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Accelerated ageing of shales of palaeontological interest:
Impact of temperature conditions
Giliane P. Odin, Frederik Vanmeert, Koen Janssens, Hervé Lelièvre, Jean-Didier Mertz, Véronique Rouchon
To cite this version:
Giliane P. Odin, Frederik Vanmeert, Koen Janssens, Hervé Lelièvre, Jean-Didier Mertz, et al.. Ac-
celerated ageing of shales of palaeontological interest: Impact of temperature conditions. Annales
de Paléontologie, Elsevier Masson, 2014, 100, pp.137 - 149. �10.1016/j.annpal.2013.12.002�. �hal-
01435164�
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Original article
Accelerated ageing of shales of palaeontological interest:
Impact of temperature conditions
Vieillissement accéléré de schistes argileux d’intérêt paléontologique : impact des conditions de température
Giliane P. Odin
a, Frederik Vanmeert
b, Koen Janssens
b, Hervé Lelièvre
c, Jean-Didier Mertz
d, Véronique Rouchon
a,∗aCentredeRecherchesurlaConservationdesCollections(Muséumnationald’Histoirenaturelle,ministèredelaCultureetdelaCommunication, CentreNationaldelaRechercheScientifique),CRCC,USR3224,CP21,36,rueGeoffroy-Saint-Hilaire,75005Paris,France
bDepartmentofChemistry,UniversityofAntwerp,Groenenborgerlaan171,2020Antwerp,Belgium
cCentredeRecherchesurlaPaléobiodiversitéetlesPaléoenvironnements(Muséumnationald’Histoirenaturelle,CentreNationaldelaRecherche Scientifique,UniversitéPierre-et-Marie-Curie),UMR7207,CP38,8,rueBuffon,75005Paris,France
dLaboratoiredeRecherchedesMonumentsHistoriques(ministèredelaCultureetdelaCommunication,CentreNationaldelaRechercheScientifique), LRMH,USR3224,29,ruedeParis,77420Champs-sur-Marne,France
a rt i c l e i n f o
Articlehistory:
Received31January2013 Accepted19June2013 Availableonline24January2014
Keywords:
Shale Pyrite Sulfide Degradation Fossil Sulfate Conservation
a b s t r a c t
ThepalaeontologicalcollectionsoftheMuséumnationald’Histoirenaturelle(MNHN,Paris,France)and theMuséumd’Histoirenaturelled’Autun(MHNA,Autun,France)includemanyfossilspecimensoriginat- ingfromtheargillaceousshalesoftheAutunbasin(Saône-et-Loire,France).Thesefossilsarepreserved withinsedimentaryrockscontainingunstablesulphidecompounds,suchaspyrite,whichmaydeterio- rateincontactwithwaterandoxygen.Thisalterationprovokescrystallineefflorescenceandcracks,thus compromisingthepreservationsofthefossils.Thisworkconstitutesthefirststepofaprojectthataims tounderstandthemechanismsofalterationofthesematerialsinordertodefineconservationguide- linesforpalaeontologicalcollections.Forthispurpose,eightdamagedspecimensoriginatingfromthe PermianAutunbasin(Saône-et-Loire,France)wereselectedandanalyzedbyX-raydiffraction(XRD), Ramanspectroscopy,scanningelectronmicroscopycoupledtoenergydispersiveX-rayspectrometry (SEM/EDS)andX-rayabsorptionspectroscopyatthethresholdofthesulphurKa-edge(XANES).This methodologyenabledthecharacterizationofthematricescompositionandthechemicalnatureofthe alterations.Subsequently,wehavesoughttoreproducebyartificialageingthealterationphenomena encounteredinthecollections.NewshalesampleswerecollectedonsevenoutcropsofthesameAutun basin.Theywereanalyzedandsubjectedtoartificialageingat50%relativehumidity(RH)andattemper- aturesrangingbetween40◦Cand90◦C.Ourworkshowsthatdamagedspecimensandnewlycollected shalehaveasimilarmineralogicalcomposition.Yetthecrystallineefflorescencematerialformedonthe surfaceofdamagedspecimensbelongstotheironsulphategroupwhereasgypsumpredominateson artificiallyagedshalesamples.Reproducingthealterationsobservedonspecimensbyartificialageing remainsthereforeproblematic.Additionally,itappearsthatthetemperatureofageingcontrolsthenature ofthedamage:at40◦C,manysamplesaremechanicallydamagedwhereasnoorminorcrystallineefflo- rescenceoccurs.At90◦C,itistheoppositetendencythatisobserved.Finally,mechanicaldamagesdo notseemtobecorrelatedwiththedevelopmentoftheefflorescence:sampleswithefflorescentcrystals generallydonotshowclearlyvisiblecracks;thosethatseemmostfragmenteddonotshowanyvisible efflorescence.
©2013ElsevierMassonSAS.Allrightsreserved.
Motsclés: Schistesargileux Pyrite
r és um é
Lesschistesargileuxdubassind’AutunfontpartieintégrantedescollectionsdepaléontologieduMuséum nationald’Histoirenaturelle(MNHN,Paris,France)etduMuséumd’Histoirenaturelled’Autun(MHNA,
∗Correspondingauthor.
E-mailaddress:rouchon@mnhn.fr(V.Rouchon).
0753-3969/$–seefrontmatter©2013ElsevierMassonSAS.Allrightsreserved.
http://dx.doi.org/10.1016/j.annpal.2013.12.002
Sulfure Dégradation Sulfate Conservation
Autun,France).Cesrochessédimentairescontiennentdenombreuxfossiles,malheureusementsujetsà d’importantesdégradations.Eneffet,lescomposéssoufrésqu’ilscontiennent,enparticulierlessulfures defertelslapyrite,s’oxydentpeuàpeu,provoquantdescraqueluresetdesefflorescencescristallines quicompromettentlaconservationdesfossiles.Cetravailconstituelapremièreétaped’unprojetvisant àmieuxcomprendrelesmécanismesd’altérationdesfossiles,afind’optimiserlesconditionspropicesà leurconservation.Pourcela,nousavonssélectionné,danslescollectionspatrimoniales,unensemblede huitspécimensendommagésissusdubassinpermiend’Autun(Saône-et-Loire,France)etavonsprocédéà leurcaractérisationàl’aidededifférentsoutilsanalytiques:diffractiondesrayonsX(DRX),spectroscopie Raman,microscopieélectroniqueàbalayagecoupléeàunesonded’analyseélémentaire(MEB/SDE)et spectroscopied’absorptionauseuildelaraieKadusoufre.Desdonnéespréliminairessurlacomposition desencaissantsetlanaturedesaltérationsontainsipuêtreacquises.Parlasuite,nousavonscherché àreproduireparvieillissementartificiellesphénomènesd’altérationrencontrésdanslescollections.
