The Canadian Mineralogtst

THE CANADIAN MINERALOGTST

Journal of the MineralogicalAssociation of Canada

Volume 22 MAY 1984 Paxt 2

Canadian Mineralogist Yol.22, pp. 213-218(1984)

COLORSEM-IMAGING OF MINERALOGICALSAMPLES: SULFIDE ORES AND ZEOLITES

ARI ANTONOVSKY CSIRODivision of Chemicaland llood Technolog!,P.O. Box 56, Highett, Victoria3190, Australia MICHAEL J. SANDY GeologicalSurvey of PapuaNew Guinea,P,O. Box 778, Port Moresby,Papuo New Guineo

ABSTRAcT Ir.trRoDUCTIoN The proposed technique for mineralogical scanningelectron-microscopy(SEM) studiesutilizes back-scattered- The scanning-electronmicroscope (SEM) is find- electron and secondary-electronimages to form a com- ing wide application in mineralogical and geological posite image in color, as shown by such color imagesof studiesas relevantinstrumentation and techniques coarselyand finely-polishedsurfaces of sulfide ores and are developed(Blaschke 1976). The selection of unaltered zeolite minerals. Contrasts between minerals in signal detectorsused is crucial in determining the in- 11ts5ample, both in atomic number and in textural rela- formation obtainedfrom a sample,and a variety of tionships, are visible in theseimages. The ability to iden- detectorsmay be utilized in the examinationof a par- tify phasesand artifacts is improvedby inclusion of three (Northrup method is describ- SEM signalsin oneimage. The useof color enhancesboth ticular sample 1972).A aesthetic quality and detail of technical illustrations. ed here f61 ulilizing the increasedcapacity of color media to presentinformation obtained from several Keywords: scanning-electronmicroscopy, composite im- detectors,notably the secondaryelectron (SE) and age in color, sulfides,zeolites. back-scatteredelectron (BSE) detectors,in a single image. In mineralogical work the SE, BSE and X-ray Soutrtannn signals are cornmonly utilized for the imaging of samples(Green et al, 1979).A detector of secondary On combine, en microscopie dlectroniqued balayage electrons(Everhart & Thornley 1960)produces an (MEB), les imagesproduites par les 6lectrons rdtrodisper- sample morphology, and this is s€set les €lectronssecondaires pour former une seuleimage image that shows conposde, en couleur. On prdsentede telles imagesde sur- useful for examining the structure of rocks in an faces grossi0rementou finement polies de minerai de sul- unaltered stafe @ull 1978). A detector of back- fures et de z€olites inalt6r6es. Les contrastes entre mine- scattered,electrons {Robinson 1975, Lin & Becker raux d'un m€me 6chantillon, tant en nombre atomique 1975)reveals differences in averageatomic number qu'en relations texturales, sont visibles dans ces images. (ZJ across a sanlrple,surface,typically a polished L'identification desphases (et d'objets insolites) est facili- surface of a rock sample (Stanton & Finkelrnan par tde I'utilisation de trois signaux MEB dans une m6me 1979). The BSE image has proved to be of con- image. Les illustrations rechniquesb€n6ficient de la qua- siderablevalue in mineral and geological studies lit6 esthdtiqueet de la r6solution am6lior6eque permet l,uti- (Robinson lisation de la couleur. where elementalinformation is needed & Nickel 1979,Hdl & Lloyd 1981),"In many cases,a (Iraduit par la R6daction) combination of signals provides much more information than any one si'gnal alone @kelund & Mots

