite-(Y)areconsistentwithsiteoccu-pancies in common rock-forminggarnets:Ca,Mn,YandREEat theX site,Ti and all trivalent cations attheY site (exceptminoramountsofAlneeded tofilltheZ site),andSiat theZ site.Wheremenzerite-(Y)differsfrommorecommongarnetsisintheassignmentofthedivalentcationsMgandFe2+.TherefinementisconsistentwithMgbeingassignedtoYinsteadofXandwithFe2+beingapportionedbetweentheXandYsitestobringtheiroccupanciesuptothreeandtwocationsper formula unit, respectively.Thus, the discussionwill focuson justifying the latter twounconventionalsite-assignments.

ThegoodagreementbetweenEPMAandSREFinsitescatteringandthehighnumberofelectrons(ca.29)attheXsiteargueinfavorofassigningFe2+ratherthanMgtoXtobringitstotaloccupancytothree.IfmoreFe2+andlessMgwereassignedtoY,thediscrepancyisworsenedbetween theEMPAandSREF site scat-tering for theY site.Moreover, our assignments areconsistentwith the radii ofMgandFe2+: the smallerMgionshouldbepartitionedintothesmallerofthetwosites,i.e.,atY.Becausetherearesofewgarnetspecieswithsubstantialdivalent-cationoccupancyatY,dataonthedistributionofMgandFe2+betweenX andY aresparse and contradictory.On the basis ofMössbauerspectroscopy,Kühbergeret al.(1989)reportedamodestfractionationofFe2+intotheYsiterelativetotheXsiteinschorlomite,buttheXsiteislargelyoccupiedbyCa(2.89–2.90Caperformulaunit).Incontrast,Geigeret al. (1991a, b) citedMössbauer data as evidence thatFe2+isstronglyfractionatedintotheXsiterelativetotheYsiteinsyntheticCa-freemajorite,aninterpretationsupportiveofourmodelformenzerite-(Y).

InordertofurtherverifythedistributionofcationsbetweenX andY,we have performedX–OandY–OdistancepredictionsbasedonthecompositionobtainedbyEMPA(Table2).TheXsiteisoccupiedby50%Ca,21%Fe2+ (includingminorMn2+), and 29%Y (withminorREE). Reference distanceswere taken fromsyntheticgarnetend-members(Ca–O:andradite,Fe–O:almandine,Y–O:YAG).ThecalculatedX–Odistances

Fig. 3. Maps of six elements inmenzerite-(Y), grain #5,sectionGB50A2.Back-scattered electron image showsareamapped (white square).The following linesweremeasured:AlKa,CaKa,TiKa,YLa, ErLa, andYbLa.ColorscalestotherightoftheimagesshowhowtheimagewasadjustedtomaximizecontrastintheImageJprogram.Abbreviations:Alm: almandine,Aln: allanite-(Ce),Kfs:K-feldspar,M:menzerite-(Y),Pl:oligoclase.

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Fig. 4. Back-scattered electron images ofmenzerite-(Y).A.SampleGB50A2, grain #10.B.SampleGB94I4, grain 1.C.SampleGB50C,grain#7/1.D.SampleGB50A2,grain#1.E.SampleGB94I4B,grain#1.F.SampleGB50A2,grain#8.Abbreviations:Alm:almandine,Bt:biotite,Hbl:calcicamphibole, Ilm: ilmenite,Kfs:K-feldspar,M:menzerite-(Y),Pl:oligoclase,Qz:quartz,Z:zircon.

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formenzerite-(Y)are2.316Åand2.454Å,respectively,whichiswithin0.01Åwiththerefineddistances(Table4).CorrespondingcalculationsweredonefortheY–Odistanceinmenzerite-(Y)with16%Al,28%Mg,34%Fe3+,and22%Fe2+(includingminortransitionmetals)attheYsite.Referencedistance-dataforMgandFe2+weretakenfromend-memberforsteriteandfayalite,Alfrompyropeandalmandine,andFe3+fromandradite.The calculated distance is 2.056Å,which is 0.03Ålonger than the refinedvalue (Table4).Thisdiscrep-ancy is consistentwith the observation discussed inthesectiononMethodsthatsitescatteringatYderivedfromEMPAishigherthanthevalueobtainedbysite-occupancyrefinement.Thismayindicatethatstronglyscattering ionswith large radii were in octahedralcoordinationareoverestimatedintheEMPAowingtochemicalzoningortothelimitationsofelectron-beamsizeandpenetration.

