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The Canadian MineralogistVol.48,pp.231-235(2010)DOI:10.3749/canmin.48.1.231

IMAGING BIREFRINGENT MINERALS WITHOUT EXTINCTION USING CIRCULARLY POLARIZED LIGHT

MichaelD.hiGGiNS§

Sciences de la Terre, Université du Québec à Chicoutimi, 555 boulevard de l’Université, Chicoutimi, Québec G7H 2B1, Canada

abStract

Linearcross-polarizedopticalimagesrevealthebirefringenceofmineralgrainsandareveryusefulinpetrography.However,ifthevibrationdirectionsofthecrystalarenotat45°tothepolarizers,thentheintensityoftheinterferencecolorsisreduced,andinsomeorientations,tozero(extinction).Thisreductioninintensity,andextinction,canbeeliminatedifcircularpolarizersareusedinplaceoflinearpolarizers.TheinterferencecolorsofcolorlessmineralsthencorrespondexactlywiththoseontheMichel-Lévychart.Similarly,isogyresareeliminatedfromconoscopicimages.Thissimple,inexpensivetechniquecanbeusedwithordinarypetrographicmicroscopesandscanners.Itcanbeveryusefulforquantificationoftextures(microstructures)fromsingleimages.

Keywords:opticalmineralogy,polarizedlight,circularpolarization,texturalquantification,birefringence.

SoMMaire

Lesimagesoptiquesproduitesavecpolarisationlinéairementorthogonalerévèlentlabiréfringencedegrainsdeminéraux,ets’avèrenttrèsutilesenpétrographie.Toutefois,silesdirectionsdevibrationducristalnesontpasorientéesà45°desnicolspolariseurs,l’intensitédescouleursd’interférences’entrouveréduiteet,danscertainesorientations,jusqu’àzéro(c’est-à-dire,jusqu’àl’extinction).Onpeutéliminercetteréductionenintensité,etl’extinction,enutilisantdespolariseurscirculairesaulieudespolariseurslinéaires.Lescouleursd’interférencedeminérauxincolorescorrespondentalorsexactementàcellesdel’abaquedeMichel-Lévy.Demême,lesisogyressontéliminéesdesimagesconoscopiques.Onpeutseservirdecettetechniquesimpleetpeucouteuseavecdesmicroscopespétrographiquesordinairesetavecnumériseurs.Ellepeutêtretrèsutilepourquantifierlestextures(microstructures)àpartird’uneseuleimage.

(TraduitparlaRédaction)

Mots-clés:mineralogieoptique,lumièrepolarisée,polarisationcirculaire,quantificationdetextures,biréfringence.

§ E-mail address:mhiggins@uqac.ca

iNtroDuctioN

Microscopeobservationof transparentminerals incross-polarizedlightisafamiliartechniquediscussedinallopticalmineralogytextbooks(e.g.,Dyaret al.2008).Thesample(generallyathinsection)isilluminatedwithlinearlypolarizedlightandobservedthroughasecondlinear polarizer oriented orthogonally to the illumi-natingpolarizer.Isotropicmaterialsareextinct(nolightpasses through).Anisotropicmineralsarealsoextinctifviewedalonganopticaxis.Allotherorientationsofanisotropicmaterialswill show retardation (birefrin-gent) interference-colors,whose intensity varieswith

orientation. If the vibration directions of the crystalareparalleltoeitherpolarizer,thentheintensityoftheretardationcolorswillbezero,andthecrystalappearsextinct.Theseextinctionpositionsareusedtodetermineaspectsoftheorientationoftheindicatrixofthecrystal.Theycanalsohelptheobservertodistinguishadjacentcrystalsofthemineral.Insomecircumstances,theycanbeahindrance,however.Thisisparticularlythecaseifapetrographerwantstoimageathinsectionandexaminequantitativelythetexture(microstructure)ofthegrains.In this case, several sectionswith different orienta-tionsofthepolarizersmustbeexamined.Interferencecolorsalsogiveinformationoncrystalorientationbut,

232 thecaNaDiaNMiNeraloGiSt

again,intensityvariationswithrotationwillcomplicateimageanalysis.Thisisparticularlyaproblemforlow-birefringenceminerals,suchasquartzandthefeldspars.