Flc. l. Zeolitesfrom the coastlinenear Flinders, Victoria, Australia. Trapezohedra are analcite, radiating fibres are nalrolite. higher content of information in SEM images(Hayes near Daly River, Northern Territory, Australia. The et ql, 1969); color has been used to enhance SEM country rock consists mainly of metamorphosed techniquesin mineralogical studies, including X-ray greenschistose rocks. The mineralsobserved are, in mapping (Pawley & Fisher 1977) and order of decrezgingaverage atomic number: galena, cathodoluminescence(Obyden et ql, 1980). Actual sphalerite, chalcopyrite, pyrrhotite Fen-1So(z from colors in a specimencannot be reproducedby the 5 to 16) and actinolite, which is the ganguemineral. SEM. Therefore, the use of color can be directed The third sampleconsists of ore (Fig. 3) from a toward the presentationof compositionaland mor- massivesulfide deposit at the Woodlawn mine, 75 phological differences. km northeast of Canberra, ACT, Australia (Malone et al. 1975).It is finely banded and has been polish- MATERIALS ed only coarselyto allow for sometopographic variation. Present are pyrite, sphalerite, galena and In the presentstudy, SEM imageswere produced chalcopyrite,in order ofdecreasingabundance. Talc, from three samples;one has morphological interest chlorite and quartz ile present in the gangue. and two have compositional interest. The first is a zeolite sample (Fig. l) collected on a beach near MSTHON Flinders, Victoria, Australia (Vince 1980,Coulsell 1980).The specirnenconsists of weathered,highly The samplesof zeolite and sulfide ore were cut, vesiculargreen basalt. The outer rim of the vesicle and the ore samplespolished. After coating with car- containingthe zeolitesconsists.of magnesian chlorite. bon, they were examined in a Cambridge 54-10 Two zeolitesare.preseqt: analcite NaAlSi2O6.H2O Stereoscanmicroscope. The SE signal was detected occurs as regular trapezohedra, and natrolite with a positively biasedEverhart-Thornley detector, Na2Al2Si3O16.l.5H2O appearsas radiating, fibrous whereasthe low-collector voltage SE signal was ob- clusters(Saha 1959). tained with the samedetector operating at areduced- The second,an ore sample(Fig. 2), is finely polish- bias voltage. The BSE sipal was detestedby using ed to enhancedifferences in composition between four semiconductorqrystals (available from LeMont minerals present.It is a piece of drill core from a Scientific Inc., State College, Pennsylvania, USA) copper-zinc prospectof copper-rich massivesulfides arranged around 1[g ssiling of the specimen COLOR SEM-IMAGING OF SULFIDE ORES AND ZEOLITES 215

FIc. 2. Sulfide minerals from prospect near Daly River, Northern Territory, Australia. Color varies from dark green for actinolite to bright yellow for galena. Polished section. chamber. Other signalsaccessible from the SEM, topography. This can be seenin Figure l, in which e.9., X-ray or cathodoluminescence,could be utilized fibrous natrolite produced a strong SE (green) im- in forming the color image. age, whereas the polyhedral analcite produced a The stepsfollowed in producing the image are il- strong BSE (red) image. This occuned becausethe lustrated in Figure 4. The BSE, SE and low-collector SE signal tends to be relatively intense at edgesand voltage SE image were photographed sequentially at high take-off angles(Newbury 1975).However, through red, greenand blue filters, repectively, onto the yield of BSE is greatest in the direction of op- a single color film. Kodak Ektachrome transparen- tical reflections(Niedrig 1978),so that flat areasof cy film was used to facilitate processing (E-6 pro- the sample return the strongest signal to the BSE cess)and viewing. To compensatefor the phosphor detector positioned at the top of the specimen of tle record CRT, the strength of red exposureswas chamber. The two minerals present are clearly dif- increasedand that ofblue exposureswas decreased. ferentiatedin color on the basisoftopography alone. Contrast was reduced from that of a monochrome Whereasthey would remain distinguishableby mor- image for red and green exposuresand increasedfor phology in a monochrome image, the use of three blue exposures. The original monochrome images different SEM signalsaddsvisual inf,ormation. (Sub- can be extractedby rephotographingthe transparen- tle structural differences that are not clearly il- cy onto black-and-white film using the appropriate lustrated by a single rnonochromatic image may be filter. noticeable in a color micrograph.) The core sample from Daly River representsthe REsuLTsAND DlscussroN opposite case.Polishing removesmost topographical features, leaving differences in composition to pro- A variety of color imageswas obtained owing to vide most of the image contrast. (Such information the differing nature of the samplesin this study. In is derived from the BSE signal but, through color the caseof the zeolite specimen,very little composi- imaging, it is possible to netain SE information as tional variation was expected [^Z"" (analcite) = well.) As shown in Figure 2, the gangue mineral 20.22, Zo (natrotte) = 20.251;therefore, color dif- (green-black)is easily distineuishedfrom the sulfide ferences within the image can be related to minslsl5 becauseof a weak red iniage rculting from 2t6 THE CANADIAN MINERALOGIST

