353 The Can adian M ine ralo gi sr Yo1. 32"pp. 353-358(1994) OPTICALANISOTROPV OF CUPRITE CAUSEDBY POLISHING EUGENLIBOWITZKY Instintfi)r Mineralogieund Kristallographie der UniversitdtWien, Dr. Karl Lueger-RingI, A-1010 Wien, Austria ABSTRACT It has long beenwell known that polishedsections of cuprite, CurO, spacegtoup Pn3m, show stronganomalous anisofiopy underthe microscope.Optical investigationson bulk samples,as well as on orientedsingle crystals,confirm that all diamond- polishedsections except (1 I 1) and (100)are anisotropic. Electron channeling pattems obtained on a scanningelectron micro- scopereveal that thesemechanically polished surfaces of cuprite are always extremelydeformed. Consequently, an altemative chemomechanicalpolishing procedure with alkalinesilica solutionswas applied,and resultedin isotropicsections without exception.These samples then showedan undeformedsurfaceJayer in the electronchanneling image. Thus, the optical behaviorof cuprite correspondsto that expectedof a cubic crystal-structue. Keywords: cuprite, optical anisotropy,polishing methods,surface analysis, SEM methods,electron channeling pattem (ECP) images. Sot{N4ArRE Il est clair depuislongtemps que les sectionspolies de cuprite,CurO, $oupe spatialPn3m, observ6esau microscope montrent les effets d'une forte anisotropie.l,es 6tudesoptiques faites sur 6chantillonsmassifs, aussi bien que sur cristaux uniquesorient6s, confirment que toutes les sectionspr6par6es avec pdte de diamantsauf (1 I 1) et (100)sont anisotropes. Une 6tudi effectudepar microscopie6lectronique i balayageavec canalisationdes 6lectronsr6vdle que les surfacesde cuprite poliesm6caniquement sont inmanquablementfortement d€form6es. C'est ce qui a motiv6 le d6veloppementd'une m6thode alternativede pr6parerles sections.I1 s'agit d'un protocolde polissageavec solutions siliceuses alcalines. Les sectionsqui en rdsultentcontiennent la cuprite tr 1'6tatisotrope sans exception. La couchede surfacedans ces cas est sansd6formation, selon l'imageobtenue par canalisationdes 6lectrons. C'est donc dire quele comportementoptique de la cupritecorrespond tout.a fait d sa sffucturecristalline cubique. (Traduit par la R6daction) Mots-cl6s:cuprite, anisotropieoptique, mdthodes de polissage,analyse de surface,microscopie 6lecuonique d balayage,cana- lisationdes 6lectrons. INrnooucuox paperson ore microscopycomment that anomalous anisotropy in cuprite is common (Pecket 1992), and The anomalousoptical anisotropy noted in polished that it can be usedas a diagnosticfeature, even though sectionsof cupritehas beendescribed by many the effect is unusual,as the mineralis cubic (Ineson authorssince the early days of ore microscopy. 1989). Schneiderhtihn& Ramdohr(193 1) and Ramdohr Klemm (1962)investigated several cubic but opti- (1975) statedthat all investigatedsamples of cuprite cally anisotropicore-minerals. He found all samplesof are considerablyanisotropic, that the colors of cuprite to be distinctly anisotropic.He suggestedthat anisotropyare ink-blue to olive-green,and that the cuprite might in fact depart from cubic symmetry. effectcould be befterobserved in air thanin oil owing Criddle & Stanley(1986) publisheda set of to the intenseinternal reflectionsin oil immersion. reflectancedata for cuprite (with minimum and maxi- Uytenbogaardt& Burke (1971) claimedthat cuprite mum values),which showeda bireflectanceof up always showsstrong anomalous anisotropy and that to 2Vo.Tlte strongly anomalousanisotropy was men- the colors of anisotropyare $ey-blue to olive-gteen. tionedas well. Picot & Johan(1982) wrote that in spite of its cubic The cubic structure of cuprite (Pn3m) was symmetry,the mineral is distinctly polarized and that determinedby Bragg & Bragg (1915).Even if some the anisotropyshows green to blue tints. Even recent hemihedrallyshaped crystals suggesteda lower cubic Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/32/2/353/3446719/353.pdf by guest on 27 September 2021 354 TIIE CANADIAN MINERALOGIST symmetry,physical experiments (etch figures, lack of (1988) for preparationof sectionsof sensitivesemi- optical activity) confirmed the holohedralstructure conductors. (Bragg& Bragg1915). A recentrefinement ofcuprite, In addition, two thin sectionsof cuprite from which also confirmedspace group Pn3m, was given Siberiawere prepared to checkfor anisotropyin trans- by Restori& Schwarzenbach(1986). mittedlight. optical investigationswere peri'ormed on The presentwork was initiated by the extremely a Reichertore microscopeequipped with a halogen contradictoryresults of optical and structuralinvesti- lamp and several objectives. As the anisotropy of gations of cuprite. Moreover, since the author cupriteis verystrong, no specialsettings of themicro- investigateda similar problem on