Owing to the presence of octahedralMg andFe2+,menzerite-(Y)displayssignificantly longerY–Odistances, 2.0244(16)Å, compared to the associatedalmandine,1.9132(17)Å(Table4),whichhasAldomi-nantattheoctahedralsite.

Wehaveno indicationofTi4+orFe in tetrahedralcoordinationbasedonthestructuraldataofbothalman-dineandmenzerite-(Y).Thepopulationofsiliconwasnotrefinedinordertoreducecorrelationproblemswiththe scale factor.However, the refinement of the siteoccupancyofMgandFeattheYsiteinmenzerite-(Y)indicates esd values ofca. 1–2% (Table 2), and thussimilaresdvalueswouldbeexpectedforaSiversusTiorFe refinement.The similar and physically reason-ableUeq values for the tetrahedral sites (Table 3) donotpointtoanysignificanttransition-metalconcentra-tion. Furthermore, theZ–Odistances of 1.640Å for

almandine and 1.649Å formenzerite-(Y) (Table 4)arecharacteristic foronlySiatZ as inothergarnets,e.g.,1.628–1.645ÅforvariousCa–Mg–Fe–Mngarnets(Novak&Gibbs1971,Armbrusteret al.1992),1.648–1.649Åandsyntheticandradite(Armbruster&Geiger1993)versus 1.658–1.678Å for andradite containingsignificantTiorFeattheZsite(Armbrusteret al.1998).

TheSi–Odistances and scatteringpower atZ areinconsistentwith the presenceof significantH2O;cf.1.762Å, an average for hydrous andraditewith 1.65Siand1.35H4O4performulaunit(Armbruster1995).

Owing to the smallgrain-size, itwasnotpossibletoobtainapowderX-ray-diffractionpattern,soitwascalculated forCuKa1 radiation andDebye–Scherrergeometryfromthesingle-crystaldatausingtheLazyPulverixsoftware(Yvonet al.1977)(Table5).

Micro-X-RayAbsorptionNear-EdgeSpectroscopyandFe3+Content

ofMenzerite-(Y)andAlmandine

TheX-ray absorption spectra at the FeK-edgewereacquiredat twopoints at the centerofgrain#5onmenzerite-(Y),andweighted peak-centers werecalculated from thefits (Fig. 6).Our current calibra-tionsuggeststhatthesecorrespondtoFe3+/SFevaluesof0.553and0.571,witherrorsestimatedatÅ0.05–0.1absoluteorbetter (work isstill inprogress tospecifi-callydetermine theerrors).Almandineat theedgeofgrain#5wasalsoanalyzedtwice,andyieldedFe3+/SFe=0.166and0.190,whereasalmandineingrain#1gaveFe3+/SFe=0.082and0.077.AllanalysesweremadeusingthinsectionGB50A1.

Crystal-chemicalformulaecalculatedfromstoichi-ometry (seebelow)giveFe3+/SFe in the range0.37–

Fig.5. Profileforselectedconstituentsinmenzerite-(Y)andenclosingalmandine.A.Back-scatteredelectronimageofsampleGB50A1,grain#3,showingprofileline(red).B.Profilesforfiveconstituentsalongthe50mmtraceinA.C.ProfilesforYandYbfromB,butwithordinate(counts)expanded.Abbreviations:Alm:almandine,Ap:fluorapatite,Kfs:K-feldspar,M:menzerite-(Y),Opx:ferrosilite,Pl:oligoclase,Qz:quartz.