Dumatérielneufaainsiétécollectésurseptaffleurementsdecemêmebassind’Autun,analysé,puis soumisàdesvieillissementsartificielsavecunehumiditérelativede50%etàdifférentestempératures.
Nostravauxmontrentquelesschistesdesspécimensendommagésetceuxnouvellementprélevésont unecompositionminéralogiquesemblable,maisprésententdesefflorescencescristallinessensiblement différentes:elless’apparententpourlespremiersàdessulfatesdefer,etpourlessecondsàdessulfatesde calcium.Lareproductiondesmécanismesd’altérationparvieillissementartificield’unmatérielapriori prochedesspécimensendommagésrestedoncproblématique.D’autrepart,latempératureserévèle êtreunparamètredéterminantdutypedeladégradation:à40◦C,peud’efflorescencescristallinessont observéesmaisdenombreusesdégradationsmécaniquesontlieu.A90◦C,c’estlatendanceinversequi estconstatée.Enfin,lesdégradationsmécaniquesobservéessurleséchantillonsvieillisartificiellement nesemblentpascorréléesàl’apparitiond’efflorescences:leséchantillonssurlesquelsonobservedes efflorescencesnemontrentgénéralementpasdedégradationmécaniqueévidenteàl’œiletceuxqui semblentlesplusfragilesnemontrentpasd’efflorescencevisible.
©2013ElsevierMassonSAS.Tousdroitsréservés.
1. Introduction
Shalesmaycontainfossils,asinthecaseoftheworld-renowned andwidelystudiedBurgessShale(Gainesetal.,2012).Although shalesoftheAutunbasin(Saône-et-Loire,France)arelessknown, they have also delivereda largenumber of fossils, now stored in various museums, includingthe Muséumnational d’Histoire naturelle, Paris, France (MNHN) and the Muséum d’Histoire naturelled’Autun,Autun,France(MHNA).Someofthesefossilsare nowshowingsignificantdegradations(Rouchonetal.,2012);these arefrequentlyattributedtopyritebreakdown,hencethedesigna- tionof“pyritized”fossils.However,agreatvarietyofquiteunstable sulphidespeciescouldaswellbesuspectedtobetheoriginofthis damage.
Thealterationofpyritizedfossilsismainlyduetotheoxidation ofsulphides(sulphurofoxidationnumber−Ior−II)intosulphates (sulphurofoxidationnumber+VI).Manyinvestigationsarecur- rentlydedicatedtothedegradationofsulphidespecies(Jambor etal.,2000;Vaughan,2006;MurphyandStrongin,2009;Chandra andGerson,2010;Schoonenetal.,2010;HeidelandTichomirowa, 2011).Ithasbeeninparticularshownthatthepresenceofheavy metalimpurities(Lehneretal.,2007),microorganisms(Tupikina etal.,2009)orcarbonates(Caldeiraetal.,2010)significantlyinflu- encestheoxidationprocess.
Palaeontologistsandmuseumconservatorsarewellawareof therisksrelatedtopyritebreakdown(Howie,1977a,b)andcon- sequently developed several preventive and curative methods.
Preventivemethods,suchasdry/anoxicstorageinwater/airtight plasticbagsconstituteapromisingstrategy(CarrioandStevenson, 2002;McPhailetal.,2003;Day,2005;Buttler,2006)butnoinfor- mationisavailableregardingtheacceptableamountofhumidityor residualoxygeninthebags.Curativemethods,suchastheappli- cationofethanolaminethioglycollate(CornishandDoyle,1984;
Cornish,1986)arepoorlydocumentedwithrespecttotheireffi- ciencyandlongtermsideeffects.Theyaremoreoverinvasiveand compromiseanyfurtherdiagenesisinvestigation.
Despiteintensive studiesdedicatedtothedecayof ironsul- phide,thepreservationofpalaeontologicalspecimensduringtheir storageinmuseumcollectionsremainsmisunderstood(Fellowes
and Hagan, 2003). This work constitutes the first step of a project that aims to obtain a better insight into the physico- chemicaltransformationstakingplaceinfossilspreservedinshale after their excavation in order to define conservation guide- linesforpalaeobotanicalcollections.Forthispurpose,itappears necessary:
•tocharacterizethealterationproducts;
•toreproducethedegradationinlaboratoryconditions.
Twoapproacheswerethereforedevelopedinthiswork:firstly, originaldamagedspecimensfromtheAutunbasin(Saône-et-Loire) wereanalyzed;secondly,shalesamplesweretakenfromdifferent outcropsofthesamebasinandartificiallyagedwithdifferenttem- peratureconditions.Thepurposeistocomparethenewlyinduced alterationoftheseshalesampleswiththatformedonthedamaged specimens(Rouchonetal.,2012).
2. Materialsandmethods
2.1. Investigatedsamples 2.1.1. Damagedspecimens
Thespecimens (Table1)were selectedfromtheMNHNand MHNAcollections.Whiledamageisevident(Fig.1),itisdifficultto assesswhenandunderwhichcircumstancesitoccurredbecause thehistoryofthesespecimensisnotdocumented.
The specimens MNHN.F.6888, MNHN.F.6889, MNHN.F.6890, MNHN.F.6891andMNHN.F.6892belongtotheFlouestcollection andwerealreadydepictedinthesupportinginformationofaprevi- ousstudy(Rouchonetal.,2012).Asthesespecimensjointlyentered theMuseumcollectionin1865,itisreasonabletoassumethatthey havebeensubjectedtosimilarconservationconditions.Theyare nowstoredinthesamedrawer,inaroomwithoutairconditioning andthusexposedtotemperatureandhumidityfluctuations(Fig.2).
