1009 TIU Canadian Mirrcralogist Vol.35, pp. 1009-1019(199)
SINGLE.CRYSTAL NEUTRON-DIFFRACTION STUDYOF PYROPE IN THETEMPERATURE RANGE3(F1173K GILBERTOARTIOLI t eNoALESSANDROPAYESE Dipartincnto
d.i Scienze dellaTena.
Universithdi Mililn,
viaBonbelli23,
I-20133 Milatto, Italy
KENNYSTAHL Departmewof Chcnistry, TeclmicalUniversityof Denmnrh DK-2800 Lyngby,Denmark RICHARD K. McMULI-AN Departnent of Chzmistry,BrookhavenNatiorwl Laboratory Upton,New York11973,U.S-A. AssrRAsr Single-crystalneutron-diftaction data were collectedover the tempraftre ftrnge 3Lll73 K on a pyrope crystal from the Dora Maira Massif, WesternAlps, Italy. The snrdyexpandsthe strucure analysisof syntheticend-memberpyrope previously performedwith X-ray single-crystaldiftaction overawide tonpemnrrerange.Theneuton nuclearscattering allowsa moreaccurate definition ofthe atomicdisplacementparametenandtheir temperanuedependence. Theproblemsinvolvedin thecharacterization of theatomicdisplacementparametersin gamet-inerals, andespeciallyin pyrope,haveimplication for theconect interprctation of the physicalandthermodynamicpropertiesof the garnetstrucaue.The presentanalysisof the atomicdisplacementp,roperties by thehighercumulantsmodelin the Gram-Charlierseriesapproximationshowsthatthe anhamoniccontributionto thedynamic partofthe magnesiumsite.lisplacementis significantabove7fi) K No evidenceis foundfor staficsitedisorderin thedodecahedral Mg siteby refinenent with split-aton models,andthe interceptat 0 K re.sultingfrom linear interpolacionof thehigb-temperatre harmonicdisplacementcoefrcientsis not statisticallysignificant-The refinementof high-orderdisplacement-tensor components resuls in simificantly lower figuresof merit in the whole exploredtenperatue range. Keywords:pyrope,neutrondiftaction, rnhamonicity, crystal strucfirre,Dora Maira ma.ssif,Italy. Souraenr Desdonn6esde diftaction neuhotriqueont 6t6obtenuessur un cristal uniquedepyropeprovenantdu massifdeDora Mairz, dansles Alpes Occidentales,en ltalie, sur un intervalle de temfrature entre 30 et 1,173K. Notre 6tude s'ajoute i I'analyse strucorraleant6rieured'un 6cbantillonsynth6tiquedepyrop parditrractionX, aussieffecnr6esrnunlargeintervallerletemperaarre. La dispersiondesneutronspar les nucldipermetuneddfinition pluspr6cisede.sparamBtres de ddplacements atomiqueset de leur d6pendance sur Ia temffra$re. Les probldmesqu'implique la caract6risationde cesparam0tresclansles mindrauxdu groupedu grenat,etplusparticuliercment lepyrope,ontdesirylications pourI'interp'retation etthermodynamiques comce despropri6tephysiques de la structuredesgrenats.Notre analysedaspropri6tAsdasddplacements atomiquespar la m6thodedescumulantsde degr6plus 6lev6.lans l'approximation de la s6rie de Gram{harlier montre que la contribution anharmoniquei la partie dynamiquedu ddplacementdu site du meqn€siume.stimportantee unetemp6ra$resupdrieurei 7@ K. Nousne Fouvonsaucuneindicationde d6sordrede position statiquedansle casdu site dod6ca€driquedu Mg, suite i nos efFnementsde moddles) atomes"divis6s"; I'interceptea 0 K r6sultantde l'inlerpolation lin6airedescoefficientsde d6placementharmoniquei temffrature ebv6e n'est pas statistiquementsignificafive.I- affinementdescomposantes dedegr66lev6destenseursdeddplacementa donn6uneam6lioration simificative de la figure de merite sur I'intervalle completde temp€rature. (Ibaduit par LaR&laction) Mots-cUs:pyrope,ditrection neutronique,anhamsaisitd, .g,t"6rp gri5talline,massifde Dora Maira, Italie.
