1 Summary For measuring the density of a variety of geological materials in particular oddly shaped samples of relatively consolidated material Density is important in the context of cosmogenicnuclide measurements because the cosmic ray 64258ux is a ID: 23642
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2Methods2.1AnoteoncollectingsamplesTheideaofthiswholeprocedureistodeterminethedensityofthematerialinitsnaturalcondition.Thus,themostimportantthingistotrytogetthesampletothemeasurementwithoutdisturbingittoomuch.Ifyoucan,measurethedensityintheeld.Forweaklyconsolidatedmaterial,trytocollectthesampleinsomesortofrigidholdersoitwon'tbecrushedduringtransport.Forwetmaterial,sealsamplesinsomethingwatertightsothatwaterwillnotevaporatebeforeyoumeasuretheirdensity.Trytocollectmultiplesamples,andtrytomakethesamplesrepresentative.Ingeneral,largersamplesarebetter.2.2Collectingaknownvolumeintheeld{unconsolidatedsedimentInprinciple,itshouldbeeasytomeasurethedensityofanymaterialbycuttingoutacubeofthematerial,measuringthesizeofthecubetodetermineitsvolume,andthenweighingthecube.Inpractice,it'snearlyimpossibletocutoutaregularcubeofanynaturalgeologicaldeposit.Forsomeunconsolidatedsediments,itispossibletocollectaknownvolumeintheeld.Thisworkswellforwetsandandsilt.Itsometimesworksforglaciolacustrinesedimentandwet,clay-richglacialtill.Itgenerallydoesn'tworkforgravel,drysandandsilt,oranythingcemented.Thepreferreddeviceisasectionofaluminumpipeseveralincheslong.It'shelpfuliftheedgesoftheendsofthepipehavebeenbeveledontheoutsidetomakeasharpedge.Theprocedureisasfollows:1.Determinethevolumeofthepipesectionbyaccuratelymeasuringtheinsidediameterandlengthofthepipe.Measureasaccuratelyaspossibleusingcalipers.Ifthepipewascutbyhand,measurethelengthatseverallocationsaroundthecircumferenceofthepipeandtaketheaverage.Determinetheweightofthepipe.2.Intheeld,pushthepipeintotheoutcropfaceuntilmaterialstartstoextrudeoutthenearend.Hammeritasnecessary.Becarefultoensurethatthereisnoairspaceinsidethepipe.Digthepipeoutandcarefullyslicetheprotrudingsedimentawayfromeachend.3.Weighthepipeandsediment.It'sbesttobringtheel-cheapobalanceintotheeldwithyouanddothisonsite.Ifthisisnotpossible,wrapthesamplebyplacingsomethinghardovereitherend(properpipecapsarebest)andthensaran-wrappingandtapingthewholethingtominimizewaterlossduringtransport.Whendisassemblingitinthelab,makesurethatallthesedimentinthetubegetsweighedanddoesn'tfalloutduringcapremoval,etc.,orthatyouweighthetube,sediment,andcapstogetherandthenthecapsseparately.4.Subtracttheweightofthepipe(andcaps)fromthetotalweighttodeterminethesampleweight.Dividebythepipevolumetogetthedensity.It'sgoodtodothisafewtimesforeachunit.Themostimportantthingistogetthepipeentirelyfullofsediment.|||||||||||||||||||{UWCosmogenicNuclideLab{Densitymeasurement2|- 6.Calculatetheweightofpackingmaterial,WP=WTSP�WT�WS.CalculatethevolumeofpackingVP=WP=P.Pisthedensityofthepackingmaterial(seebelow).CalculatethesampledensityS=WS=(VT�VP).7.Dothemeasurementacoupleoftimestomakesureyoureallydidllallnooksandcrannieswiththebeadsthersttime.Notes:Metalvesselsworkbestforthis.Anythingplasticwillcausetroublewiththebeadsduetostaticelectricity.Wemeasurethevolumeofthetinsbymeasuringtheweightofwaterthatwilltinthem.It'simportanttomakesurethatthewatersurfaceisclosetotheactualtopofthetin(towhichyouwillgradethebeads){it'spossibletooverllduetosurfacetension.Weuse1-mmglassbeads(availablefromchemicalsupplycompanies).0.5-mmbeadsalsoworkOKbutareabitmessier.Wedeterminethedensityofpackedbeadsbyllingatinofknownvolumeandmeasuringtheweightofthebeads.Thedensityofour1-mmbeadsPis1.53gcm�3.It'sprobablyagoodideaforeachpersontoindependentlydeterminebeaddensitywiththeirownparticularscraping/compactingtechnique.Occasionallyit'snecessarytocleanthebeads.Wedothisbysonicatingtheminwater,rinsingthor-oughly,anddrying.Ifalotoflargechunksofforeignmaterialbuildupinthebeads,sievingmightbeneeded.Wealsousecleanbeachsand(mostlyquartz)inthe0.5-0.85mmsizerange.Weprepareitbysievingthesandtothissize,sonicatingitinwaterforapprox.1hr,thenrinsingitthoroughlyanddryingintheoven.Theadvantageofsandisthatitisinexpensiveandcanbeusedinsacricialapplications,suchasthewetdensitydeterminationmethoddescribedbelow,orwhensamplesareverypoorlycompactedandarelikelytobreakupduringtheprocessandmakeamess.Sanddoesnotcompactasreadilyasglassbeads(moreangulargrains),soit'sveryimportanttorepeatedlytapthetincontainingsandandsampleonthebenchasyouarellingit,tomakesurethesandisfullycompacted.Also,eachbatchofsandwillhaveaslightlydi erentdensitywhichwillneedtobemeasuredbeforestarting.OursandhasadensityPof1.45-1.47gcm�3.Byrepeatedlymeasuringthedensityofavarietyofsamples,includinglargequartzcrystals,whosedensity,ofcourse,weknowexactly,we'vedeterminedtheaccuracy/precisionofthistechniquetobe0.08gcm�3fortypicalmaterialswithdensitiesof1.2-2.7gcm�3.2.5Unconsolidatedsamples{stungintoavialIfthesampleiscompletelyunconsolidated,e.g.drysand,thereisonemethodremaining.Takeavialofknownvolume,forexampleoneofthesmallmetaltinsorplasticvialsinthedrawer.Usingyourngers,pressthesampleintothevial,attemptingtoduplicatethenaturalcompactionofthematerial.Formostsands,thismeanssquishingitinwithsomeauthoritytoensurethatthesandgrainsarewellpacked.Overllthevialandbladeo theexcesswiththesteelspatula.Weighsampleandvial,subtractthetareweightofthevial,anddividebythevolumeofthevial.|||||||||||||||||||{UWCosmogenicNuclideLab{Densitymeasurement4|- Figure1:Measuredwetdensitiescomparedwiththosecalculatedfromdrydensitiesusing2.Circles,wetdensitydeterminedbysaturationofdrysand,andtriangles,ofdrytill,asdescribedin2.6.Diamonds,samplesofglacialtillcollectedwetandthenoven-dried.Error-barsre ectwhatwebelievetobemeasurementprecisionasdescribedin2.4.Wetestedthisapproximationwithsamplesofunconsolidatedsandandglacialtillthatweobtainedfromdrieddrillcoreandwhosewetdensitywemeasuredasdescribedabove,aswellaswithsamplesofglacialtillthatwerecollectedwetandwhosedensitywemeasuredbeforeandafteroven-drying(Figure1).Theresultsshowthatthisapproximationseemstobeadequatewithintheresolutionofourmeasurementtechnique.|||||||||||||||||||{UWCosmogenicNuclideLab{Densitymeasurement6|-