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0.50forthe12grainsofmenzerite-(Y).Thecalculatedvalue forgrain5 (section1,Table6) is0.39(2),witha1serrorcalculatedbypropagationusingthespread-sheetmodifiedfromLocock(2008).ThisstoichiometricFe3+/SFeratio isdemonstrably lower than themicro-XANESFe3+/SFe=0.56(10)forthisgrain.Similarly,theaveragestoichiometricFe3+/SFe�0.05(Table7)fortheeuhedralalmandineislowerthanthemicro-XANESFe3+/SFeintherange0.08–0.18(10)obtainedongrains1and5,butthediscrepancyisless.

In summary, themicro-XANESFe3+/SFe valuesprovide only partial confirmation of the Fe3+/SFevalues calculated by stoichiometry, and the discrep-ancy in the case ofmenzerite-(Y) raises thequestionastowhetherweshouldusethemicro-XANESresultsto calculateFe3+/SFe ratios for all themenzerite-(Y)grains.OnedifficultyisthatinterpretationofXANESdata isnotentirelystraightforward in thepresenceofmultiplecoordinationenvironments,asinthecaseformenzerite-(Y).Wilkeet al.(2001),Berryet al.(2003)andHöferet al.(2006)havecitedthecoordinationofFeasacomplicatingfactorindeterminingFe3+/SFeusingXANES data.Moreover,we have amicro-XANESdeterminationononlyonegrainofmenzerite-(Y)outofthe12analyzed.Giventhecomplexitiesininterpretingthemicro-XANESFe3+/SFevalues,wedonotbelievethat it is appropriate to apply this one determinationtotheother11grainsofmenzeritebecauseofcompo-sitional differences among them.Consequently,wehave relied on the stoichiometric values ofFe3+/SFeso that the resultson the12grainswouldbedirectlycomparablewithoneanother.Wehavealso reliedonthe stoichiometric values for almandine tomaintaininternalconsistency.

ChemicalCompositionofMenzerite-(Y)andAlmandine

Thesalientcompositionalfeatureofmenzerite-(Y)isitshighcontentofYandHREE,e.g.,13.03–16.93wt%Y2O3and2.19–3.17wt%Yb2O3(Table6),thehighestby far reported in a natural garnet.We have basedourcalculationoftheformulaonthecrystal-structure

refinementasstatedabove,assumingfulloccupancy,12oxygenatomsandeightcations[methodbasedonstoi-chiometryofDroop(1987),Giaramita&Day(1990)].

WehavealsoassumedthatCrandVaretrivalentandMnisdivalentbasedonthefollowingevidence,whichis sufficient forourpurposes,because theseelementsarepresentinsuchlowconcentrationsinmenzerite-(Y):

Cr:Geigeret al. (2000) reportedonly trivalentCrinpyropesynthesizedat1050°C,25kbar; indeed,Crisalmostexclusivelytrivalentinmetamorphicsilicateminerals.

V:Geiger et al. (2000) reported tetravalentV inpyropesynthesizedat950–1050°C,25kbar,inonecasewithVmetal toconstrainoxygen fugacity,but foundthat theconcentrationofV3+ is“significantlyhigher”thanthatofV4+.

Mn:At oxygen fugacities corresponding to 1–2log10unitsmoreoxidizingthantheQFMbuffer,Mnisexpectedtobepredominantlydivalentinmenzerite-(Y),as it is in silicates and oxides synthesized at 600°C,4 kbar at these oxygen fugacities (Abs-Wurmbach&Peters1999).

Although the precision of the averaged analyticalresults [standard deviations are given inTable 6 forthegrainusedtodefinemenzerite-(Y)]justifiesgivingformulaunitsonlytothenearest0.01cations,wehavetabulatedcationproportionstothethirddecimalplacesoastominimizeaccumulationofrounding-offerrors.

Incalculatingaformulaformenzerite-(Y)andiden-tifyingend-members,wefollowHatert&Burke(2008)infirstconsideringtherelativeproportionsofthecationvalences at each of the three sites, then the relativeproportionofcationsforeachvalenceatthatsite.TheZsiteisdominatedbythetetravalentcation,Si.TheYsiteisdominatedbydivalentcationsingrains#5and#6

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(sectionGB50A1)andbytrivalentcationsintheother10 grains (Fig. 7a).Of the twodivalent cations,MgexceedsFe2+ inall theanalyzedgrains,whereasFe3+isthedominanttrivalentcationinthesegrains.TheXsiteisdominatedbydivalentcationswithCa>(Fe2++Mn2+)inalltheanalyzedgrains(Fig.7b).