The specimens MHNA-PAL-000316, MHNA-PAL-000155 and MHNA-PAL-000167(Fig.1)werecollectedinthenineteenthcen- turybyAnatoleDesplacesdeCharmasseanddonatedtotheSociété d’Histoire naturelle d’Autun around 1900. In 1964, they were
Table1 Mineralogicalcompositionofdamagedspecimens.ThecompositionofthematriceswasdeterminedbyX-raydiffractionandscanningelectronmicroscopycoupledtoenergydispersiveX-rayspectrometry(SEM/EDS).The compositionoftheefflorescencewasdeterminedbyX-raydiffractionandRamanspectrometry.Allmatricesquotedinthistablepresentamatteaspect.ThematricesofthespecimenMNHN.F.6889andMHNA-PAL-000167show anadditionallayerthatisnotquotedinthistable.Itisshiny,amorphous,composedoforganicmatterandhastheaspectofvitrinite. Compositionminéralogiquedesspécimensendommagés.LacompositiondesmatricesaétédéterminéepardiffractiondesrayonsXetparmicroscopieélectroniqueàbalayagecoupléeàunesonded’analyseélémentaire(MEB/SDE).La compositiondesefflorescencesaétédéterminéepardiffractiondesrayonsXetparspectrometrieRaman.Touteslesmatricesréférencéesdanscetableauontunaspectmat.LesmatricesdesspécimensMNHN.F.6889etMHNA-PAL-000167 comportentunecouchesupplémentairenonréférencéedanscetableau.Cettecoucheestbrillante,amorphe,composéedematièreorganiqueetprésentel’aspectdelavitrinite. OriginPeriodPermian GeographicalareaAutunbasin,Saône-et-Loire,France HistoryInstitutionMuséumnationald’Histoirenaturelle(MNHN),ParisMuséumd’Histoirenaturelle(MHNA),Autun Collection(entrance)Flouest(1865)Charmasse(around1900) HousingStoredinindividualplasticbags,andinawoodencabinet,Brogniart’sroom,drawer#M17(sincethe1990s)–noairconditioningOnplasticracks,inanairconditionedroom(45%±5%R.H.and20±2◦C)sincethe2000s ReferenceInventoryMNHN.F.6888MNHN.F.6892MNHN.F.6889MNHN.F.6890MNHN.F.6891MHNA-PAL-000316MHNA-PAL-000155MHNA-PAL-000167 ReferenceinRouchonetal,2012IGHJK––– FossilSphenopterissemialata (Geinitz,1858)Weiss, 1868 Walchiasp.SyringodendronspPecopterisarborescens (Schlotheim,1804) Brongniart,1828 Callipterisnaumannii (Gutbier,1849)Kerp, 1988.
FernFishNofossilprint LocalisationLeRuet,TavernayLeRuet,TavernayLeRuet,TavernayLeRuet,TavernayLeRuet,TavernayIgornayIgornayUnknown ShalematrixQuartz SiO2
xxxxxxxnotanalysed Kaolinite Al2Si2O5(OH)4
xxxxxxx Peakat14Å(smectite,chlorite...)xxxxxx Peakat10Å(illite,muscovite...)xxxxxx Mineralofthefeldspargroupxxxx Dolomite CaMg(CO3) 2
x Pyrite FeIIS2
xx Gypsum CaSO4·2H2Oxxxxxx Mineralofthejarositegroupxxxxx ElementdetectedbySEM/EDSAl,Si,S,K,CaFe,TiAl,Si,S,K,CaFe,Mg,Ti, ZnAl,Si,S,K,CaFe,Ti,Na, Cl,ZnAl,Si,S,K,CaFe,Mg,Ag, GdAl,Si,S,K,CaFe,Mg,ZnAl,Si,S,K,CaFe,TiAl,Si,S,K,CaFe,P,Na,Ti EfflorescenceGypsum CaSO4·2H2Ox Melanterite FeIISO4·7H2Ox Rozenite FeIISO4·4H2Oxxxxxxxx Szomolnokite FeIISO4·H2Oxxxx Mohrite (NH4) 2FeII(SO4) 2·6(H2O)xxx Boussingaultite (NH4) 2Mg(SO4) 2·6(H2O)xxx Copiapite FeIIFeIII4(SO4) 6(OH)2·20(H2O)x Metavoltine K4Na4FeII0.5Zn0.5FeIII6(SO4) 12
O2·20(H2O)x Jarosite, KFeIII3(OH)6(SO4) 2
x Ammoniojarosite (NH4)FeIII 3(OH)6(SO4) 2
xxx Natrojarosite NaFeIII 3(OH)6(SO4) 2
x
Fig.1.GeneralviewofMuséumd’Histoirenaturelled’Autun(MHNA)specimens.a:
MHNA-PAL-000316;b:MHNA-PAL-000155;c:MHNA-PAL-000167.Aphotographic reportofMNHNspecimensisavailableelsewhere.
VuesgénéralesdesspécimensduMuséumd’Histoirenaturelled’Autun(MHNA).a: MHNA-PAL-000316;b:MHNA-PAL-000155;c:MHNA-PAL-000167.Undossierpho- tographiquedesspécimensduMNHNestdisponibleparailleurs.
Rouchonetal.,2012.
donatedtothecityofAutunandjoinedtheMHNAcollection.In 2000,theymovedtoanair-conditionedroomwithstablehumidity andtemperatureconditions(Fig.2).
Mostdamagedspecimenspresentahomogeneousmatteaspect, buttwoofthem(MNHN.F.6889andMHNA-PAL-000167)havean additionallayerthatappearsshiny.
