Optical Anisotropv of Cuprite Caused by Polishing

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,canalisationdes 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 as in a) after a "Mastermet" finish; isotropic. c) (1 11) sectionof a cuprite octahedronfrom Comwall after a "Mastermet" finish. d) (100) sectionof a cuprite cubeftom Banat.Romania after a'Mastermet" finish.

the grainscould be observed(except for the changing Even with delicateadjustments of the microscope red internal reflections).ECP imagesrevealed that all (polarizers slightly uncrossed,daylight filter diamond-polishedsamples had an extremelydeformed removed),anisotropy was not observed.The weak surface-layer,i.e., an ordered,cubic crystalstrucfure bireflectanceof the bulk sampleshad disappearedas could not be observedat the surface(Fig. 1a). Since well. However, the grain boundariesin the bulk the informationof the electronchanneling pattern samplesof cupritewere cleady visible owing to the stemsfrom the uppermostsurlace layer (about40 nm enhancedrelief causedby the 60 minutes of thick in cuprite), knowledgeabout further propagation 'oMastermet"finish. This chemomechanicalmethod of the deformationcannot be obtained.Also, differ- of polishing even producedperfect, scratch-free encesin the intensity of deformationbetween random surfacesin the very soft inclusionsof nativecopper in orientationsand the (111)and (100) sections could not the samplefrom Siberia.Besides, an experimentwith be verified.A summaryof observationsis presentedin only 30 minutesof the "Mastermet" finish in the case Table l. of the bulk samplefrom Siberiaresulted in very weak, On the other hand,all samplesthat had been but still visible,anisotropy effects. ftnished with the "Mastermet" procedurefor 60 min- ECP imagesshow a perfect, undisturbedsurface in utesproved to be isotropicunder the microscope. all samples subjected to 60 minutes of the

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/32/2/353/3446719/353.pdf by guest on 27 September 2021 356 TIIE CANADIAN MINERALOGIST

TASLE 1. SU!'MABY OF OBSEBVATIONS ON CUPBITE 1990).Even in an investigationof the influenceof the different methods of polishing on tfie reflectance of Ssuple oprioal sElit silicon (Pauly 1986),one finds only mechanical dGcctfrdo! bobavtor study methodscompeting for the ultimate result. Polishing with alkaline silica solutionsis usually only described Cupdte bulk &qr!€f stlh lanaloEly ulsotlopto no EcP in the literature on material sciences preparation odated gmtru, [e.g., steDalsrd tptlsh of electrical and optical materials:Fynn & Powell Standsld Boltshed cprlts, (111) boflgptu rc ECP (1988),specimen preparation for the SEM: Lloyd (1937)1,and it is known to provideperfect, undis- Stariard pollshed oprlta, {100) i8otlopto ,to ECP turbed surfaces,even of soft materials.Since this Cuprlt€ bulk eEples wtth mdoDly lsouoplo porfeot, methodworks with colloidal nMiatolBstt silica dispersedin a ortsnt€d 6nl$, fnrrh shalp ECP mediumof pH = 9.8,it may be possiblethat chemical- nlilaat€ret' fralshsd opdto, (1U) &otlst,fo padeot, ly sensitivematerials are affectedby etching,dissolu- shar'.I' ECP tion, etc. This phenomenonwas observedin tle case 'ltagtsmetn fld8hsd aupltte, (100) tsotroplc pedst, of aurostibite,AuSb2, which shows an almost perfect sher? ECP surfaceafter the standardpolish, but which became considerablyanisotropic and in which the surface Cupr.tte bulk saExrte, tlln seodou leotlot ls becamedeformed after the ooMastermet"finish (Libowitzky L994a).Consequently, since polishing Ecps elerc(ron ohqnnEltng Fattsm. with alkalinesolutions is not alwayssuccessful, resulting surfacesshould alwaysbe examinedby ECP or other surface-sensitivemethods. Of course,the "Mastermet"finish cannotbe appliedroutinely to "Mastermet" procedures.The patternsof a random sectionsfor ore microscopy,since this will result in sectionofcuprite (Fig. lb), a (1ll) sectionofan ocra- high relief, which is not desirablefor studiesof ore hedral crystal from Cornwall (Fig. lc), and a (100) parageneses.But the methodshould be consideredfor sectionof a cuprite cube from Banat,Romania specialpurposes such as quantitativemeasurements of (Fig. ld) show sharp zonesthat reflect the symmetry reflectanceon ore minerals,and other investigations ofthe respectiveface. The picturesare only disturbed (like the presentone), which require a structurally by a few black spotsand lines that stemfrom holes or undisturbedsurface. grain boundariesin the surfaceof the polished Why do mechanicalmethods of polishing cause sections.Differences in the quality of tlre surface of anisotropyeffects in cuprite? One might well suggest different sectionscould not be observed. that deformationof the surfacelayer of a crystal will A final investigationof two thin sectionsof the result in a randomly distorted,isotropic surface- bulk samplefrom Siberia also confirmed that cuprite structure.But since the mechanicalproperties of a is cubic and henceisotropic: with crossedpolarizers, crystal face (e.9., scratchhardness) mirror the crystal the samplesappear dark; the extinction doesnor symmetry, and since the deformation causedby changeupon rotation of the stage. mechanicalpolishing conformsto the mechanical propertiesof that face, there is strong evidencethat DlscussroN randomly orientedcrystal f.aces(hkl) show an anisotropic deformationof the surface,thus resulting The results of the presentwork clearly prove that in optical anisotropyeffects. This also explainswhy the real optical behavior of undeformedcuprite is the (l1l) and (100) sectionsof cupriteappear iso- isotropic,and hence consistent with its cubicstructure. tropic. In spacegroup Pn3rn,the (11 1) face has a Solely mechanicalmethods of polishing (like careful three-foldaxis of symmetry(3m), andthe (100) face diamond-polishing)can causesevere damage in the has a four-fold axis of symmetry (4mm). Both the surfacestructure of solid materials,which may result threefold and the fourfold symmetry preclude aniso- in unexpectedand misleadingeffects. In the caseof ropic optical phenomena,even if the surfaceis cuprite, theseeffects are responsiblefor the strong deformed according to the symmetry of the face. In anomalousanisoropy. Polishingwith alkaline silica pyrite (Stanton1957, Libowitzky 1994a),which solutionsavoids or removessurface deformations and belongsto a lower space-group(Pa3), or.ly the (111) allows the study of the undisturbedstructure of the face (face symmetry3, trimetric) is isotropic, whereas surface. the (100) section (face symmetry 2mm, drsymmetic) Concerningchemomechanical polishing with is considerablyanisotropic. In the spinel grolp, Fd3m stronglyalkaline media,there seems to be only limited (Libowitzky 1994b)the isotropicfaces are (111) and knowledge of this method in the literature on ore (100)as in cuprite,which stronglyconfirms the rule of microscopy,even though the topic of sampleprepara- mechanicaldeformation of the surfaceconforming to tion is dealt with on a broad scale(e.g., Laflamme the face symmetriesof a crystal.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/32/2/353/3446719/353.pdf by guest on 27 September 2021 OPTICAL ANISOTROPYOF CI,?RITE 357