IlrnopucnoN A substantial mormtofwo'rkhasbee,lrdevotedo gunetgroupmineralsin thepast.They areimportantphasesin high-pressureand high-temperaturemetamorphicand igneousassemblages, andseveralfeaturesoftheircrystal I E-mail address:
[email protected]
shadows chemistryandphysicalpropertiascastdisquieting uponourabilityto int€rFetthedetailsof thecrystalsttrcfl:re of a garnetat a fundamental level. Unresolvedproblems,for example,concernthethermodynamicpropertiesof garnetsolid-solutions(Crassey l9Sl,Geiga etal.1987,Berman1990,Koziol& Bohlen
1010
THE CANADIAN MINERALOGIST
1992),especiallyalongthe pyrope- grossular,pyropeorderof magnitudelessthantheminimrrmsingle-crystal almandine, almandine-spessartine, andalmandine-grossula volume useftrl for neufron diftaction from anhydrous joins. The debatednon-idealityof thesesolid solutions compounds.We thereforehad to rely on large natural (Ungareniet oI. 1995),in particularthoseinvolving the singlecrystalsof pyrope.TheDoraMairapyrope(Chopin pyropecomponent, couldin principledependon structural 1984,Schertlet al. l99I), which occursasmegablasts subsitedisorder Aenann 1962,Cressey1981,Hofueister in high-gradeblueschistrocks, is the only reported & Chopelas1991,Pilati et aI.1996),on non-harmonic occurrenceof pyropewith a small amountof almandine components. Themegablass, vibrational behaviorof the cationsin the dodecahedral md grossulaassolid-solution site (Winkler et al. l99l), on cationclusteringat a local up to 10-20cm in size,arecomposedof a pyropemanix inclusionsof othermineralphases, level(Gangulyalal 1993),oronthenonlineadeformation s6atqiningabundant of theXpolyhedron(Qtartrui etaJ.1995).It is remarkable mainly muscovite,staurolite,andkyanite. that mostauthorsfail to reproduceandinterpretin detail Prior to datacollection,thick slidesof the composite werecheckedforcrystalqualityusingtexture theexperimenalobservations in somegarnetend-members,megablasts pyropeandalmandinein particular.Thepoblemsconcern analysisona Huber5-circlepowderdiftactometer.Details eithertheanomalouslyhighheatcapacityobservedatlow of the texture analysishave beenreportedpreviously temperature(Kieffer 1980,Haselton& Westrum1980, (AmisanoCanesiet al. 1994).Acrystalof 2.83x2.87 x Hofineister& Chopelas1991),or theanomalously large 3.02 mm3was finally extractedfrom the centerof a alomicdisplacementparameters of theMg andFecations slabcut throughthe megablasts. observedby diffraction methods,and not reproducible Electron-probemicro-analysison different areas'of by simulations of lanicedynamicsQatela aL 191, Winkler thepyropematrix closeto theregionof crystalextraction et al. l99L,Platr et al. 1996). resultedin the formula (Mgo.s2Feo.osCao.og)AlzSigOrz The crystalstructureof pyropeM$AIzSisO17,,Ia34 andindicateda remarkablechemicalhomogeneityof the Z= 8)basbeenthoroughlyinve*igatedby X-rayditfraction matrix crystal. Q.emann&Znmann 196I, Gibbs& Smith1965,Meagher 1975,Armbruster etaL 1992,Ambruster& Geiger1993, Epnnnrn-rer (@rgeretaL l992). Sawada193) andFTIR