Mostpetrographersandmineralogistsconsiderthatvariable intensity of retardation colors and extinctionareunavoidable,butthereexistsasimple,inexpensiveopticaltechniquethatcanrevealallanisotropiccrystalsinasection,exceptforthosewithaverticalopticaxis.Thecircularlypolarizedlighttechniqueisnotnew,butdeservestobemorewidelyapplied.

circularlyPolarizeDliGht

Conventional petrographicmicroscopes use twolinear polarizers to produce images of anisotropicminerals. This is generally termed cross-polarizedlight (XP).Unpolarized light passes through a linearpolarizer,whichonlyallowsasingledirectionofpolar-ization.This polarized light then passes through thesample,generallya thinsection,where it is split intotwoorthogonalcomponents.Wherethesecomponentsemergefromthecrystal,thereisadifferenceinphasecalled the retardation.The components are combinedin a second, orthogonal polarizer,where interferencecolorsareproduced.Thecolorsarecontrolledby thebirefringenceofthemineralinthedirectionofpropaga-tionofthelight.However,theintensityoftheinterfer-encecolorwilldependon theorientationof the lightcomponentswithrespect to thepolarizers: if theyareparallel,thennolightwillpassthrough,andthemineralissaid tobeatextinction.Hence, the intensityof theretardationcolorwillchangeasthestageisrotated.

It is commonly taught that retardation colors allfall on theMichel-Lévy chart. In fact, theMichel-Lévy chart shows the brightest interference-colors,visiblewherethevibrationdirectionsareat45°tothepolarizers.Ifthestageisnotrotated,thenmostoftheinterference colorswill be less intense than those ontheMichel-Lévychart.Thisisthesituationinasimplephotomicrograph.

Theproportionofmineralgrainsthatareatextinc-tion in a samplewithout preferredorientation canbecalculatedreadily.Therearetwooriginsforextinction:1) alignment of the optic axis close to the vertical,and2)alignmentofthevibrationdirectionsparalleltothe polarizers.Althoughboth effects apply to uniquedirections, in reality extinction extends over a bandoforientations.Thewidthofthisbandwilldependonmanyfactors,butprincipallythebirefringence.Forthepurposes of calculation, the band is considered to be±5°,avaluetypicalforlow-birefringencemineralslikequartzandthefeldspars.Forthefirstsituation,considerasphereofradius180/pmm;eachdegreeonthesurfacehasalengthof1mm.Theareaatextinctionaroundanopticaxisispr2,here78.5mm2.Theareaofthesphereis4pr2,here41250mm2.Thepercentofextinctposi-tionsforabiaxialmineralisthen0.76%,andhalfthatvalueforauniaxialmineral.Forthesecondsituation,

thecalculationisevensimpler. Ifamineral isextinctfor±5° of each90° segment, then10/90, or 11%,ofcrystalswillbeextinct.

Thetechniquepresentedhereusescircularlypolar-ized (CP) light.This simple treatment is for a singlewavelengthoflight.Asbefore,unpolarizedlightpassesthrough the lower polarizer (Fig. 1). It then passesthroughaquarter-waveplateorientedwithafastdirec-tionat45°tothepolarizationdirection,whichsplitsthelight into twocomponentswithaphasedifferenceof90°.Theelectricvectorofthislighttracesoutahelicalpath,hencethetermcircularlypolarizedlight.TheCPlight thenpasses through thesample.Here thediffer-ences in indexof refraction for different orientationsorthogonaltothepropagationdirectionwillmodifytheshapeofthehelixandmakethelightellipticallypolar-ized.The light thenpasses througha secondquarter-waveplatewithitsfastdirectionorthogonaltothatofthefirstquarter-waveplate.Theellipticalpolarizationisthenresolvedbackintolinearlypolarizedlightwitha different orientation from the initial polarizer.Thisthenpassesthroughtheanalyzingpolarizer,producingasimilarresulttothefirstcasewithlinearlypolarizedlight.Thereisnoextinctionforanyorientationofthecrystalbecauseallpolarizationorientationsarepresent.Indeed,thereisnochangeintheintensityofthefinalinterference-colorswithrotationofthesample.