Frc.3. Sulfide depositfrom Woodland mine near Canberra,ACT. Coarselypolish- ed section. a) SE image showing topographic features resulting from differences in hardnessbetween minerals present. b) Image showing differences in atomic number between minsnls p1esen1.c) Color image derived from BSE and SE im- a6!es.The dust particles are clearly distinguishable.

its low Zu. As Zu, increasesfrom gangueminerals remains relatively even. The combination of red through to galena, the BSE yield increases,and so (BSE) and green (SE) forms areasof the imagerang- the red image,componentis enhanced"At the same ing from light green for pyrrhotite to orange for time the SEyield is relatively constant for a polish- sphalerite and yellow for galena. ed sample, and so the intensity of the green image Sometopographic contrasthween mineralscould COLOR SEM-IMAGINC OF SULFIDE ORES AND ZEOLITES 2t7

VIDEO CRT COLORCAMERA AMP FILTER DETECTOR - LCV-SESIGNAL f)--{ r-ffi^i E.R'HlJ!?i^{l'-'* V Ftc.4. Arrangementfor producing a colored image from three distinct SEM images.

be observed,particularly betweengalena, which is suchas that of the zeolites,in which morphology is relatively soft, and pynhotite which is relatively a primary concern,a color imagetends to be a more hard. The relationship betweenthe minerals (e.g., aestheticaly pleasingmedium for illustrative material the position of physicalboundaries relative to com- for technical presentation. Particular features ofin- position gradients and boundaries)is more easily terest are easily highlighted, though other features determinedwhen physical information is included of lessinterest, e.9., pits, dust particlesand specimen in the micrograph concurrentlywith compositional charging, may also becomemore apparent. information. The colors in the SEM image cannot be made to Topographic contrast was also observedin the correspondwith natural coloration. However,with ore samplefrom the Woodlawn mine @ig. 3). In- a greater capacity for display of information and completepolishing during preparationintroduced a detail, the colored picture is a meansof extending topography based on relative hardness of the the versatility of the scanning-electronmicroscope minerals present.The dark regions in the secondary- in mineralogicalresearch. electron image (Fig. 3a) consist of three distincr phasesdistinguishable by topography: a soft gangue mineral (chlorite) that had been eroded during polishing, a hard ganguemineral (quartz) that had RSFERENcBS not beeneroded, and foreign particlescollected on BLAscrKE, R. (1976): the surface.leeatized chargingwas observed where Signals excited by the scanning bean. In Electron Microscopy (H.R. the conductive coating in Mineralogy was insufficient (the bright Wenk, ed.). Springer-Verlag,Berlin. greenobjects indicate charging of material not part of the sampleat the time of coating, e.g., dust par- Buu, P.A. (1978): Observationson small sedimentary ticles.) The BSE image (Fig. 3b) distinguishedthe quartz particles analysed by SEM. Sconning Elec- sulfide minerals from the gangue and from each tron Micros, 197E, 821-826. other by their relative brightness,which is dependent on atomic number. Thesedetails were more easily Coumerl, R. (1980): Noles on the zeolite-collecting perceivedin the compositeimage in color (Fig. 3c), area of Flinders, Victoria, Australia. Aust. Mineral. 33,163-167. in which the six mineralspresent, dust and charging could all be observed. EKELUNo,S. & WrnLsFoRs,T. (197Q: A systemfor the quantitative characterization of microstructures by CouctusroNs combined image analysis and X-ray discrimination Considerable information for mineralogical in the SEM. Scanning Electron Micros, 1976, 4r7-424. studiesis provided by the SEM through different imaging signals.A color film can effectively store three El'nnuanr, T.E. & TuonrLev, R.F.M. (1960):Wide distinct images of a single sample, which can be in- band detectorfor micro-micro€unperelow-energy dividually retrieved by photographic means.A com- electroncurrents. J. Sci.Instrum. 37,24G248. posite of BSE and SE imagesin one color micrograph illustrates both compositional and textural relation- GnreN, D.A., DnNnr, P.B. & Fnrornrcrsot, R.G. ships betwe€nthe minerals within a sample.In rocks (1979):The application of heavy metal staining exhibiting large differences in atomic number be- (OsO) and BSEI for detectionof organicmaterial gas tween constituents,such as sulfide ores, material and in and oil shales.Scanning Electron Micros. t979,495-5N. topographic variations (either naturally occurring or induced by preparation methods) shown vrithin a Her.l, M.G. & Lr-ovo,G.E. (1981):The SEM ex- single micrograph serve to distinguish one mineral aminationof geologicalsamples with a semiconduc- from another. These variations can be perceived tor back-scatteredelectron detector. Amer. Mineral. more easilythrough a color image. In other cases, 66,362-368. 2t8 THE CANADIAN MINERALOGIST