spectroscopy All structuralsudiesperformedaflow orhighte,mperafirre Collection of neutron-diflrattion data failed to revealanyevidenceofsubsiteMg disorder,but insteadindicate large anisotropyof the Mg harmonic Neutron-diffractionintensity datawere collectedat vibration. The recent study of Paveseet al. (1995) 30, 100 and293K at the NeutronResearchl,aboratory considerablyextendedthetemperahrerangeinvestigated Studsvik,Sweden,andat295,773,973,and 1173K with single-crystalX-ray diffraction, and the findings at the High Flux BeamReactor,BrookhavenNational supporttheseconclusions. Rnthermore,itpresentsevidence Iaboratory Brookfiaven,NewYork;thesamecrystalwas of weakanharmonicvibrationaleffecs of theMg cation usedfor all experiments. at high temperature.Most authorsnow seemto agreeon The requiredlow temperaturewas obtainedusing a Hecooler,installed the "dynamic" natureof the disorderof the Mg atomin CTI Cryogenicstwo-stepclosed-cycle pFope, thoughtherestill areclaimsthat a modelbased on statisticalsubsitedisordercanadequatelyexplainthe TABLE I. DPERIMENTAL PARAMETERSFOR COLLFCTION OF anomalousexperimentalfeaturesobserved@lJaaet aL SINGLE{RYSTAL NzuTRON.DIFFRACTION DATA AT VARIOUS 1996). TEMPERATT'RFSAND DETAIIJ OF REFINEMENT Webaveundertaken asingle-crystal neutrondiftaction sody of pyropeoverawidetemperarure range,in anatte, pt T(K) 30 t00 2931 295j 973 lt73 295" 773 to obtainsolid experimentalevidencefor the posnrlated anharmonicbehaviorof Mg in pyrope.In addition,this m-o.hl. 6(D0) 6\220) clt22o) BdlW2) Bq0o2) B{002) B{002) 8{002) r(A) l2ts0 12150 t2Ls0 t.0452 LoK2 t.046.2 Lo62 t.046'2 study is intendedto charactetizethe dependenciesof €[u(A) ll.44l 11.442 11.450 11.455 1r.457 ll.5l2 11536 11.559 displacementsof the nuclei on temperatureili a means @Eode ?}-20 dr-20 ?>-20 ?r-20 6-20 7D-2O 6-20 7E-2O @@gle 2.5'* 2.0'A 2-5'* 1.555+1555+ lJ55+ 1.555+ lJ55+ of distinguishingbetweenstaticanddynamisdisorderof 3.0oil* 2.50*i 3.0"*+ Z855tg€2.t55190 2t55tS02.855t90 2t5ns0 t@ (sho/t) 0.79 0.79 0.79 0.88 0.88 0.88 0.88 0.88 atomsin the structure. '!|@rcfl 584 609 82 610 1435 Eol 840 848 Selpt-e
indEp.idR@sins us€drcIt'n td rs. w ,iP tr URF
210 214 254 216 254 0.019 0.010 0.009 0.011 0.932 39 4@ M 956 787 0.0194 0.01, 0.0135 0.0185 0.t3t2 0.02t6 0.0t42 0.0145 0.0194 0.t3t2 0.614 0.V242 o.tnl 0.0242 o.MlO 0.t314 o.mn 0.024[ 0.02l.3 0.A427
255 257 EA 0.030 0.m3 0.C37 ,{89 501 477 0.t26 0.@04 0.0280 0.0293 0.t309 0.t297 0.G46 0.0361 0.0330 0.936a 0.0376 0.0354
Available neutron beamsare much weaker than X-ray beamfluxes availableat laboratoryor advanced sources,in termsof totalflux at thesample.It is therefore ' olleaio of rcftrae d@ d|@ withcu fue toryqtue o eyoce necessary to performdiftaction experimentsat reactors *' dds @Ilerti@ dt@ Eqqffi with thrqydrl iEidetheEi@frd@. F. wwgod rellerti@ usrd iu the refi!ffi orpulsedneutonsourceswithrelativelylrgesinglecrysals.< 120 in qdq to @lude wk uobsved rcf,eim@ h tie @ge 0.83 < [Ftobryfl@b)l ffi hcLdhgthitd- ed ftuth
