CopperDiffusioninPlagioclase

JohnL.Emmett&TroyR.Douthit

CrystalChemistry

August21,2009 Therehasbeenpublicconjecturethattheredandesineandlabradoritegemsinthemarketplacearenotnatural,butareinsteadrelativelycolorlessmaterialthathasbeendiffusedwithcopperinthelaboratorytoproducetheredcoloration.Thefirststepinelucidatingthissituationistodeterminewhetherornotcoppercanbeeasilydiffusedintotheseplagioclases.TomakethisdeterminationweconductedasetofexperimentsfromJunethroughOctoberof2008,usingavarietyofandesine/labradoriteplagioclases.Inwhatfollows,themethodologyfortheseexperimentsandtheirresultsarepresented. Ourgeneralapproachforexploratorydiffusionexperimentsistoembedthetestsamplesinarefractoryoxidepowderthathasbeendopedwiththeelementwhichwewishtodiffuse.Theembeddedsamplesarethenheatedtothemaximumtemperatureconsistentwiththematerialsinvolvedandheldatthattemperatureforsufficienttimetobeabletoobserveasignificantdiffusionlayer,ifoneforms. Thefirstdecisiontomakeinplanningsuchexperimentsisthetemperatureatwhichthediffusionexperimentswillberun.Inthegemologicalworld,diffusioncolorationwasfirstencounteredwiththediffusionoftitaniumintopaleblueorcolorlesscorundumtoproduceadeepbluecoloration(Kane,Kammerlingetal.1990).Thiswasfollowedbyberylliumdiffusionintocorundumtoproducemanydifferentcolorationeffects(EmmettandDouthit2002).Itiswellknownthattheseprocessesareconductedatveryhightemperatures(1750‐1900°C)andthusitisthoughtbymanythatalldiffusionprocessesrequiresuchtemperatures.Thisisnotthecase.Thediffusioncoefficientforanypairofmaterials(substrateanddiffusant)increasesexponentiallywithtemperatureuptothemeltingpointofthesubstratewhenmixingorcompoundformationusuallydominatestheinteraction.Aruleofthumbisthatdiffusionratesscalebythefractionofthesubstratemeltingtemperature,measuredfromabsolutezero,atwhichtheexperimentisconducted.Thusdiffusionintocorundumwithameltingpointof~2050C(2323K),at1850°C(2123K)isat~90%ofthemeltingpoint.Ifonethenconductsadiffusionexperimentonamaterialwitha1250°Cmeltingpoint,onewouldexpectroughlysimilardiffusioncoefficientsat~1110°C. Toevaluatewhattemperaturewecanconductdiffusionintoplagioclasesweneedtoexaminetheirphasediagram,showninfigure1.

Figure1.Phasediagramoftheplagioclasefeldspars(Bowen1913).

Figure1showsthattheplagioclasesarecomprisedofasolidsolutionofalbiteandanorthiteandthusthemeltingcharacteristicsareafunctionofcomposition.ThelowerofthetwocurvesinFigure1istermedthesolidus,andbelowthiscurvethematerialisfullysolid.Theuppercurveistermedtheliquidus,andabovethiscurvethematerialisfullyliquid.Betweenthesetwocurvesisaregioninwhichbothliquidandsolidmaterialcoexist.Whatisinterestingisthatthesolidphaseandtheliquidphaseinthisregionareofdifferentcomposition.Ifoneheatsaplagioclaseof50%anorthiteto1350°C,andthenwaitsforequilibriumtobeestablished,thesolidphasewillhavethecompositiongivenbythesoliduscurveat1350°C,whichisabout67%anorthite,whiletheliquidphasewillhavethecompositiongivenbytheliquiduscurveat1350°C,orabout25%anorthite.Ifthecooldownfromthisstateisrapid,hoursinsteadofmonths,theresultingsolidwillbeamixtureofcrystalswithdifferentcompositions.Sincethetwocompositionshavedifferentindicesofrefraction,themixturewillscatterlightleadingtotranslucencyinsteadoftransparency.Thusinplanningadiffusionexperimentitisimportanttostaybelowthesolidus.Sincethecompositionofoursamplescoversarangearound50%anorthite,wemuststaysafelybelow1250°C. Thereisasecondconsiderationinchoosingthetemperatureandthatisthereactionbetweencopperoxideandplagioclase.IfacopperdiffusionexperimentisconductedinairtheprimarycopperspeciewillbeCuO.InprincipleCuOcanreactwithplagioclasestoproducealiquidphasebelowthemeltingpointofCuO(~1235°C)andbelowthesolidusoftheplagioclase.