Circularpolarization isonlyproducedforasinglewavelengthoflightinthissetup,fourtimestheretarda-tionofthequarter-waveplates.Fortypicalquarter-waveplates, this is in the range560–620nm, i.e.,orangelight.However,despitethislimitation,theCPtechniqueworks remarkablywell for polychromatic light, asdepartures fromcircularity of polarization areminor.The interference colors of colorlessmineralsmatchexactlythoseoftheMichel-Lévychart,withoutthelossinintensityobservedinXPimages.

TheCPtechniquecanalsobeappliedtoconoscopicobservations.Indeedoneoftheearliestreferenceswasin this context:Craig (1961) described the “BenfordPlate”forthestudyofinterferencefigures.SuchfiguresinCParesimplerthantheconventionalimagesinthattheylackthefamiliarisogyres.However,theystillhavemelatopesandisochromes.Hencetheycanbeusedfordistinguishinguniaxialandbiaxialminerals,aswellasfor the determination of optic signs.TheCPmethodhasalsobeenusedtomakebombsights(Gunter2003).

One should note that CP does not completelyreplacelinearpolarizers.Extinctioncanhelpdistinguishadjoiningmineralgrains,anditgivestheorientationofvibrationdirections.Alsoalinearpolarizerisnecessaryfor the identification of pleochroic colors.Combina-tionsofalinearpolarizerandcircularanalyzer,andtheinverse,havebeenexploredprincipallybyresearchersinmaterials science andbiology (Glazeret al. 1996,Oldenbourg&Mei1995).Thesemethodshaveconsid-erableadvantagesfortheanalysisoflow-birefringencematerials.

iMaGiNGbirefriNGeNtMiNeralSwithoutextiNctioN 233

aPPlicatioNofthecPtechNique

Somemicroscopemanufacturerscansupplycircularpolarizers and analyzers, but generally thesemustbe improvised.Quarter-wave plates (also known asretarder plates) aremanufactured fromquartz,micaandplastic.The twoquarter-waveplates shouldhavethesameretardation:140to155nmaretypicalvalues.Thoseforpetrographicmicroscopesarereadilyavail-able and are generallymade of quartz.Most petro-graphicmicroscopeshaveapositionforaquarter-waveaccessoryplatebetweenthesampleandtheupperpolar-izer.However,veryfewmicroscopeshaveapositionfortheotherquarter-waveplatebetweenthelowerpolarizerand the sample. In somemicroscopes, anunmounted20-mm-diameter wave plate can be placed on thecondenser.Otherwise,a lowerquarter-waveplatecanbeplacedunderthesampleonthestage.Ofcoursethe

stagecannotberotated,butthisisnotnecessaryusingtheCPset-up.Aholdercanalsobemadethatholdsthefilterunderthestage.

Scanners areveryuseful for imagingboth regularand largewhole thin sections. Inexpensive flat-bedscannerswithanupperlightsourcecaneasilyachieve10mmresolution,andslidescannerscansurpass this(Tarquini&Armienti2001).Forthistypeofimaging,thepolarizingandquarter-waveplatesneed tobe thesame size as the thin section. Polarizingmaterial isavailableasrigid,mountedsheets,butunmountedmate-rial reduces the scanner–slide distance.Quarter-waveplasticretarderfilmisalsoavailablewithgoodopticalquality.Bothmaterials are inexpensive and availablein large sheets. Suitable sources of thesematerialsare available at http:// geologie.uqac.ca/~mhiggins/cp_images.htm

fiG.1. Circularlypolarizedlightobservationsofanisotropicmaterials.Thedirectionsofthelinearpolarizersareindicatedbyhatching.Thefastdirectionofthequarter-waveplatesisindicatedbyanarrow.Petrographicmicroscopestypicallyhaveallofthesecomponentsexceptthelowerquarter-waveplate.Asimilarsetupcanbeusedonaflatbedscannerwithanupperilluminator.