Heres,T.L., GleEsER,P.M. & PewLev,J.B. (1969): PewLrv, J.B. & Ftsuen, G.L. (1977): Using Information contentof EM images:color and deptl simultaneousthree-colour X-ray mapping and information as vehiclesfor subjectiveinformation digital-scan-stopfor rapid elementalcharacteiza' transfer. Proc. 27th Ann. Meet., Electron tion of coal combustionby-produds. J. Micros, ll0, MicroscopySoc, Amer., 4lG4l l. 87-101. Lrr, P.S.D.& Bncxnn,R.P. (1975):Detection of BSE Roanrsol,t,B.W. & Nrcrrl, E.H. (1979):A usefulnew with high resolution.'ScanningElectron Micros, technique for mineralogy: the backscattered- 1975.61-70. electron/low-vacuummode of SEM operation. Amer. Mineral. 64' 1322-1328. Meror.n,8.J., Or,csns,F., CucsHI,F.G., Ntcnores, T. & McKev, W.J. (1975):Woodlawn copper-lead- RosnlsoN,V.N.E. (1975):Backscattered electron im- zinc deposit./n EconomicGeology of Australia and a$ing. ScqnningElectron Micros. 1975' 51-60. PapuaNew Guinea.1. Metals(C.L. Knight, ed.). Austral. Inst. Mining Metall., Mon. 5,701-710. Sana,P. (1959):Geochemical and X-ray investigation of natural and syntheticanalcites. Amer. Mineral, Nrwnunv,D.E. (1975):Image formation in the SEM. 44,300-313. .Irr Practical ScanningElectron Microscopy(J.I. Goldstein& H. Yakowitz,eds.). Plenum, New York. SreNror, R.W. & Ftxru-uen, R.B. (1979): Petrographicanalysis of bituminouscoal: optical Nrcomc,H. (1978):Physical background of electron and SEM identificationof constituents.Scanning backscattering,Scanning l, 17-34. ElecffonMiqos. 1979' 465-470, NonrHnur,D.C. (1972):Interaction of electronswith VrNcB,D.G. (1980):Zeolites and other mineralsin solids. /z The Use of the ScanningElectron vesiclesin the NewerBasalt of the Melbournearea, Miooscope(J.W.S. Hearle, J.T. Sparrow& P.M. Victoria.Awt. Mineral.32, 155-161. Cross,eds.). Pergamon, Oxford. OayDBl,l,S.K., Separlr, G.V. & SrtvaK,G.V. (1980): Observation of long persistenceluminescent materials using colour TV SEM. Sconning 3' Received November 8, 1982, revised manuscript ac' l8l-192. cepted May 18, 1983.