These observations suggest thatmenzerite-(Y)consists largely of two end-members, {Y2Ca}[Mg2](Si3)O12,whichhasnotbeenreportedpreviously,andandradite, {Ca3}[Fe3+2](Si3)O12,where {}, [] and ()encloseoccupantsattheX,YandZsites,respectively,as recommended byGeller (1967). Because chargebalanceallowsonly2YatX forSi=3atZ,wehave

drawntheboundaryinFigure7bbetween{Y2Ca}[Mg2](Si3)O12 and{Ca3}[Fe3+2](Si3)O12 at0.333 insteadof0.5.Thus,grains#5and#6plotinthefieldwherethe{Y2Ca}[Mg2](Si3)O12end-memberisdominant.More-over,Mg/(Mg+Fe2+)coverstherange0.54–0.66attheYsite,implyinganimportantrolefortheend-member{Y2Ca}[Fe2+2](Si3)O12,whereas the presence ofAlat theY site andFe2+ at theX site implies a role foralmandineinadditiontoandradite.Asthecompositionsofthegrainsplotclosetotheboundariesbetweentheendmembers{Y2Ca}[Mg2](Si3)O12and{Y2Ca}[Fe2+2](Si3)O12ontheonehand,and{(Ca,Fe)3}[(Fe3+,Al)2](Si3)O12ontheother(Fig.7),thegeneralformulafor

Fig. 6. Micro-XANES spectra ofmenzerite in the center of grain 5 (top) contrastedwiththeedgeofthegrain,whichisalmandine.Bothspectrahaveprominentpeaksatca.7111.6eVand7113.5eV,but theareasaredramaticallydifferent,givingrise toweightedpositionsof7113.1eVformenzeriteand7112.6eVforalmandine.

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these grains contains both theY-bearing andY-freeend-members, that is,{(Y,REE)(Ca,Fe2+)2}[(Mg,Fe2+)(Fe3+,Al)](Si3)O12.Zoninginmenzerite-(Y)grainscanbeinterpretedasvariationsintherelativeproportionsof the two end-members {Y2Ca}[(Mg,Fe2+)2](Si3)O12consideredtogetherand{(Ca,Fe)3}[(Fe3+,Al)2](Si3)O12inwhichtheproportionoftheYbanalogueof{Y2Ca}[(Mg,Fe2+)2](Si3)O12remainsrelativelyconstant(Figs.3,5b).

Analternativeapproach tocalculateend-memberswasadvocatedbyRickwood(1968),becauseincontrasttomost silicateminerals, garnet end-memberpropor-tionsconstituteanunder-determined system from the

pointofviewoflinearalgebra.Aspresentlyconstituted(e.g.,Back&Mandarino2008), there aremore end-members in the garnet group than oxides, resultingin ambiguity.Rickwood (1968) emphasized that thesequenceofcalculationinfluencestheresults,andthere-fore recommended thatall investigatorsuse thesamesequence so that their resultswould be comparable.Locock(2008)expandedRickwood’sworktoincludealmostallcomponentspresentinnaturalgarnet.OfthepossibleY-bearingcomponents,only{Y3}[Al2](Al3)O12wasincluded.Withthediscoveryofmenzerite-(Y),thespreadsheethadtobemodifiedtoincludethe{Y2Ca}[Mg2]Si3O12(menzerite)anditsFe2+analogue{Y2Ca}