Light source

Lower polarizer

Linearlypolarizedlight

Circularlypolarizedlight

Ellipticallypolarizedlight

Linearlypolarizedlight

Upper polarizer(analyzer)

Lower quarter-wave plate

Upper quarter-wave plate

Sample

Observer / camera

234 thecaNaDiaNMiNeraloGiSt

exaMPleS

Thefirstexampleshowstheapplicationof theCPmethod to low-birefringenceminerals.A sample ofvesiculardacite lava fromTaapacavolcano, inChile,containsabundantplagioclaseandminorsanidine.Animageof the feldsparcrystalswasneededso that thecrystal-size distribution could be calculated (Higgins2006).The simplestway to produce such images isbythresholdingtheimage.Itisdifficulttodistinguishconsistently the feldspars from vesicles in unpolar-

izedlight,asbotharebright.Inlinearcross-polarizedlight(XP),thefeldsparsareclearlydistinguishedfromvesicles,butsomecrystalsareatextinction(Fig.2A).InCP, almost all crystals are revealed, except for asmallproportionwithverticalopticaxes(Fig.2B).ThedifferencesbetweentheimageshavebeenbroughtoutinFigure2C.ImagesofthefeldsparsinFigures2Aand2Bwereproducedbythresholding.TheseimageswerethencoloredandcombinedtoproduceFigure2C.Inthisimage,crystalsinblackarepresentinbothimages;bluecrystalsareonlypresentintheCPimages.Thefewred

fiG.2. Thinsectionimagesobtainedusingaflat-bedscanner.A)Linearcross-polarizedimage(XP)andB)circularcross-polarizedimage(CP)ofadacitelava.MorecrystalsarevisibleintheCPimage.C)DifferenceimageforplagioclaseinA)andB).CrystalsvisibleintheCPimageonlyareshowninblue.D)Linearcross-polarizedimage(XP)andE)circularcross-polarizedimage(CP)ofagabbro.

iMaGiNGbirefriNGeNtMiNeralSwithoutextiNctioN 235

crystals,whichareonlypresent in theXP image,areprobablyduetoslightdifferencesinthethresholdusedtomakeeachimage.

A second example involvesmineralswith higherbirefringence.Figures2D (XP)and2E (CP) areof agabbrofromtheSkaergaardintrusion,ineasternGreen-land.The rock contains plagioclase, olivine, clinopy-roxeneandanFeoxide.Intheconventionalimage(XP),somecrystalsofalltransparentphasesareatextinction,andmanyothershaveintensitiesofretardationcolorssignificantlylessthantheirmaximumvalues.IntheCPimage,veryfewtransparentcrystalsareatextinction,andtherestshowretardationcolorsthatresemblethosein theMichel–Lévychart. It isconsiderablyeasier toidentifymineralsinthisimage.

coNcluSioNS

Thecircularpolarization(CP)techniqueisausefuladdition to the petrographer’s toolkit. It is especiallyusefulforproducingimagesforillustrationandquan-tificationoftextures.Itiseasilyappliedtopetrographicmicroscopes for detailedwork and to scanners forimagesofwholethinsections.

ackNowleDGeMeNtS

IthankJ.M.Derochette,whodrewmyattentiontotheBenford plate technique.MickeyGunter andW.RevellPhillipsgaveclearreviews.MytexturalresearchissupportedbyaDiscoverygrantfromNSERCCanada.

refereNceS

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Glazer,a.M., lewiS, J.G.,&kaMiNSky,w. (1996):Anautomaticopticalimagingsystemforbirefringentmedia.Proc. R. Soc. London, Ser. A: Mathematical, Physical and Engineering Sci.452(1955),2751-2765.

GuNter,M.e. (2003):A lucky break for polarization: theopticalpropertiesofcalcite.ExtraLapis English4,40-45.

hiGGiNS,M.D.(2006):Quantitative Textural Measurements in Igneous and Metamorphic Petrology. CambridgeUniver-sityPress,Cambridge,UK.

olDeNbourG, r.&Mei, G. (1995):New polarized-lightmicroscopewith precision universal compensator. J. Microscopy180,140-147.

tarquiNi,S.&arMieNti,P.(2001):Filmcolorscannerasanewandcheaptoolforimageanalysisinpetrology.Image Anal. Stereol.20(Suppl1),567-572.

Received September 1, 2009, revised manuscript accepted November 26, 2009.