practicalin correlative microscopy of a mineralised silicate...

Funded by the European Union

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Practical in correlative microscopy of a mineralised silicate rock

Introduction:

Whereas themajorityofGeologyandEarthSciencestudentsobtain training inplane-polarisedandcross-polarisedmicroscopywiththepetrographicmicroscope,educationwith the reflected lightmicroscope ismuch less common. This is in part because theadditionofareflectedlightsourceandpolariseraddsubstantialcosttotheacquisitionandmaintenanceofteachingmicroscopes.Moreover,EconomicGeology,themainuserofreflectedlightmicroscopy,isnowadaysnolongerpartofthecurricula.

Petrologicmicroscopyhasreceivedanewleaseonlifewiththeintroductionofmotor-controlled stages and high-resolution digital cameras that are used with dedicatedsoftware to produce seamless stitched mosaic images of entire petrographic thinsections.Theseimagesaregigabyte-sizedfilesthatcanbeweb-hostedandviewedandnavigatedwithsoftwareakintosatellite-basedmapbrowsing.OneofthemostsuccessfulimplementationsofthisneweducationaltoolistheUKVirtualMicroscope(UKVM)forEarthSciencesProjectbytheOpenUniversity.Itpermitsthelayeringofplane-polarised,cross-polarisedandreflectedlightimagefilesandtherebyprovidesaccessofreflectedlight microscopy to students enrolled in courses that do not have access toreflected lightmicroscopes.ThepracticalfeaturedhereishostedontheUKVM.

Asecondtechnologicalinnovationthathasoccurredoverthelastdecadeishigh-spatialresolution (down to <1 micron x <1 micron pixel) elemental mapping of polishedpetrographicthinsectionswithfield-emission-gunscanning-electronmicroscopes(FEG-SEM)equippedwithlargeareaenergy-dispersiveX-raydetectors.Theoutputsfromsuchelementalmapsare imagesof thinsections inwhich thecolourbrightnessrepresentselementalabundance. Individualelemental imagescanbeoutputtedorseveralcanbecombined into false-colour multi-element images. These elemental images can becoordinatedwithmicroscopicimagesandthecombinedinformationgivesstudentsandresearchersunparalleled informationregardingthemineralogyandchemistryof theirspecimens,potentiallyopeninganewerainchemicalpetrography.

This practical is an on-line open-access educational output of the H2020 EID‘Metalintelligence’andprovidesanexampleofhowthesetechnologiescanbecombinedfor next generation learning and teaching of petrography. The rock selected for thepractical is a feldspathic pyroxenite from the Merensky Reef, a chromitite-sulphide


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mineralisedportionoftheca.2GaoldBushveldComplex,SouthAfrica.ThesamplewascollectedbyBalzKamberin2017intheEasternLimboftheBushveldComplex,fromtheDerBrochenprojectareaheldbyAngloAmericanPlatinumCorporation.

The practical introduces some of the useful educational features of the UKVM andaims to help develop the critical skill of combining observational datawith chemicaldataandphasediagrams. Thepractical could be used in conjunctionwith lectures onlayered igneous complexes, magmatic sulphides, and/or electron microscopy-basedchemical imaging. A sample copy of a solved practical is available upon request [email protected]. The practical is aimed at students with some familiaritywith microscopy and should provide sufficient scope to fill a half-day practical timeslot.Thetwosuitesofthinsectionimagescanbeaccessedatthislink:

https://www.virtualmicroscope.org/content/feldspathic-pyroxenite

https://www.virtualmicroscope.org/content/feldspathic-pyroxenite-b


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Part1:Thesilicateframework

YouareencouragedtofamiliariseyourselfwiththeUKVMfeaturesbywatchingthisshortvideo:

http://www.geolab.ie/learning-2/intro_vm1/

Task 1:

Using a combination of plane-polarised light (PPL) and cross-polarised light (XPL)identifythetwomainsilicatemineralsthatconstitutethethinsection.TheMgmapshowsthatthedominantsilicatephaseisbright(high)inMgOandthereforeeitherenstatite-richorthopyroxeneorolivine.Pleasepasteascreenshotintotheprovidedboxthatshowsdiagnosticfeaturesthatallowyoutoconfidentlyidentifythephase.Pleaseannotatethefeaturesonthescreenshotandprovideascale.