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Fig.7. Plotofmenzerite-(Y)compositionsintermsofsiteoccupancyandcationvalence(moleproportions).NumbersrefertograinsinsectionGB50A1usedforthecrystal-structurerefinement(SREF),opticalmeasurements,andmenzerite-(Y)sensustricto,including grain #5 used as the holotype to characterize themineral.Open circlesindicatetheotherninegrainsanalyzedinsectionsGB50A1andGB50A2(Table6).The general formula refers to {(Y,REE)(Ca,Fe2+)2}[(Mg,Fe2+)(Fe3+,Al)](Si3)O12.(a)Thecornersare labeledby thedominantcationforagivenvalenceat theY siteinmenzerite-(Y).Nomineral is specified for theTi field because of the complexsubstitutions inTi-rich garnets. (b)Thedivalent cations are split intoCa-dominantandFe2+-dominantfields.TheY+REEoccupancycannotexceed2/3owingtocharge-balanceconsiderationsunlessSi<3apfu.Thegeneral formulaapplies to theentirelengthoftheboundarybetweenandraditeandmenzerite-(Y),butissplitintwosoastonotobscurethedatapoints.

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[Fe2+2]Si3O12 (“menzerite-Fe”) end-members, bothmeetingtheattributesofavalidend-memberformula(Hawthorne 2002). For calculating the proportion ofY-bearing components, the REEwere added toY.Menzeriteisdominantinrecalculatedcompositionsof10ofthe12grainsanalyzed,andandraditeisdominantin theother two (Table8).Overall, theend-membersandradite,almandine,{Y2Ca}[Mg2]Si3O12[menzerite-(Y)], {Y2Ca}[Fe2+2]Si3O12, and {Fe2+3}[Fe3+2]Si3O12(“skiagite”)compose71–83%of theanalyzedgrains,withmorimotoite and “yttrogarnet” being themostabundantcomponentsintheremainder.

The composition ofmenzerite-(Y) varies fromgraintograin,buttheonlyregularvariationswithY+REEaretheincreaseofMg(Fig.8)andthedecreaseintotaltrivalentcations.Inaddition,individualgrainsarezoned,commonlywithMg,Ti,YandEr(toalesserextent)increasingtowardtherim,butwithAlandCadecreasingandYbremainingrelativelyconstant(e.g.,Figs.3,5b).Bothgrain-to-grainvariationandzonationof individual grains are best understood in terms ofvariation of Fe-bearingmenzerite-(Y), {Y2Ca}[(Mg,Fe2+)2]Si3O12,versusandradite–grossular,{(Ca,Fe)3}[(Fe3+,Al)2](Si3)O12.

Fig.8. PlotofMgversussumofY+REEinmenzerite-(Y).Numbersrefer tograinsin sectionsGB50A1 used for the crystal-structure refinement (SREF), opticalmeasurements,andmenzerite-(Y)sensustricto,includinggrain#5usedastheholotypetocharacterizethemineral(Table6).Aleast-squaresfittoallthepointsexceptSREF(grain#2),whichwasanalyzedunderdifferentconditions,givesMg=0.34(Y+REE)+0.19,R2=0.79.

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Euhedral grains of almandine, including severalovergrowingmenzerite-(Y), contain up to 2.7wt%Y2O3(atoneanalyticalpoint;inmostcases,analyzedareascontainlessthan1.8wt%Y2O3)andupto0.68wt%Yb2O3 (e.g., Table 7), the contents of whichincreasefromrimtocore,exceptforadipinYcontentnexttothecontactwithmenzerite-(Y)insomegrains

(Fig. 5c). Incorporation ofY andREEappears to belargelythroughincorporationofmenzerite-(Y)anditsFe2+analogueratherthanthroughincorporationofthe“yttrogarnet” component (YAG), {Y3}[Al2](Al3)O12,and its Fe3+ analogue,YIG, {Y3}[ Fe3+2]( Fe3+3)O12(Fig.9).Thatis,Ti+V+Cr+(Al– ZAl)decreasesto1.855andY+REE+Fe2++Mn+Mg+Careaches

Fig. 9. Plot of compositions of almandine associatedwithmenzerite-(Y) in sampleGB50A(section1).Theplotteddataareaveragesof3to10analyses(e.g.,Table7),exceptforthepointrichestinY(oneanalysis).Euhedral:isolatedeuhedralalmandinegrains,insomecasessurroundingmenzerite-(Y);symplectite:symplectiteofalmandinewithvermiculesofquartz.Thegeneralizedcomponents{Y2Ca}[(Mg,Fe2+)2](Si3)O12and {Y3}[(Al,Fe3+)2]((Al,Fe3+)3)O12, ormenzerite and “yttrogarnet” substitutions,respectively.A.PlotofthedeficitattheYsite,equalto2–[Ti+V+Cr+Sc+(Al– ZAl)],versustotalY+REEperformulaunitofeightcationsand12atomsofoxygen.B.PlotofSiversustotalY+REE,usingthesameformulabasis.