2.1.2. Newlyexcavatedshalesamples
ThecrystallineefflorescencepreviouslyobservedontheMNHN specimenswasnotsystematicallylocatedonthefossilsalonebut wasalsodistributedintheshalematrix(Rouchonetal.,2012).It
Fig.2.Temperatureandrelativehumidityofthestorageareas.Muséumnational d’Histoirenaturelle(MNHN),Brongniart’sroom,#M17drawer,betweenOctober 2011andNovember2012(up)andMHNA,air-conditionedvaultbetweenDecember 2011andDecember2012(down).
Températureethumiditérelativedeslieuxdestockage.MNHN,salleBrongniart,tiroir
#M17entreoctobre2011etnovembre2012.(enhaut)etMHNA,caveclimatiséeentre décembre2011etdécembre2012(enbas).
wasthereforenotfoundnecessarytocollectshaleslabsthatcontain fossils.
Samples,severaldecimetresinsize,weretakenfromsevendif- ferent outcropsof theAutunbasin (Table2). Itsstratigraphyis welldocumented(ElsassDamon,1977;Châteauneufetal.,1980;
Marteau,1983).Theydonotshowanyvisiblefossilsandpresenta matteaspect.
Astheyweretakenfromoutcroppingshalelayers,thecollected slabsmighthavealreadybeenexposedtooxygenandthussub- jectedtosome“primary”oxidation.Thisoxidationwasvisibleon onesampleonly,whichcamefromSaint-Léger-du-Boisandpre- sentedafewspotsofgypsum.Nevertheless,sampleswerehoused inairandwatertightplasticbags(ESCAL,LongLifeforArt,Ger- many)withoxygenscavengerbags(ATCOFTM,LongLifeforArt, Germany)for 3monthsbeforethestudy.Thisconditioningwas efficientinlimitingdrying,butoxygendepletionwassometimes ineffective,probablybecausetheplasticdidnotwithstandpiercing fromthesharpshaleedges.Itwasalsonecessarytocheckregularly theamountofoxygenwithanoximeter(Canal111,Vigaz,Visciano, France).
Somespecificityshouldbenoticed:firstly,theslabsoriginating fromtheMusesitedonotbelongtothefamousfishlayer(Gand
Table2
Locationofshalematerialcollectedforthisstudy.
Localisationdesaffleurementssurlesquelsontétéprélevésleséchantillonsdeschistesargileux.
Geographicallocation GPScoordinates Autunianstratigraphy
North East Altitude(meter)
Millerya 46◦59′09,96′′ 4◦16′51,27′′ 307 Millerybedding
Surmoulina 47◦00′29,14′′ 4◦19′21,14′′ 297 MainlayeroftheSurmoulinbedding
LaComaillea 46◦58′37,75′′ 4◦13′50,42′′ 319 MainlayeroftheSurmoulinbedding
Musea 47◦01′36,22′′ 4◦22′54,44′′ 322 Musebedding
Bois-des-Grands-Miens 47◦01′54,15′′ 4◦21′50,71′′ 328 Musebedding
Saint-Léger-du-bois 47◦01′00,34′′ 4◦26′54,16′′ 319 Igornaybedding
Igornay 47◦02′58,75′′ 4◦22′42,49′′ 327 Igornaybedding
ThedepositsofLaComailleandSurmoulincorrespondtothemainlayerofSurmoulin(Châteauneufetal.,1980);theMuséumnationald’Histoirenaturellespecimenscoming from“LeRuet”probablycorrespondtothislayer.
aLocalitiesofthehistoricalAutunianstratotype.
etal.,2010),butweresampledapprox.onetotwometresabovethis layer.Secondly,theoutcropofthesiteofLaComailleissituatedvery closetothelocality“LeRuet”wheretheMNHNspecimenswere collected.TheslabscomingfromLaComailleprobablycorrespond tothesamelayerastheMNHNspecimens.
2.2. Acceleratedageing
Threesetsofnewlyexcavatedshalesampleswereartificially agedatthesamelevelofrelative humidity(50%RH)yetatdif- ferent temperature values: 40◦C, 70◦C and 90◦C. Ageing was performed in closed vessels with an experimental setup simi- lar to that depictedin the ASTM D6819standard (Sawoszczuk et al., 2008); however,silica gel wasusedinstead ofpaper for themonitoringofhumidity(PROSorb,LongLifeforArt,Germany;
pre-conditionedat50%RH).Duringageing,thetemperatureand humidityconditionsweremonitored bysmallsensorsplacedin thevessel(Hygrobutton,ProgesPlus,USA).
Duetotheheterogeneousnatureofshale(BerrowandReaves, 1981;Subasingheetal.,2009),eachslabhasbeensplitintoeight samples,eachapproximately2to3cmwideand0.5to1cmthick.
Thesesampleswerethen dispatchedineighttubes.Asaresult, eachtubecontainedsevensamples,eachofthemoriginatingfrom adifferentoutcrop.Theeighttubeswereclosedandplacedtogether inthesameovenatagiventemperature(40◦C,70◦Cor90◦C).They wereregularlyremovedfromtheovenandopenedforafewdaysin ordertobephotographed.Thesilicagelwasreplacedandthetube placedbackintheoventopursuetheageing.Inall,168samples wereartificiallyagedforatotalperiodof16weeks.
2.3. Characterisationtechniques
2.3.1. Visualassessmentduringartificialageing
Visiblechangesduringartificialageingwereevaluatedforeach samplewithmacro-picturestakenwithadigitalcameraplacedon topofa4300Klightbooth,specificallydesignedforsmallsamples;
seeelsewherefordetails(Rouchonetal.,2009).Thecomparisonof picturestakenbeforeandduringageingenabledtheidentification ofchanges.Thisvisualassessmentallowedtopinpointthemost obviousdamages.
2.3.2. X-raydiffraction(XRD)
X-raydiffraction(XRD)wasusedtoidentifycrystallinephases present inshalespecimensand alsoinnewlyexcavatedshales.
Small samples, several millimetres large, were taken from the slabsandcrushedinanagatemortar.Onsomesamples,XRDwas usedadditionallyfortheidentificationoftheefflorescencewhen thelattercouldbesampledinsufficientquantity.Analyseswere conductedwithaconventionalCu-Kapowderdiffractometer(D2
Phaserdiffractometer)equippedwithaCeramicKFLX-raytube sourceanda 1D-linearLynxEyedetector,usingstandardcondi- tions:30kV;10mA;2urange:3◦ to65◦;timescan0.2sstep−1; 3068steps;angularspeed200radmin−1.Phaseswereidentified withtheJointCommitteePowderDiffractionStandardusingthe EVAsoftware(Bruker,Germany).
