CHAPTER 2 THE PREPARATION OF SAMPLES FOR ORE …

1 CHAPTER2 THE PREPARATION OFSAMPLESFOR preparationof polishedsurfacesfree fromscratches, fromthermalandmechanicalmodificationofth esamplesu rface,andfromreliefis essentialfortheexamination,identificatio n,andtexturalinterpretationof pre-paredonmanytypesofmaterials withrelativelylittle effortusinga widevarietyofmechanicalandmanualprocedur es. However, oresamplesoftenpresentproblemsbecausethe y mayconsistofsoft, malleablesulfidesor evennativemetals intimatelyintergrownwithhard,andsometime sbrittle, sili-cates,carbonates, oxides, orothersulfides. Weatheringmay co mplicate theproblembyremovingcementsandinterstiti almineralsandbyleavingsa m-ples thatarefriableorporous. Delicatevug fillings alsocauseproblems withtheiropenvo idspacesandpoorlysupportedcrystals. Alloysandbeneficiationproductspresentthe irowndifficultiesbecauseofthe presenceofadmixedphasesofvariablepropert iesandfinegrainsizes. In thischapter,thegeneralproceduresofsample selectionandtrimming, casting, grinding,andpolish-ing(and,inspecialcase s,etching)requiredtopreparesolid or particulatesamplesforexaminationwith the ,andalsopolishedanddoubly polishedthinsec-tions,whichareusefulin thestudyoftranslucentorcoexistingopaquea ndtranslucentspecimens,is described.

2 Thesearemattersaboutwhichall stu-dentsofreflected-lightmicroscopyshou ldbeaware, the unevensurface of a sectio n resulting fromsoft phase sbeingworna way morethanh ardphasesduring polish - - - - - - - - - - - - - - - - - ---- PREPARATION OFPOLISHEDSECTIONST heproceduresinvolved in thepreparationofpolishedsectionsvary indetailfrom onelaboratorytoanother, dependingonboththe type ofmaterialto bestudiedandtheactualobjectivesofthe study. A typicalsequenceofsteps isoutlinedin describedingreaterdetail Initial PREPARATION and Casting of SamplesCoherentmaterialsare easily cut to size with sawbladeby water, kerosene,or acuttingoil topreventheatingofthespecimenduringcutti ngis suspectedofcontainingphasesthatareeither soluble( ,metalsulfatesorchlorides)or reactive withwater ( ,some rare sulfides inmeteorites),it will benecessaryto conductallcuttingandsubsequentoperations usingappropriatenonreactivefluids (oils,ORESAMPLECUTTING,TRIMMING&GRINDING ON3011mMILLPRODUCT/CONCENTRATEMATERIALDI SPERSEDONBASE-PLATEMOUNTING& BYHANDORVIBRATORYPOLISHERFIGURE Flowsheetoutliningthe steps in the prep ~.)

3 ~~--~--PREPARATIONOFPOLISHEDSECTIONS19al cohols,kerosene,etc.). In allcases,it isnecessarytoremovecuttingoilsandwateran dto mayreact with the ,in time,bleedoutontothesurfaceandcreateseri ousprob-lems inconductinganalysis wherehigh ndshapeofthesampleto bemountedis so mewha tarbitraryandmustbedictatedby thenatureofthematerialandtheintendedstud y:mostworkersfindthatcircularpolishedsec tions from to5 em be inone-ortwo-piececylindricalplas-tic,pol yethylene,ormetalmolds( ) are used. athincoatingofwaxorothernonreactivelubri catingagent( siliconstopcockgrease. orvaseline)is extremelyhelpfulin theremovalofspecimensoncethecastin a the totalthicknessofthepolishedsectiongenera lly neednot exceed1-2 oftengovernedby the workingheightb etweenthemicroscopestagea nd the eci men iscoherentandhasa lowporosity, it may bec ut to thedesiredsizea n ,h owever. specime nsare cast inamountingresinto facilitatehandlingandtominimizetheproble msof' flatsurfaceonthespec ime n by cut-tingandgrindingpriortocastingis avery useful allows thesa m p le to lie flat on thebottomofthemold; irregularspecimens may trapairbubblesandrequireextensivegrindin gofthepolishedsection useful to bevel the edgesofthesu rface to bepolished:thishelps topreventmaterialbeingdislodgedfrom.