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3.145atomsper12O,suggestingthattheYsiteisnotfullyoccupiedbytrivalentcationsandTi,whereastheXsiteisoverfilledwithdivalentcations.ThehigherCacontent in the euhedral almandine compared to otheralmandine (Fig. 10) is consistentwith incorporationofmenzerite.Because theX site is dominantlyoccu-piedbyFe2+ in almandine, additionof themenzeritecomponents should increase theCacontent at a ratioofCato(Y+REE)=0.5.Calciumcontent increaseslinearlywithY+REEcontent,butthemeasuredratiois1.035(Fig.10),possiblyduetoadditionofandraditeconcomitantlywithmenzerite.Directmeasurementbymicro-XANESconfirmsthestoichiometriccalculationsshowingthatFe3+/SFeissignificantlylowerinalman-dine than in associatedmenzerite-(Y).The dominantcomponentsintheeuhedralalmandinearealmandine,grossularandpyrope,andtheproportionofmenzeritereaches6.7%,whereas “yttrogarnet”doesnot exceed1%(e.g.,Table9).

Incontrast,theYandHREEcontentsofalmandinein symplectiteswithquartzandaugiteorwithbiotiteandquartzareclosetothedetectionlimitontheelec-tronmicroprobe;theCacontentsofthisalmandinearesensiblylower(Fig.10).Nonetheless,grossularisstillthesecondmostabundantcomponent(Table9).

DefinitionofMenzerite-(Y)anditsRelationshiptoOtherSpeciesofGarnet

Following the dominant valency rule (Hatert&Burke2008),menzerite-(Y)isdefinedasagarnetrichinYandHREEinwhichdivalentcationsaredominantattheYsite,Mgisthedominantdivalentcationatthis

site, andY isdominantamongY+REE.Majorite istheonlyothernaturalgarnetspeciesinwhichdivalentcationsaredominantattheYsite,butitisdistinctfrommenzerite-(Y) in thatY contents are negligible, andsignificantSioccupiestheYsite(e.g.,Smith&Mason1970,Maoet al.1982).Theend-memberformenzerite-(Y)is{Y2Ca}[Mg2](Si3)O12;itsFe2+analoguewouldbe{Y2Ca}[Fe2+2](Si3)O12,whichhasnotyetbeenfound.

However, the 12Y-rich garnet grains analyzed inthis study are intermediate in composition betweenmenzerite-(Y)plusitsFe2+analogueontheonehand,and andradite plus a “skiagite” component on theother,i.e.,closetotheformula,{(Y,REE)(Ca,Fe2+)2}[(Mg,Fe2+)(Fe3+,Al)](Si3)O12. Onlytwooftheanalyzedgrains, namely #5 and #6, qualify asmenzerite-(Y)underthisdefinition,andeventheircompositionsplotclose to the boundarywith andradite [menzerite-(Y)sensu stricto inFig.7].Theother10, includingthoseused for single-crystalX-ray diffraction and opticalcharacterization, are, strictly speaking, andradite thatis rich in themenzerite-(Y) component plus its Fe2+analogue (Fig. 7). Grains ofmenzerite-(Y) sensu stricto are indistinguishable fromY-rich andradite inthin section and back-scattered electron images, andcan only be identifiedwith a complete, quantitativechemical analysis.Consequently, all grains similar tomenzerite-(Y) sensu stricto in composition, opticalappearanceorelectronscatteringintensityarereferredtomenzerite-(Y)inthepresentpaper.