2.3.3. Ramanspectroscopy(RS)
Ramanspectroscopy(RS)isapowerfultoolforthespeciation ofsulphatemineralsandwasalreadyusedfortheanalysesoffos- sildegradationproducts(Rouchonetal.,2012).Whenthecrystals werepresentinaquantitytoosmalltobeanalysedbyconventional XRD,theywerecharacterizedbyRS,bytheuseofaRamanmicro- scope(inVia,Renishaw,UK;532nm,0.5to2.5mW,50×objective) followingaprotocolalreadydescribedelsewhere(Rouchonetal., 2012).Asshaleisahighlyfluorescentmaterial,themeasurements wereperformedonmicrosamplestakenfromthesurfaceofdam- agedshales.
2.3.4. Scanningelectronmicroscopy/energydispersive spectrometer(SEM/EDS)
Elemental analysis by SEM/EDS was mostly undertaken on shale matrices in order to map elemental distributions onthe microscale(withanapproximatelateralresolutionof5microme- ters)andtoidentifycorrelatedelements.Inaddition,itwasuseful tocharacterizemorepreciselysomeefflorescencematerialssuchas boussingaultite,mohriteandjarositetypesulfates.Measurements were conducted without any specific sample preparation with alowvacuumscanningelectronmicroscope(JEOL,JSM-5410LV) coupledwithan X-rayprobe (Oxford Link Pentafet).Elemental mappingswererecordedwiththefollowingexperimentalparame- ters:acceleratingvoltage,20kV;pressure,20Pa;workingdistance, 20mm;aperture,2;acquisitiontime,1hour.Alldataweretreated withtheLinkISISsoftware(OxFord).
2.3.5. SK-edgeX-rayabsorptionnearedgespectroscopy(XANES) XANESappearstobeapromisingtoolfortheanalysisofsulphur speciationinrocksandminerals(Prietzeletal.,2003;Mikhlinand Tomashevich,2005;Jugoetal.,2010;Boyeetal.,2011).Anumberof preliminaryexperimentswerealsoundertakenontheLUCIAbeam lineoftheSOLEILsynchrotron(Saint-Aubin,France)toevaluatethe feasibilityofXANESfortheanalysisofsulphurinshalematrices.
Thesamplewasplacedinavacuumchamber(210−2atm)and themeasurementswereperformedinX-rayfluorescencemode withthefollowinggeometry:thesamplesurfacewastilted20◦ withrespecttotheincidentbeamandthedetectorwasoriented perpendiculartotheincidentbeam.SK-edgeXANESspectrawere recordedintheenergyrangefrom2440to2600eV,employing175
Fig.3.SK-edgeX-rayabsorptionnearedgespectroscopypreliminarytesting:eval- uationofthestabilityofsamplesunderthebeam.Fourspectrawereconsecutively recordedonthesamespotonsampleMNHN.F.6889.Nomodificationisobserved betweenthefirstandthefourthspectra,meaningthattheexperimentalconditions donotprovokesulphurreductionunderthebeam.
Testspréliminairesd’absorptionauseuildelaraieKadusoufre:évaluationdelastabilité deséchantillonssouslefaisceau.Lesquatrespectresontétésuccessivementenregistrés surunmêmepointdel’échantillonMNHN.F.6889.Lasuperpositionparfaitedesspectres montrequ’iln’yapasderéductiondesoufresouslefaisceau.
datapointswithaspacingof0.2eVbetween2465.2eVand2500eV and65datapointswithaspacingof1eVelsewhere.
Theanalysisofsulphurrichspeciesbysynchrotronradiation isgenerallylimitedbythefactthatsulphurispronetoreduction underhighintensitylight beams,thusleading tomeasurement artefactsifthisreductionoccursduringtheacquisition(Wilkeetal., 2008).Onourspecimens,thisdifficultywasovercomebytheuse ofadefocused1×1mmbeamthatlimitstheincidentlightflux persurfaceunit.AsshownonFig.3,severalconsecutiveacqui- sitionscouldbemadeonthesame positionwithoutnoticeably changingthespectrum;thelatterremainsclosetothatofgypsum (CaSO4·2H2O).
SK-edgeXANESmeasurementswereperformedonallthematte matricesoftheMNHNspecimens.Severalspectrawererecorded oneachmatrixandthemostcharacteristicspectrawerecompared tothedatacollectedonsulphurreferencesoravailableintheliter- ature(Cotteetal.,2006;Bohicetal.,2008;Almkvistetal.,2010)or indatabases(ID21,2011).
3. Results
3.1. Damagedspecimens
3.1.1. Mineralcompositionofefflorescence
Themineralcompositionsoftheefflorescence,asdetermined byRamanspectrometry,arereportedinTable1.Mostefflorescence belongstotheironsulphategroupandmorespecificallytothefer- roussulphategroup(rozeniteandszomolnokiteareencountered everywhere).Ferricsulphates,suchascopiapite,jarositeormeta- voltinearealsodetectedinsomeefflorescencebutarepresentin muchloweramounts.
3.1.2. Mineralcompositionofmatrices
Themineralcompositionsofthematrices,as determinedby XRDmeasurements(Fig.4),arereportedinTable1.Theshinylay- ersarenon-diffractingandaremainlycomposedofanamorphous organicmatter,suchasvitrinite,asconfirmedbyasmearslide.In theshinylayerofMNHN.F.6889,somequartzinclusionsoflessthan 20micrometerswidearedetectedbyXRDandSEM/EDS.
Incontrast,allmattelayersaremainlycomposedofinorganic matter:theycontainquartz,kaoliniteandseveralundefinedclay minerals(smectite,chlorite,illite,muscovite,etc.)(Table1).Afew samplesadditionallycontainfeldsparsandcarbonates(dolomite).