4 Ortrappedbetween,thesampleandmountingmed ium. Also. it may be desirableto cutandmountasa m ple inspecificorientations, becauseplucking(thetearingofmaterialfrom thesurfaceduringpolishing)may occu friable,fractured,porous, orcon-sistsofloosegrainsorpowders, impregnationis a usefulproceduretopreventgra readily accomplishedbyplacingthemoldcontainingth especi-B~--~ . ~~.FIGURE used for thecastingofcylindricalpolishedsections. (A)Two-piece cup. (B) Plasticringthatbecomes aportionof thepolishedsection afteremplacementof the sample and OFPOLISHEDSECTIONS menandmountingresinundervacuumfor severalminutes;this drawsairfrom the voids,andthesubsequentreleaseofthevacuum tendsto drive themountingresinintothe drive the resin into importantthattheproceduresinvolvedinspec imenpreparationpre-serve ( ,anilite,CU7S4,anddjurleite, )breakdownbelow100 Candthatmanyotheroremineralshaveunknownt hermalstabilitiespointsto thenecessityofusingcold-settingmountingm edia,requiringneitherheatnorpressureforp repara-tion.

5 Olderproceduresthatrequireelevatedtemper aturesandhighpressure( , the useofbakeliteas amountingmedium)andthatare stillcommonlyused ( ,chromite,magnetite,pyrite)but mayprofoundlymodify lowtemperatureorhydrousmineralassem-blag es. Evenheatingforbriefperiodsattemperatures as low as 100 C willcausedecompositionofsomephasesandind ucetwinningorexsolutioninothers. Accordingly,virtuallyallmineralogicallab oratoriesnow use cold-settingepoxy resins orplastics; epoxy resinsaretranslucentandnearlycolorless, whichfacilitateslabeling,becausepaperlab elscanbe set in the resin with insam-plesbetterthanhigher-viscosityones -animportantpointwhenoneis deal-ing with friable also to be used forelectronmicroprobeanalysis(seeAppendi x3) orsomeotherhigh-vacuuminstru-ment,onemus t becarefultochoosea resinthatdoes Grinding and Polishing of SpecimensOncea samplehasbeencut to anappropriatesizeandcastin amountingmedium,it orvariable-speedrotarylapequipmentdesign edto prepareonesampleat a anylaboratory.

6 Experiencedworkersinvariablydevelopfavor ite"tricks"andminormodificationsbutfollo w to removesurfaceirregularities,toremovecast -ing resinthatcovers thesample, toreducethickness,topreparea smoothsur-face forfurtherwork,andto remove anyzoneofmajordeformationresultingfromin itialsamplecutting( ).Manyworkershavefoundthatfixedabrasives (suchasadhesive-backedemerypaperordiamon dembeddedinmetal or epoxy) aresuperiorto looseabrasives, becausethelattertendtorollandleaveirregu lardepthscratchesratherthanplaningoffa ofin tensedeformation-Zoneof c crosssectio nofa polycrystallinesa mp leillustratingthenatureofdeformationfrom the cut orcoarselygroundsurfacedowntotheu ndeforrncdsa m nddepthofdeformation varies withthenatureofthesa mp designatedin : theequivalenceoftermsis nd6OO-mesh silico ncarbidehaveprovenadequateinpreparingmos tsurfaces,althoughthe400-meshabrasiveis reallyneededonlyforhardermaterialsandman ysectionsmadeupofsoftsulfidescanbe startedona eachsteptopreventanyheatingofth e sp eci me norto avoidcontaminationby thecarry-overofa b ra sive fromonestepto the sho uld bethoroughlywashedandcleaned(with available)betweensteps,andit anoperator'shands.

7 Becauseabrasivesandpolish -ingcompoundsarereadilyembeddedin the ridgesofone' coveredwhennot in use topreventcon-taminationbystrayparticlesa ndairborne ,oneis well advisedtostopthepolishingandcoatorimpreg natewith amountingmediumandthenbeginaga in;otherwise,thepluckedgrainsprohahlywil lcontaminate allgrindinga n dpolishinglaps and leavethepluckedsa m p les arefulattentiontothegrindingsteps isimportantin thepreparationofgoodpolisheds urfaces ,becausethesestepssh o u ldprovidea incompactspeci-mens,thefracturingfrom ahigh-speeddiamondsawmay extendI mmo a 600-meshabrasivecompoundorwith1,200-mesh abrasiveusedmanuallyona glassplate;thematte-likesurfaceofatypica lsampleat grainsizeandshapeins ph a le rite. areactually moreeasily seenmegascopically whenasamplehasa good600- or fa cereadilyrevealsthe nd involvingthe useofa fabricloadedwithISa nd6 IJmabrasives, the samplefor finalpolishing:theappearanceo f thesa m plea fter theISand6 IJmsteps is sho wnin Figures bestaccomplishedu singdiamonda b ra sivesSTAGE2Co.