The deformedlayers of the cuprite surfaceobvious- made on spherically ground crystals.It seemsworth- ly do not representa new oosingle-crystalphase"; while to check for the possibleinfluence of deforrned otherwise,the ECP imageswould show an ordered surface-layerson the observedX-ray intensitiesof structure.Rather, the unstructuredECP images of cuprite.However, surface deformations also may be standardpolished cuprite indicate that the deformed deleteriousto orientation contrastimages with the surface-layeris composedof extremelysma1l domains SEM (-loyd 1987)and to the preparationand investi- of cuprite in different (but not o'random".as this might gation of sensitivesemiconductor materials (Fynn suggest"isotropic") orientations. & Powell 1988).Consequently, surface-sensitive A more difficult questionconcerns the high inten- methodslike ECP shouldbe usedto ensurethat the sity of the optical anisotropyeffect. Whereas material itself is investigatedrather than the disturbed anomalousanisotropy effects are weak and not always layersat the surface. visible in most cubic ore minerals (Klemm 1962), the effect can alwaysbe observedwith high intensityin ACKNOWLEDGEMENTS cuprite. The only minerals describedby Klemm (1962)as considerablyanisotropic without exception The helpful discussionswith A. Beran and are cuprite and cobaltite. The latter has been refined J.Tnmannare greatly appreciated. G. Giesterprovided in spacegroup Pca2y @leet & Burns 1990),but all the sampleof syntheticcuprite. The commentsof crystallographicwork on cuprite has confirmed the J.C. Rucklidgeand M.E. Fleet, who kindly refereed cubic symmetry(Restori & Schwarzenbach1986), the manuscript,helped to improvethe paper. even though Klemm (1962) suggestedthat it might deviatefrom cubic symmetry.One possibleexplana- RSFERENcES tion for the strong anisotropymight be that cuprite is extremely brittle. Thus mechanicaldefects propagate AcRANovIcH,V.M. & GtNzsunc,V.L. (1966):Spatial easily into the upper layersof the surface.It is Dispersion in Crystal Optics and the Theory of Excitons. interesting to note that similar, but weaker effects Wiley-Interscience,London. of anisotropyoccur in standardpolished spinel- (1915): group minerals (chromite,magnetite, franklinite; Bnaco,W.H. & Bnacc, W.L. X-Raysand Crystal Bell andSons, Ltd., London. Libowitzky 1994b),as well as in standardpolished Structure. sectionsof pyrite and sperrylite (Libowizky 1994a). & SraNLsv,C.J. (1986):The One common feature among these minerals is the Cnrpot-s,.A.J. Quantitative Data FiIe for Ore Minerak. IMNCOM. British Museum ratler low amount of metallic bonding in their struc- (Natural History), London. tures. even wherethese minerals conform to the usual o'ore mineral" propertiesand they absorbvisible light Frrrsov, G.V. & ZHuKov,S.G. (1992):Influence of the to a considerableextent. Another feature is their poor specimenpreparation method on the results of X-ray behavior during grinding: even soft grinding produces structural analysis.Sov. Phys. Crystallogr. 37(4), a considerableproportion of holes in the surfaceof 461465. thesematerials. One differencebetween cuprite and pyrite or spinel is that cuprite showsa cleavagealong Fr-rE'r,M.E. & BuRNs,P.C. (1990): Structure and fwinning of (111),whereas the latterdo not. Anotherdifference is cobaltite.Can. Mineral. ?A,1 P-723. that cuprite is ratherweak in comparisonwith the hard mineralsof the pyrite and spinel groups. FyNN,G.W. & Powell, W.J.A. (1988): Cutting and The anomalousanisotropy effects have also been Potishing Optical and Electonic Materials (2nd ed.r. attributedto second-orderphenomena of spatialdis- Adam Hilger, Bristol, U.K. persionin cuprite(Agranovich & Ginzburg1966). On the other hand.these authors claimed that theseeffects GIESTER,G. (1992): Synthesesand crystal structuresof gr. 201, are usually small and negligible in classicalcrystal TlCu(OH)SOaand TICu(OH)SeO a. Z. Kristallo optics.Only in the caseof anomalouseffects like 59-67. anisotropyin cubic crystals, which cannot be P.R. (1989): Introduction to Practical Ore explainedby other optical theories,can spatial disper- INEsoN, Microscopy.Longman, London, U.K. sionbe consideredas a causefor visibleeffects. _#tx?;"lllxTll;1"tri,TT,H:.:,fi:ffi::i': *,.J*,,*l;"t';:),j:;ir";,i::::f:,r;,5;,5:?it*:" gations, but also to other fields of research on crys- of talline materials, like in X-ray structure analyses LAFLAMME,- J.H.G. (1990):The preparationof materialsfor spherically ground single crystals (Fetisov & Zhukov .i"r*.opi" stuiy. /n Advarrced^Microscopic Studiesof 1992).This is of specialinterest, since the most recent Ore Minerals (J.L. Jambor& D.J. Vaughan,eds.). refinementofcuprite,whichfocusedoncalculationof Mineral, Assoc.Can., Short-CourseHandbooklT, charge density (Restori & Schwarzenbach 1986), was 37-68.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/32/2/353/3446719/353.pdf by guest on 27 September 2021 358 THE CANADIAN MINERALOCIST