Thepresenceofpyrite ishighlighted,butthis mineralcouldbe assessedinonlytwo(outoftheeight)specimens(Fig.4)while sulphatealterationphases(gypsumandamineralofthejarosite group)wereidentified byXRDin manyof thematrices. Asthe numerousSEM/EDSmappingsrecordedonthematricesdidnot evidence the presence of grains rich in sulphur and iron, we
Fig.4.X-raydiffractionspectrarecordedonthespecimenmatrices.
Diffractogrammesenregistréssurlesencaissantsdesspécimensendommagés
Table3
EnergyofreferencecompoundsatthemaximumoftheSK-edge.ThecompositionofmineralswasconfirmedbyX-raydiffraction,withtheexceptionofpotassiumsulphite, whichisacommercialproduct(SigmaAldrich®).TheenergycalibrationwasperformedongypsumbyfixingtheenergyoftheSK-edgeabsorptionmaximumat2482.5eV.
Energiesaumaximumduseuild’absorptiondelaraieKdusoufremesuréessurdescomposésderéférence.LacompositiondesminérauxaétéconfirméeparDRX,àl’exceptiondu sulfitedepotassiumquiestunproduitcommercial(Aldrich®).Lesénergiesontétécalibréesparrapportàl’énergiedumaximumduseuild’absorptiondugypse,fixéeà2482,5eV.
Compounds o.n.ofsulphur EnergyatthemaximumoftheSK-edge(eV)
Gypsum,CaSO4·2H2O +VI 2482.5
Anhydrite,CaSO4· +VI 2482.6
Rozenite,FeIISO4·4H2O +VI 2482.7
Melanterite,FeIISO4·7H2O +VI 2482.7
Jarosite,KFeIII3(OH)6(SO4)2 +VI 2482.8
Potassiumsulphite,K2SO3 +IV 2478.2
Marcasite,FeIIS2 −I 2472.0
Pyrite,FeIIS2 −I 2472.1
Pyrrhotite,FeIIS −II 2470.3
reasonablysupposethatpyriteandjarositearepresentintheshape ofsmallcrystalslessthan5mmlarge.
3.1.3. Determinationofsulphurspeciationinthematrices
TheenergiesofSK-edgemaximaarereportedonTable3for severalreferenceminerals.Asexpected,thesevaluesshowasub- stantial shift betweenthe most reduced sulphur species (such aspyrrhotite,o.n.−II,2470.3eV)andthemostoxidizedsulphur species (such asjarosite,o.n. +VI,2482.8eV). Themost charac- teristicspectrarecordedonthesurfaceofthedifferentspecimen matrices arereportedonFig.5: allthesespectrashowa sharp absorptionedgepositionedat2482.5±0.1eV,meaningthatsul- phurismostlypresentassulphates(o.n.+VI)(Table3)(ID21,2011).
In most XANES spectra, a post-absorption edge pattern is noticeablebetween2484eVand2486eV.Asitispresentongyp- sum/anhydritespectraandabsentfromallothersulphatespectra, itisconsideredtobeasignatureofcalciumsulphate.Thisobserva- tionisconsistentwithSEM/EDSmapsinwhich20to100mmlarge grains,richincalciumandsulphuraredetected,mostlynexttothe surfaceandsparselypresentintheinnerpartofthematrix.These crystalsarenotvisiblewiththebinocularmicroscope.
SomeoftheXANESspectrashownospecificfeatureintherange 2484to2486eV(asspectrumCofFig.5a).AsXRDevidencedthe presenceofmineralsofthejarositegroup,theseXANES spectra mayreasonablybeattributedtoironsulphate.
Fig.5.ExampleofX-rayabsorptionnearedgespectroscopyspectrarecordedon matricesofMuséumnationald’Histoirenaturelle(MNHN)specimen.SpectraA, B,CandDwererespectivelyrecordedonspecimenMNHN.F.6888,MNHN.F.6889, MNHN.F.6890andMNHN.F.6892.
ExemplesdespectresXANESenregistréssurlesspécimensduMNHN.LesspectresA,B,C etDontétérespectivementenregistréssurlesspécimensMNHN.F.6888,MNHN.F.6889, MNHN.F.6890etMNHN.F.6892.
Althoughsulphurismostlypresentassulphatespecies,asmall XANESpatternintherange2472to2474eVisnoticedforallspec- imens,attestingthepresenceofreducedsulphurspecies(Fig.5b).
Thissignalseemstobecomposedoftwoabsorptionedgesrespec- tively positioned at 2472.1eV and 2473.9eV. The first edge is similartothatofpyriteormarcasite,i.e.,ironsulphidesofo.n.−I;
itis notpossibletodistinguishwithXANESbetweenthesetwo
Fig.6.Apparitionofcracksduringartificialageing.
Exemplededélitementdesschistesaprèsvieillissementartificiel.
Table4
ListofthedifferenttypesofdamagethatareobservedonartificiallyagedshalesamplescomingfromtheSurmoulinsite.Ageingwasperformedat50%ofrelativehumidity.
Theweeknumberscorrespondtothefirsttimedamagewasobserved.“No”meansthatnodamagewasobservedafter16weeksofartificialageing.
ListedesdifférentstypesdedommagesobservésaprèsvieillissementartificielsurleséchantillonsprovenantdeSurmoulin.«No»signifiequ’aucundommagen’estobservéaprès 16semainesdevieillissementtandisquelenombredesemainefaitréférenceàladated’apparitiondudommage.
Temperature 40◦C 70◦C 90◦C
Typeofdamage Fissuration Efflorescence Fissuration Efflorescence Fissuration Efflorescence
Tube1 Week14 No Week8 No No No
Tube2 Week7 No No Week10 No Week10
Tube3 Week9 No No Week6 No Week6
Tube4 No No No Week8 No Week8
Tube5 Week13 No Week13 No No Week10
Tube6 Week12 No No Week15 No Week15
Tube7 No No No Week14 No No
Tube8 No No No Week16 No No
minerals because they possess similar signatures. However, as pyritewasdetectedbyXRD(Table1),itseemslikelythatthisfirst edgeismostlyrelatedtopyrite.Thesecondedgedoesnotseemto berelatedtoironsulphidesandremainsunattributed.