Usingthemeasuringtoolinthelowerrightcorner,determinethelengthsandwidthsandaspectratiosfor15ofthesegrainsanddeterminethemedianandstandarddeviationsofthethreeparameters.Recordyourvaluesinthetablebelow.Consideringthatthisisanigneous rock, comment on whether these grains are more or less equigranular thantypicalsilicates.


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Length Width Aspect ratio Comments:

Grain 1

Grain 2

Grain 3

Grain 4

Grain 5

Grain 6

Grain 7

Grain 8

Grain 9

Grain 10

Grain 11

Grain 12

Grain 13

Grain 14

Grain 15

Median

Standard deviation


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Task 2:

Turning your attention now to the second-most abundant mineral, use the chemicalimagesofMg,FeandcombinedAl-Mg-Catoidentifythismineral.

Paste a screenshot (XPL) into theprovided box and highlight thediagnostic optical properties of thismineral.

Together,thetwomineralsmakeupbetween80and90%ofthethinsectionandtheirmutual grain boundaries are typical of one type of cumulate. With reference to theimages below (after Wager and Brown, 1953), what kind of cumulate is this rock.Makeyourcaseina2-3sentenceparagraph,puttingforwardyourkeyobservations.

(A) Ortho-,(B)Meso-,and(C)Ad-cumulatetextures.Thestippledlinesshowextentoforiginalcumulatecrystals,whichexperiencedprogressivelymorepost-cumulusgrowthfrom(A)to(C).


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Task 3:

In addition to the two main silicates, there are three additional silicates of lesservolumetricabundance.ThefirstofthesehasasimilarappearanceinPPLasthedominantsilicatebutitdiffersinchemistry.ItbecomesmostvisiblewhentogglingbetweenthePPLimage and the false colour Al-Mg-Camaps. Identify themineral and comment on itspropertiesinPPLandXPL.Finally,putitintotheorderofcrystallisationsequenceoftheothertwominerals.

ThesecondminorsilicateistheonlymineralwithproperpleochroiccolourinPPL.Itiseasy to identify. Comment on its spatial distribution throughout the thin section,particularlywithreferencetotheopaqueminerals(blackinPPL).Isthismineralpartoftheoriginalcrystallisationsequenceorisitalateaddition?Finally,judgingfromtheMgand Fe maps, does this mineral have a higher or lower Mg/Fe ratio than the maincumulatesilicate?


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Thefinalsilicateisbyfartheleastabundant,haslowreliefandgreyinterferencecolours.Itispresentinthisframe:

https://www.virtualmicroscope.org/rock_sample?asset=bushveld_elements/index.html?x=46.06&y=17.38&zoom=0.43&s=1

Identifythemineralandcommentonitsrelationstotheotherminorsilicatesandtheopaques.


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Part2:Theopaqueminerals

Ifyouhaveneverusedreflectedlight,familiariseyourselfitbycomparingPPLandREFimages.Thekeyobservation,ofcourse, is thatphasesthatappearopaqueinPPLnowhavereflectivecolour.Pleasenotethattheholesinthethinsection(lookinggreyinPPL,blackinXPL)areblackinREFlight.Itisdifficulttocapturethetruecoloursinreflectedlightusingadigitalcameras.Furthermore,theappearanceofhuesofreflectivecoloursdependsonillumination,theuseofmonochromators,etc.Asaresult,itisnotstraight-forwardtocomparethecoloursfrompublishedphotosoratlaseswiththoseseenhere.TheUKVMalsohasthedisadvantageofnotbeingabletoshowcross-polarisedREFlightimages,whichexposeanisotropieswhichcanbedistinctivefeaturesofopaqueminerals.