Llnowtrzrv, E. (1994a):Anisotropic pyrite: a polishing REIMER,L. & PrrrrunKoRN,G. (1977): Rasterelek- effect.Phys. Chem.Minerals (in press). tronenmikroskopie(2nd ed.). Springer, Berlin, Germany.

(1994b):Optical anisotropy in rhe spinelgroup: a REsroRI,R. & ScswenzeNBAcH,D. (1986):Charge density polishingeffecL Eur. J. Mineral.6,187-194. in cuprite,CurO. Ac ta Cry s tallo g r. B.42,201 -208.

Lt-oyo,G.E. (1987):Atomic numberand crystallographic SCHNETDERHOHN,H. & RauooHn, P. (1931):Lehrbuch der contrastimages with the SEM: a reviewof backscattered ErTmikroskopie.Gebri.ider BornfAger, Berlin. electrontechniques. Mineral. Mag. 51,3-19. SrANroN,R.L. (1957):Studies of polishedsurfaces of pyrite, Per.n-y,H. (1986):Effects of differentpolishing merhods on andsome implications. Can. Mineral.6, 87-118. the reflectanceof silicon. TschermaksMineral. Petrosr. Mitt.35,26l-273. UYTENBoGAARDT,W. & BURKE.E.A.J. (1971): Tabtes for Microscopic ldentification of Ore Minerals. Dover PEcKEr,A. (1992): The Colour of OpaqueMinerals. John PublicationInd., New York. Wiley & Sons,Chichesler, U.K.

Ptcor, P. & Jouex, Z. (1982): Atlas of Ore Minerals. Elsevier.New York.

RAMDoHR, P. (1975): Die Erzminerale und ihre Verwachsungen (4th ed.). Akademie-Verlag, Berlin, Received May 7, 1993, revised manuscipt accepted July 14, Germany. 1993.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/32/2/353/3446719/353.pdf by guest on 27 September 2021