8 ,,~C,md''' ""d'"1 STAGE~ !I:"e3/0210110)00200JO.,( ~JOMICROHSIZE'20II!jIj10'8765 FIGURE grit, mesh,andmicronsize. Siliconcarbideabrasivepowders are generally classified in mesh or grit plotted on theleft of thegraph. Theright-handordinateis scaled for emery con-version tomicronsize, locate the size range on theproperordinateandtransposethispointto the correctgraphline whilelocatingthispointon theabscissa. Reverse thisprocedureto transposemicronto grit size.(Diagramcourtesy ofBuehlerLtd.)22 PREPARATIONOFPOLISHEDSECTIONS23 TABLE of MeshorGrit SizeAbraslvesMeshorGritSize1002002603254 006001,2001,8003, ,00060, ahardnaplessfabricwithoutexcessiveweight pressingdownon thata 1-2cm strip ofdiamondpastebeplacedonthepolishinglapa t rightanglesto thedirectionofmovementandthatsubse-quent dispersalon thesurfaceofthe lap bedonebydabbingthediamondaboutwith acleanfingertip. Polishingwith611mabrasivesis the mostimpor-tantstageandshouldbecarriedout fairly slowly with thesurfacejustsuf-ficientlylubricatedfur asmoothactiona ndwithouttoomuchweightappliedto continueduntilnodeepscratchesare seen in may be useful tostudythesectionat this stage, sinceinformationlostlaterinpolishing( grainboundariesof isotropicmin-erals) , caremustbetakenin thisandsubsequen t , usingabrasivesoflessthan611m,removesonly a very smallamountofthespecimensurfaceandshould producea relativelyscratch-freesurface( ).

9 Thisstep may beaccomplishedusingIurn dia-mondembeddedin by urndiamondcloth, or byusinga-A1203( urn) ory-A1203( urn)suspendedinwateron ,magnesia,andceriumoxidecanbeusefulin finalpolishing, veryusefulin the finalpolishingandwhena quickbuffingofa slightlyoxidizedsurfaceis required, surfacesofcoexistingchalcopyrite(light)a ndsphalerite(dark)afterabrasionby(a)600- meshsiliconcarbide;(b)15 IIdiamond;(c)6 IIdia-mond;(d)IIIdiamond;(e) :if)y-Al203on silk in avibro-polisherbutwithouttheintermediate stepsofIS. 6,andIIIdiamond(widthoffield=520 urn). (Continued) (Continued)26 GRINDING ANDPOLISHI NG EQUIPM ENT27 Some laboratoriesprepare sectio ns usin gmachines capa bleofpoli hingsix ormore specime nssim ulta neous ly ongroovedcast-iro n,copper, o r l eadlaps (see the following section).Groovedlap srequ ire cons tantresu rfacingbut, if properlymaintain ed , allow prep ar at accompany eachtypeofmach in e, butthe polishing (see the following sectio n)is efficienta ndeffec-tive in the finalpolishingofmany typesofspec ime lityofthesu r-faceandth e time necessarytoachievea satisfactory polishd epen ds on thepriorgrindingsteps, thehardness,andthehomogen eityofth e specime n.

10 Thelapofthevibropolisheris usuallycoveredwitha tautpieceof finelywoven silk,nylon ,orotherhardfabric; the unitis filled withwater to a depthof2-3ern anda urn y-alumina. Becau sethese veryfinegrainedcompoundsremovesurfacelay ersonly slowly,ca reful priorgrindingdownto the 600-meshabrasive size, or finer,is necessa ryto ens ureflatnessanduniformityofsamplesu rface; otherwise,polishing, whic h for thecommonsulfidesandoxidestakes only afewhours, will require severaldaysandeventhenproducesonlya drawbackofvibropolish-ingunitsis thetendencyforpolishedsurfacesto develo p cons ide rab le relief( ),especially whensoftandh ardminerals arein hreliefmayobscurecontactrelationships betweenminer als,maymak eth eidentificationofsoftrimmingphasesandinc lusion sdifficult, a nd may handi-capattemptsatphotomicroscopyandele ctron microprobe ana lysis. Never-theless, thesim plicityofoperation, easymainten an ce, a ndca pa bilityofhandlingmanysa m plessimultaneou slymake vib ropolishe rs typesofequipmentarecommonlyused in the grinding and polishingofspecimensfor oremicroscopy.