Task 1:

Familiariseyourselfwiththethreesulphides–pyrrhotite,chalcopyriteandpentlandite–presentinthisrock.Exampleimagesaregivenbelow.

Amassofanhedralpyrrhotite(Fe1−xS(x=0to0.2))surrounding euhedral pyrite. The pyrrhotitehas inclusions of chalcopyrite (see below forbetterimage).Thenamepyrrhotiteisderivedfrom the Greek pyrrhos, flame-coloured orpyhrrhotes, "redness," in allusion to colour.In this photo, it has a slightly reddish hue.It is rimmed by a grey mineral, which ismarkasite.

This image shows golden chalcopyrite(CuFeS2) within bright grey magnetitecrystals (Fe3O4) and interstitialquartz/feldspargangue(darkgrey).


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This image shows a corona of brokenpentlandite ((Fe,Ni)9S8) crystals surroundingapyrrhotite(fromDuranetal.,2016).Notethelackofaredhueinthepentlandite,whichhasa more metallic lustre in reflected light.However, it is often difficult to tell thedifferencebetweenpentlanditeandpyrrhotiteonaccountofsimilarcolour.

Task 2:

Asyouwillhavenoticed,thethreesulphidesinthisrockarecloselyassociatedspatially.TheirmutualrelationshipwillbeexploredinTask3.Here,weareinitiallyinterestedinthe spatial arrangement between the silicates and the sulphides as awhole.Make aninterpretativedrawingofthisfieldofview:

https://www.virtualmicroscope.org/rock_sample?asset=bushveld_elements/index.html?x=34.4&y=2.69&zoom=0.5&s=2

Use the PPL, REF, Mg, Si and S images in junctions. Begin with outlining the majorcumulatesilicate frameworkandworktowards filling the interstitialspace.Usingthisapproach,formulateasequenceofcrystallisation.Forthetimebeing,donotworryaboutthesequenceofcrystallisationofthethreesulphides,treatthesulphidesasawhole.


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Task 3:

Theclosespatialassociationofthethreesulphideshasstronggeneticsignificance.Itmostgeneralterms,itrelatestotheprocessofexsolution.Simpleexamplesofexsolutionarealklai-feldspars,whereitiscommonfortheNa-richphasetoexsolvefromtheK-richhostupon cooling. The resulting intergrowth of the two feldspars is called perthite. Mostreaderswillbefamiliarwiththisphenomenon,whichisdescribedhere:

https://en.wikipedia.org/wiki/Perthite

In the specific case of these sulphides, exsolution is more complex. What is quitestraightforwardisthattheprecursorphasetothepresentlyobservedsulphideswasaFe-Ni-Cu-S mineral called mono-sulphide-solution (MSS). This is the most prominent S-phaseinmagmaticsystems.Itexistsasaliquidtoquitelowtemperatures(afewhundreddegrees C), and as a solid solution is quite tolerant of a wide range incompositionalvariabilitybetweenFe:NiandFe:Cu.InS-richsystems,itcoexistswithanadditional S-phase (liquidof vapour) and inFe-rich systems it coexistswith anFe-Nialloy.However,inthesampleathand,wecanfocusontheMSS.AstheMSScoolsfrom550degreesCto100 degrees C, the size of the stability field of the MSS narrowsconsiderably andexsolution of pyrrhotite, chalcopyrite and pentlandite begins. Thedecreasing stabilityfieldwithdroppingTisusuallyshowninternaryFe:Ni:SorFe:Cu:Sdiagrams(seebelow)andismoretrickytoshowforthefullFe-Ni-Cu-Ssystem.

DiminishingstabilityfieldofMSSwithdecreaseofT.Relevantabbreviations:Pn=pentlandite;Po=pyrrhotite;Py=pyrite)intheFe-Ni-Ssystem(fromGonzalez-Jimenezetal.,2018).