3.2. Artificialageingofnewlyexcavatedshales 3.2.1. Visualassessmentofartificialageing
Theexaminationof thephotographs beforeandafterageing madeitpossibletoassessthedegradationofsamples.Twotypesof visibledamagebecomeapparent:macroscopiccrystallineefflores- cenceandfissuration.Efflorescenceinitiallyincreaseswithageing timeandthenstabilizes.Fissurationisassessedbythefactthat shaleslabssplitandflakeaway(Fig.6).
Foreachsample,thetypeofdamageandthetimewhenitwas becomingvisiblewasdocumented.Thisapproachshowsthatall shalesamplesmaygetdamagedwhatevertheoutcroptheyorig- inatefrom.However,the reactivityof thesamplessignificantly differsfromoneoutcroptoanother:thelessreactivesamplescor- respondtothesitesofIgornay,MilleryandBois-des-Grands-Miens whilethemostreactiveonesoriginatefromthesitesofLaComaille, Muse,Saint-Léger-du-BoisandSurmoulin.
Additionally it wasnoticed that samplesof thesame origin behavedifferentlyfromeachothereveniftheyareagedinsimilar temperatureconditions.Table4summarizesthedamagesobserved onthe24samplesfromtheSurmoulinsiteandclearlyillustratesthe differencesamongsamples:at40◦C,three(outoftheeight)sam- plesappearundamagedafter16weeksofartificialageingwhiletwo samplesalreadygotdamagedafterlessthan10weeksofageing.
Theseobservationsconfirmthenecessitytoconsideralarge numberofsamplesforidentifyingoveralltendencies.Despitethe discrepanciesobservedamongsamplesofthesameorigin,based uponthedamageassessmentof168samples,itispossibletohigh- lightafewgeneralbehaviours.
Firstly,noefflorescenceisvisibleonthesamplesthatareshow- ingobviouscracksandviceversa(Table4).Thisfeatureisnoticeable whatevertheoriginofthesamples.ItisclearlyillustratedbyTable5 thatreportsthetotalnumberofsamplesaffectedbyeachtypeof
Fig.7. X-raydiffractionspectrarecordedonshalematricesbeforeartificialageing.
Diffractogrammesenregistréssurlesencaissantsdeschisteavantvieillissement.
damage.Inthistable,itcanbeseenthatonlytwosamples(outofthe 168)simultaneouslyshowefflorescenceandcracks.Thisbehaviour appearscontrarytoourexpectations,asonewouldapriorisuppose thattheemergenceofefflorescencewouldstressthematrixdueto thecrystallizationpressureduringsaltgrowthandmakeitmore fragile.
Secondly,anincreaseoftemperaturewasexpectedtoenhance alltypesofdamages.Thisisnotthecase:atlowtemperature(40◦C), fissurationprevailswhereasathightemperature(90◦C),efflores- cencegrowthprevails(Table5).ThesamplesfromtheSurmoulin site(Table4)illustratethistendency:at40◦C,five(outofeight) samplesgetfissuredwhilenoefflorescenceappears;at90◦C,efflo- rescenceisnoticeableonfive(outofeight)sampleswhereasnone ofthesamplesappearfractured.
Table5
Summaryofthenumberofnewlyexcavatedshalesamplesthatexhibitdifferenttypesofdamageafter16weeksofartificialageing.Thistabletakesintoaccountalltheshale samplesthatwerecollectedonthesevenoutcropsoftheAutunBasin(seeTable2).
Résumésdesdommagesobservésaprès16semainesdevieillissementartificiel.Estprisencomptedanscetableaul’ensembledesprélèvementsdeschistesargileuxréaliséssurles septaffleurements(Tableau2).
Temperature 40◦C 70◦C 90◦C
Fissurationonly 15 3 1
Efflorescenceonly 6 29 25
Fragmentationandefflorescence 0 1 1
Novisibledamage 35 25 31
Totalnumberofsamples 56 56 56
Table6
CompositionofshalesamplesbeforeartificialageingasdeterminedbyXRDandSEM/EDS.
Compositiondesschistesargileuxavantvieillissementartificiel,déterminéeparDRXetMEB/SDE.
Attribution Saint-Léger-du-bois Muse Surmoulin LaComaille
XRD Quartz,SiO2 x x x x
Kaolinite,Al2Si2O5(OH)4 x x x
Weakpeakat14 ˚A(smectite,chlorite...) x x
Weakpeakat10 ˚A(illite,muscovite...) x x x x
Albite,NaAlSi3O8 x
MineraloftheFeldspargroup x x x x
Dolomite,CaMg(CO3)2 x
Mineralofthejarositegroup x
SEM/EDS Elementsdetectedinallsamples Al,Si,S,K,Fe Al,Si,S,K,Fe Al,Si,S,K,Fe Al,Si,S,K,Fe
Otherelements Mg,Ti,Ca P,Ca,Ti Mg,P,Ca -
SEM/EDS:scanningelectronmicroscopycoupledtoenergydispersiveX-rayspectrometry;XRD:X-raydiffraction.
Table7
Crystallineefflorescenceobtainedbyartificialageingat90◦C.EfflorescentcrystalswerecharacterizedbyRamanspectroscopyand(ifnecessary)SEM/EDS.
Efflorescencescristallinesobtenuesparvieillissementartificielà90◦C.LesanalysesontétéeffectuéesparspectroscopieRamanet(aubesoin)parMEB/SDE.
Ramanattribution Saint-Léger-du-bois Muse Surmoulin LaComaille
Numberofsamplesshowingefflorescenceat90◦C 5 6 5 3
Gypsum,CaSO4·2H2O x x x x
Jarosite,KFeIII3(OH)6(SO4)2 x x
Barite,BaSO4 x
SEM/EDS:scanningelectronmicroscopycoupledtoenergydispersiveX-rayspectrometry.