ThecoupledsubstitutionsXY+YMg=XCa+YAlandXY+YMg=XCa+YFe3+relatemenzerite-(Y)togros-sularandandradite,respectively,whereashomovalentsubstitutions,i.e.,Mg=CaandFe=CaattheXsiteare

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requiredtorelateittopyropeandalmandine.Previousinvestigators foundYandREE to be incorporated invariousgarnetsthroughthecoupledsubstitutionsXY+ZAl=XMn+ZSi,XY+Z(Fe3+,Al)=XCa+ZSiorNa+Y=2(Ca,Mn)attheXsite(Jaffe1951,Yoder&Keith1951,Kasowski&Hogarth1968,Enamiet al.1995,Røhr et al. 2007). In contrast,Wang et al. (2003a,2003b) suggested the substitution (Y,REE) =Al toexplain compositional variations inY-bearing spes-sartine from theXihuashan granitic complex,China.Ourplotofthesespessartinecompositions(E.S.Grew,unpubl.data)suggestsanalternativeexplanationfortheinverserelationbetweenY+REEandAl:XY+YFe2+=XMn+YAl,i.e.,incorporationoftheFe2+analogueofmenzerite-(Y),{Y2Ca}[Fe2+2](Si3)O12.

We are not aware of either end-member {Y2Ca}[Mg2](Si3)O12 or {Y2Ca}[Fe2+2](Si3)O12 having beensynthesized,althoughseveralY–Mg–AlgarnetslackingCa have been (Ito 1967, Suwa et al. 1970, 1971).Reinen(1964)reportedthesynthesisofaGeanalogueofmenzerite-(Y),{Y2Ca}[Mg2]Ge3O12,whereasSetluret al. (2006)synthesizedagarnetapproachingtheLuanalogue,withthenominalcomposition{Lu2Ca}[Mg2]Si3O12anda=11.9758(3)Å,butaRietveldrefinementgavetheLu:Caratioas1.5:1.

ConditionsofFormationandOriginofMenzerite-(Y)

Microstructures suggest thatmenzerite-(Y) is anearly-formedmineralthatwassubsequentlypreservedby being armored by almandine and K-feldspar.Althoughmenzerite-(Y) is no longer in contactwitholigoclase, quartz, ferrosilite,magnetite and ilmenite,we presume thatmenzerite-(Y)was originally stablewith these phases.Menzerite-(Y)was probably alsostablewithaugite,eventhoughithasnotbeenfoundinthepatcheswherecoarse-grained,presumablyprimaryaugiteispresent.However,euhedralY-bearingalman-dine,with aK-feldsparmoat, does occur in such apatch;wesuggestthatthisalmandinemarkstheformerpresenceofmenzerite-(Y)becauseof itssimilarity incompositionandappearancetoalmandinethathascoresofmenzerite-(Y).Thereisnoevidenceforalmandinebeingpresentwhenmenzerite-(Y)wasstable;itisonlyfoundasanovergrowthonmenzerite-(Y).Thisabsenceissurprisinggiven therelativelyhighbulk-rockFe2+/(Fe2++Mg)=0.65(E.S.Grew&J.H.Marsh,unpubl.data).Conditionsformenzerite-(Y)crystallizationareestimated to beP ~ 7–8.5 kbar andT ~700–800°C(Grewet al.2010,Marshet al.inprep.).

Theabsenceofcontactsofmenzerite-(Y)withthemajorphasesmagnetite,oligoclase,quartz,augiteandferrosilite implies thatmenzerite-(Y)was no longerin equilibriumwith theseminerals at later stages ofthemetamorphism.Wesuggestthatthebreakdownofmenzerite-(Y)isassociatedwithpartialmelting,whenitmay have dissolved incongruently in themelt togiveperitecticalmandine.Theincongruentdissolutionresultedincuspateoutlinesandevensplitsinglegrainshavinganatoll-likeappearance(Fig.5a),whereasthesharpedgesandsmooth facesof theeuhedralalman-dine (Fig. 4) are suggestive of crystallization fromamelt.Themelt subsequently crystallizedK-feldsparin themoats.Alternatively,K-feldspar could havebeenaperitecticmineralthatformedwithalmandine,or even have exsolved from ternary feldspar in thematrix.Conditionsfordissolutionofmenzerite-(Y)areestimatedtobeP�8.5–9.5kbarandT�800–850°C(Grewet al.2010,Marshet al. inprep.).BasedonacomparisonwithdiagramsinPetríket al.(1995),Noyeset al.(1983)andLindsley(1991),theFe3+/SFe=0.36measuredinallanite-(Ce)andupto9mole%proportion