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Consideringthespatialarrangementofthethreesulphidesfromanareaonthesectionofyourownchoice(differentfromthefieldofviewshowninTask2),andusingtheFe,Ni,Cuimagesinaddition,developahypothesisfortheorderofformationofthethreesulphidesandtheexsolutionrelationship,focussingonthequestionofwhichphasemighthaveexsolvedfromwhich?


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Part3:Chemicalinformationandoriginoftheplatinum-group-elements

Task 1:

ThechemicalimagesprovidedontheUKVMshowelementalconcentrationasbrightnessofcolour.Theunderlyingdata,however,arequantitativeelementalconcentrations. Inelementalimageswithverystrongconcentrationcontrastsandstrongpartitioningoftheelementintoonephase,detailsareonlyvisiblewithalog-scalecolourrange.Regardlessof these limitations, it is possible to interpret the various colour shades as relativeelemental concentrations. Explore whether the Fe and S compositional data forpyrrhotite, chalcopyrite andpentlandite given in theTable below (fromBallhaus andRyan,1995)fitwiththerelativeshadesintheelementalmapsofFeandS.


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Task 2:

ThemaineconomicvalueoftheMerenskyReefisnotitsCuandNimineralisationbutitssensationalenrichmentinplatinum-group-elements(PGE).ThescientificdebateabouttheoriginofthePGEmineralisationcontinues.ThePGEsarehostedwithinthesulphidesthatyouhaveexplored,eitherassolidsolutionor,moretypically,asmicroscopicnuggetsofsulphidesormetalalloys.OneschoolofthoughtisthatthePGEswereenrichedinasulphidemeltthateventuallyformedtheMSSphase(e.g.Campbelletal.,1983)whereasanotherproposalisthatatleastsomeofthePGEswereoriginallyhostedinahydrousfluid phase and did not precipitate from the MSS (e.g. Ballhaus and Stumpfl, 1986).Argumentsinfavouroflatterrestonthespatialarrangementoflatecrystallisingphases.Using themaps of Cu, Si and the Si-K-S false-colour composite to test whether fine-grained chalcopyrite is preferentially associated with late-crystallising phases and iftheselatecrystallisingphasescontainpetrographicevidenceforthepresenceofafluidphase.Use2-3screengrabstosupportyourargumentation.


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References:

Ballhaus, C.G. and Stumpfl, E.F., 1986. Sulfide and platinum mineralization in theMerenskyReef: evidence fromhydrous silicates and fluid inclusions. Contributions toMineralogyandPetrology,94(2),pp.193-204.

Ballhaus,C.andRyan,C.G.,1995.Platinum-groupelementsintheMerenskyreef.I.PGEinsolidsolutioninbasemetalsulfidesandthedown-temperatureequilibrationhistoryofMerenskyores.ContributionstoMineralogyandPetrology,122(3),pp.241-251

Campbell,I.H.,Naldrett,A.J.andBarnes,S.J.,1983.Amodelfortheoriginoftheplatinum-rich sulfide horizons in the Bushveld and Stillwater Complexes. Journal of Petrology,24(2),pp.133-165.

Duran, C.J., Barnes, S.J. and Corkery, J.T., 2016. Trace element distribution in primarysulfides and Fe–Ti oxides from the sulfide-rich pods of the Lac des Iles Pd deposits,WesternOntario,Canada:constraintsonprocessescontrollingthecompositionoftheoreand the use of pentlandite compositions in exploration. Journal of GeochemicalExploration,166,pp.45-63.

González-Jiménez, J.M., Deditius, A., Gervilla, F., Reich,M., Suvorova, A., Roberts,M.P.,Roqué,J.andProenza,J.A.,2018.NanoscalepartitioningofRu,Ir,andPtinbase-metalsulfides from the Caridad chromite deposit, Cuba. American Mineralogist, 103(8),pp.1208-1220.

Wager, L.R. and Brown, G.M., 1953. Layered intrusions.Medd. dansk geol. Foren,12,pp.335-349.

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