Itisnecessarytoavoidtheformationofliquidphasesforthereasonoutlinedabove,butmoreimportantlybecausewiththeformationofaliquidphasethereisnolongeranycontrolofthesolubilityofCuionsinthecrystal,andnewcompoundsmightformaswell.TherearenophasediagramsforplagioclaseandCuO,butwecangainsomeinsightbylookingatthephasediagramsforCuOandtheconstituentoxidesofplagioclase,Al2O3,SiO2,andCaO.Liquidphasesarepresentinthesephasediagramsattemperaturesaslowas1020°C.Theselowtemperaturesindicatethatwemaynotbeabletoavoidtheappearanceofaliquidphaseandthuswillhavetocontrolcoppersolubilitybyemployingaverylowcopperconcentration.Fromtheforegoingconsiderations,wechoseacopperdopantconcentrationof1%andatemperaturerangeof1000–1200°Cforthediffusionexperiments. Choosingtherefractoryoxidecarrierforthecopperisthenextissue.Againweareconcernedaboutchemicalreactionsbetweentheplagioclaseandtheoxidecarrier.FortheoxidecarrierweconsideredAl2O3,ZnO,MgO,andZrO2.Testswererunbyembeddingpolishedplagioclaseplatesintoeachoftheseoxidesandheatingat1200°Cfor100hoursinair.Inallcasestherewasdamagetothepolishedsurfacesindicatingchemicalreaction.However,therewasonlyveryminimaldamagewithZrO2,soitwaschosenforthediffusionruns. Choosingadurationforadiffusionrunisalwaysacompromisebetweendoingexperimentsrapidlyanddiffusingforlongenoughthatthediffusionlayeriseasilyvisible.Estimatingthataneconomiccommercialprocesswouldprobablynotexceedabout2000hours(~3months),andnotingthatroughstoneshavebeenexaminedthatareintherangeof~2cmthick,wecanguessatadiffusioncoefficientoftheorderof10‐7cm2/secorlarger.Sincediffusiondepthonlyincreaseswiththesquarerootoftime,oneweeklongdiffusionrunsshouldgiveusa2‐3mmcoloreddiffusionlayer,perhapsmore. Fortheseexperiments,materialfromfourdifferentsourceswasmadeavailabletous.PlagioclasefromtheCasaGrandeMineinMexicowasprovidedbyBillBarkerofBarkerandCompany.MaterialfromthePonderosaMineinOregonwasprovidedbyJohnWoodmarkoftheDesertSunMiningCompany.MaterialfromMongoliawasprovidedbyJerrySiskofJTV.MaterialfromtheminesnearPlush,OregonwasprovidedbyBartCurranofColumbiaGemHouse.EricBraunwartandBartCurranofColumbiaGemHousearrangedfortheroughtobesawedintoplates3‐4mmthickandpolishedonbothsurfaces. Eachdiffusionruntestedfivepiecesfromeachofthefourlocationscitedabove.Weconsiderfivepiecestobetheminimumforameaningfulexperiment,and20wouldhavebeenbetterifavailable.ThestoneswereembeddedinZrO2containing1%coppermetal(‐300mesh),andplacedinaluminacrucibles.Allsampleswereheatedfor160hours.Threetemperatureswereused–1170°C,1100°Cand1000°C,withallrunsinair.Polishedsurfacedamageoccurredforallrunsasexpected,butwasminimalat1000°C.Allsampleswererepolishedfollowingdiffusion.Theresultsareshowninthephotographsoffigures2‐12. Twoconclusionsareimmediatelyclear–coppercandiffuseveryrapidlyinsomeplagioclasesamples,andawidevarietyofphenomenologyisproduced.Insomecasesitappearsthecopperdiffusesbybulkorlatticediffusion,whileinothersclearlypipeorshortcircuitdiffusionispredominant.Thelatterisshownbythefactthatthecoppertravelsrapidlyalongdislocationbundlesorotherstructuralimperfections.Thisisparticularlyclearinfigures

3and5.Insomecasesthegreencolorationoftenseenwithinnaturalmaterialisalsoobservedwithinredregionsproducingdarkcoloredareas. Thediffusionrateappearstofallrapidlywithtemperatureasshownbytheminimalcolorationofthesamplesdiffusedat1000°C.However,evenherethereisconsiderablevariationsampletosample.Thisisshowndramaticallybythesinglesampleinfigure11thatissuchadeepredcolorthatitappearsblackinthephoto. Theseexperimentswererunsolelytoanswerthequestion“doescopperdiffuserapidlyinplagioclase?”Theanswerisclearlyyes.Understandingthephenomenologywouldrequiremuchmoreextensiveexperimentationandanalysis.Clearlythediffusionisrapidenoughthataneconomiccommercialprocesscouldhavebeeneasilydeveloped.

Bowen,N.L.(1913)."Themeltingphenomenaofplagioclasefeldspars."AmericanJournalofScience,4thseries35:577. Emmett,J.L.andT.R.Douthit.(2002,September4)."Berylliumdiffusioncolorationofsapphire:Asummaryofongoingexperiments."RetrievedMarch20,2003,2003,fromhttp://agta.org/cnsumer/gtclab/treatedsapps04.htm. Kane,R.E.,R.C.Kammerling,etal.(1990)."Theidentificationofbluediffusion‐treatedsapphires."Gems&Gemology26(2):115‐133.

Figure 2. Mongolia Samples, 1170°C.

Figure 3. Mexico Samples, 1170°C.

Figure 4. Plush, Oregon Samples, 1170°C.

Figure 5. Mexico Samples, 1100°C

Figure 6. Ponderosa Mine, Oregon, 1100°C.

Figure 7. Mongolia Samples, 1100°C.

Figure 8. Plush, Oregon Samples, 1100°C.

Figure 9. Mongolia Samples, 1000°C.

Figure 10. Mexico Samples, 1000°C.

Figure 11. Ponderosa Mine, Oregon, 1000°C.

Figure 12. Plush, Oregon Samples, 1000°C.