3.2.2. Compositionofnewlyexcavatedshalebeforeandafter ageing
Analysesweremostlyperformedonsamplescomingfromthe sites of Surmoulin, Saint-Léger-du-Bois, Muse and La Comaille, becausetheyappearedtobethemostreactive.Themineralogi- calcompositionoftheshales,asdeterminedbeforeageingbyXRD (Table6, Fig.7), is similartothat of theoriginal specimens. It includesquartz,kaolinite,feldsparsandclayminerals.Pyriteisnot evidencedalthoughironandsulphuraredetectedinallsamplesby SEM/EDS.
The samples coming from the Surmoulin site are rich in dolomite,whichwasexpected,asthelayerofSurmoulinisthe onlyoneoftheAutunbasinthatcontainssignificantquantitiesof carbonates(ElsassDamon,1977).Aminorquantityofjarositeis additionallydetectedinthesamplesoftheSaint-Légersitedespite thefactthat:
•noefflorescenceisvisible;
•thesesampleswerepreservedinanoxicconditionsof0.5±0.2%
O2untiltheanalysis.
Wethinkthatthismineralcouldalreadyhavebeenpresentin theinnerpartoftheshalebeforeexcavationandwasformedduring aprimaryoxidationthatoccurredonsite.
Someofthesamplesthatweremechanicallydamagedduring theageingat40◦CwereanalysedbyXRD.Theirdiffractionpat- ternsweresimilartothoseofunagedsamples.Nonewcrystalline phasewasdetectedconsistentwiththeabsenceofvisibleefflores- cence.
Table7 summarisesthecompositionof thecrystallineefflo- rescencethatisappearingat90◦C:asexpected,theefflorescence correspondstothesulphategroup.However,despitethefactthat iron waspresent inallmatrices,ironsulphatesarefoundtobe minorphases.Onlyafewcrystalsofjarositewerefound,which werescarcelyvisibleonthemacro-pictures.Inthesamplesorigi- natingfromtheLaComaillesite,somesmallcrystalsofbarite,of theorderof10micronsinsize,weredetectedbySEM/EDS(Table7, Fig.8a).Besidethisminoroccurrenceofjarositeandbarite,allefflo- rescencecrystalsappeartobegypsumandshowalargevarietyof
crystallinemorphologies:thinneedles,organisedinstarshaped structures(Fig.8a),largeparallelbeams(Fig.8b),isolatedcrystals showingwelldefinedplanaredges(Fig.8c)orconglomeratesof crystalsofvariousshapes(Fig.8d).
4. Discussion
4.1. Characterizationofdamagedspecimens
MostoftheelementsdetectedinthematricesbySEM/EDSare ingoodagreementwiththemineralphasesdetectedbyXRD.Sili- conandaluminium,thetwomajorelements,mostlycorrespond toquartz andkaolinite that arethetwo majormineralphases.
Theseelementscanadditionallybefoundinfeldsparsandother clay minerals (Table 1, peak at 14 ˚Aand 10 ˚A) in combination withmagnesium,sodium,potassium,calciumandiron. Sulphur canberelatedtosulphateorsulphideminerals,suchasjarosite orpyrite.Titaniumistheonlyelementdetectedinallsamplesby SEM/EDSwithoutbeingattributedtoaspecificmineral.Thismay beexplainedbyitsabilitytosubstitutesiliconincrystallinephases (Hartman,1969).
Thisconsistencyshouldnot conceal thecomplexity ofshale materials,whicharecomposedoforganicandinorganicmatterand ofmineralandamorphousphases.XRDisnotthemostinforma- tivemethodforthedeterminationofamorphousphasesandthe elementsthatarerelatedtoacrystallinephasemayadditionally bepresentinotheramorphousphases.Thisistypicallythecase ofsulphurandiron.Theoccurrenceofpyritewasdemonstrated byXRDonlyinsomespecimens.However,ironandsulphurwere detectedinallmatricesbySEM/EDSandasmallabsorptionedgeat 2472.1eV,whichmatcheswithironsulphidecompoundsofo.n.−I, waspresentinthegreatmajorityoftheSK-edgeXANESspectra.
Allthisinformationtendstoevidence(butdoesnotdemonstrate) thatthereisasmallfractionofpoorlycrystallisedironsulphidestill remaininginthematrices,despitethefactthattheywereexcavated morethanacenturyago.
Anadditionalsmallabsorptionedgeat2473.9eVissuggestive ofothersulphidecompoundsthatremainunidentified.Thesemay possiblycorrespondtozincsulphidessuchassphalerite,showing
Muse
Saint Léger Surmoulin La Comaille
Fig.8.Thedifferentmorphologiesofgypsumobservedonartificiallyagedshales.Binocularmagnifierpictures(left)andscanningelectronmicroscopybackscatteredelectron modepicture(right).ThesmallwhitespotsobservedonFig.8-a(right)correspondtobaritecrystals.
Morphologiesdesefflorescencesdegypseobtenuesparvieillissementartificiel.Imagesobtenuessousloupebinoculaire(àgauche)etparMEBenmodeélectronsrétrodiffusés(à droite).LespetitspointsblancsprésentssurlaFig.8-a(àdroite)correspondentàdescristauxdebaryte.
anabsorptionedge closetothisvalue(2473.8eV) (ID21,2011);
availableelementalmeasurements(ElsassDamon,1977)indicate thatthematricescontaintracesofzinc(100±20ppm).Yet,the presenceofsphaleriteappearsdoubtfulas:
•our SEM/EDS measurements evidenced small grains of zinc, approx.10mminsize,thatwerenotassociatedwithsulphur;
•sphaleritewasnotevidencedbyXRD.
Thisindicatesthatzincsulphide(ifpresent)isbelowthedetec- tionlimitofXRDandSEM/EDSandispresentinapoorlycrystallized form.
Othercandidatesfortheattributionofthe2473.9eVabsorp- tionedge(Table8)canbefoundamongorganicsulphidesofo.n.
−II:thiol,thioether,thiopheneorthianthreneexhibitanabsorp- tionmaximumoftheSK-edgenear2473.9eV(Bohicetal.,2008;
Almkvistetal.,2010),meaningthatthisenergycouldbeconsid- eredasasignatureofC-S-CorC-S-Hgroups.Amorequantitative