pyrite recently scopewere necessary. The polarizerswere slightly (Libowitzky 1994a),a polishingeffect was suspected uncrossed,and the blue daylightfilter wasremoved, to as the causeofthe anomalousoptical anisotropy of intensifytheopticaleffectsforatestoftheisotropyof cuprite. the (111) and (100) sectionsas well as of the "Mastermet"fi nishedsamoles. Exppnnanvrer_ Electron channelingpattern (ECP) images were recordedon a JEOL JSM-6400 scanningelectron A syntheticpolycrystalline sample of cupritepro- microscope(SEM) at 30 kV and 8 mm distancefrom videdby Giester(1992), a polycrystallinebulk sample the objectivelens. Since cuprite is a semiconductor, of cupritefrom Siberia,as well as orientedsingle sampleswere not carbon-coatedbut only enclosed crystalsof cupritefrom Cornwall(octahedra)" Siberia with a carbonsuspension for electricalconnection to (octahedra)and Banat, Romania(cubes), were em- the sampleholder. All ECP imageswere recorded beddedin cold epoxy resin. They were carefully with a semiconductordetector of back-scatteredelec- groundon abrasivepaper, mesh 1000, until a smooth, trons.The rockingangle ofthe beamwas about120o, scratch-freesurface was obtained.Two methodsof the analyzedarea was about 0.2 mm diameter.The polishingwere applied. ECP gives a picture of the crystallographiccontrast of In the standardprocedure (simply calledthe "stan- the uppermostsurfaceJayer. The depth of penetration dardpolish"), the groundsamples were polished with (and information)depends on the atomic numberof 9, 6, 3, and 1 pm diamondpastes on a nylon polishing the materialinvestigated; in cuprite,it amounlsto cloth usingpolishing oil as a lubricant.The final step about40 nm. A descriptionof the ECP techniqueis in polishing was performedwith 0.25 pm diamond given by Reimer& Pfefferkom(1977) andby Lloyd pasteon Microcloth@(Buehler Ltd.). Sampleswere (1987). held by handpressing them gently againstthe rotating A chemicalanalysis by meansof a LINK energy- disk of a polishing machine.Every step of the dispersionX-ray analyzerconnected to the SEM procedurewas applieduntil scratchesand other showedthat the samplesof cuprite contain no foreigtr visible deformationson the surfacehad beenremoved elementsabove detection limits 0.1 \tt.Eo). (usually5 to 15 minutes).At present,this "standard polish" procedureis the accreditedmethod to prepare RssuI-rs sectionsfor opticalmicroscopy (Laflamme 1990). Ir is also known to be the safestmethod to achievemirror- Since optical anisotropyhas been describedas a like surfaceson nearly any material (Fynn & powell generalfeature of polished sectionsof cuprite (see 1988).Nevertheless, as the presentinvestigation and above),only two diamond-polishedbulk samples the papersof Libowitzky (1994a,b) show, surface (from Siberiaand the syntheticone) were examined deformationscannot be avoided if mechanical under t}te microscope.In almost all grains,the distinct methodsof polishingonly areused. to stronganisotropy, with blue to greencolors, was The successfulprocedure ofpolishing by which the verified, even where the anisotropyeffects interfered structureof the surfacewas not deformed,because with intensebut variableinternal reflections. A very deformedsurface-layers were removed,was as fol- weak bireflectancecould be observedat the contactof lows:The Microcloth@lined bowl of a smallpolishing somegrains with obviously different orientations.This machine(Buehler Ltd.) was filled up to about5 mm effect has been quantified with a set of reflectance with an alkalinesilica solution(Mastermet@, Buehler valuesby Criddle& Stanley(1986). The samplefrom Ltd.). The previously"standard polished" samples Siberiaalso showednumerous tiny inclusionsof were polishedin this bowl, with minimum pressure native copper,which appearedscratched and rough, applied for at least 60 minutes. Finally, they were thoughthe sampleshad beenpolished with 0.25 pm flushedthoroughly with sufficient water to avoid crys- diamond. tallization of the very concentrated,alkaline polishing The diamond-polished(lll) and (100)sections of agent.This technique(which will be simplycalled the the orientedsingle crystalsproved to be isotropic "Mastermet" finish) combineschemical and soft without exception.On rotating the stage,even with mechanicalabrasion. It was recommendedby Lloyd slightly uncrossedpolarizers and daylight filter (1987)for SEM applicationsand by Fynn & Powell removed,no variationin t}te color and briEhtnessof Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/32/2/353/3446719/353.pdf by guest on 27 September 2021 OMCAL ANISOTROPYOF CTIPRITE 355 Fro. l. ECP imagesof cuprite. Operaringconditions: 30 kV, objective distance8 mm. a) Standardpolished sample of cuprite from Siberia; optically anisotropic.b) The same,randomly orientedsample