Fig. 10. The Ca andY + REE contents in almandineassociatedwithmenzerite-(Y)insamplesGB50A,GB50CandGBI4.The plotted data are averages of three to 10analyses (e.g.,Table 7), except for the point richest inY (one analysis).Euhedral: isolated euhedral almandinegrains,insomecasessurroundingmenzerite-(Y),AlmCpxsympl:symplectiteofalmandinewithvermiculesofquartzÅ augite,AlmBt sympl: symplectite of almandinewithplanarinclusionsofquartzandovergrownbyplateletsofbiotite.Lineisaleast-squaresfittothedataforeuhedralalmandine:Ca=1.035(Y+REE)+0.663,R2=0.784.

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ofhematitesolid-solutioninilmenite(E.S.Grew,J.H.Marsh&M.G.Yates,unpubl.data)suggestanoxygenfugacityroughly1–2log10pointsmoreoxidizingthanthequartz–fayalite–magnetite(QFM)bufferfortheformationofmenzerite-(Y).

Conclusions

The discovery ofmenzerite-(Y) and the garnetcomponents{Y2Ca}[Mg2](Si3)O12and{Y2Ca}[Fe2+2](Si3)O12inassociatedalmandineaddsanewdimensiontotheroleofYandHREEingarnet.Menzerite-(Y)mayhavebeenpreviouslyoverlooked,becauseitresembleshercyniteoptically.Theoccurrenceofmenzerite-(Y)atBonnetIslandsuggeststhatitsformationandpreserva-tion require a distinctive set of circumstances, suchas theabsenceofalmandineduringat leastonestageof themetamorphic evolution,moderately oxidizingconditionsandgranulite-faciesP–Tconditions.WearelesscertainwhetherahighbulkY+HREEcontentiscritical.Thehostgranulitecontains80.4ppmY,12.7ppmDy,9.85ppmEr,and11ppmYb(E.S.Grew&J.H.Marsh, unpubl. data on sampleGB50), severaltimesgreaterthanmostgranulites.

Yttrium+REEsubstitutionfordivalentcationsattheXsiteinmenzerite-(Y)andassociatedalmandineisbalanced incharge largelyby substitutionofdivalentcationsfortrivalentcationsattheYsite,i.e.,X(Y,REE)+YR2+=XR2++Y(Al,Fe3+),themenzeritesubstitutioninFigure9.OthersubstitutionsthathavebeenproposedforY+REEincorporationingarnet,namely,X(Y,REE)+Z(Al,Fe3+)=XR2++ZSi,the“yttrogarnet”substitution(e.g.,Jaffe1951),X(Y,REE)+XNa=2XR2+(Semenov1963, Enami et al. 1995) or substitutions involvingvacanciesattheXsite(Quartieriet al.1999a,1999b),play a subordinate or no role in theBonnet Islandgarnets.ThepossibilitythatthemenzeritesubstitutioncouldalsobeimportantinthespessartinereportedbyWanget al.(2003a,2003b)suggeststhatthissubstitu-tioncouldbewidespreadinmetamorphicandigneousgarnetsoverall.

Acknowledgements

WethankFrédéricHatert,Vice-Chairman(changesin existing nomenclature) of theCNMNC IMA, forhiscommentsonthenomenclatureusedinthispaper.Comments byCharlesGeiger, an anonymous refereeandRobert F.Martin led to substantial improvementof themanuscript. JHM received support from aUniversity ofMaineDoctoralResearch Fellowship.TheresearchofESGandMGYwassupportedbyU.S.National Science Foundation grantsOPP–0228842,EAR 0837980 andMRI–0116235 to theUniversityofMaine,andEAR–0809459toM.D.DyaratMountHolyokeCollege.

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Received December 15, 2009, revised manuscript accepted September 10, 2010.

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