1961ApJ. . .134. .9 95W higher temperaturesandpressuresbyafalling-temperatureand/orincreasing-pressureprocess. several kindsofnaturalandman-madediamondsInthiswaysometentativehypothesiscanbemade and highpressures(above55000bars).Thelargerterrestrialcrystalscouldhaveformedatsomewhat laboratory experienceindiamondformationhasbeenaccumulatedtomakepossiblethecomparisonof The blackpolycrystallinemeteoriticdiamondscouldhavebeenformedatlowtemperatures(1200°C) diamonds wereformedathighpressuresandtemperaturesinthepresenceofaniron-alloycatalyst. about theoriginofnaturaldiamonds,bothmeteoriticandterrestrial.Itisproposedthatmost kerk etal.1959;Hall1960). precludes directexperimentaldeterminationsofthepertinentquantities,evenifdia- proposed naturaldiamond-formingregions,bothdeepintheearthandfaroutspace, parison betweenman-madeandnaturaldiamondsshouldpermitustodiscardsome monds werebeingformedatthismoment.Thebestthatwecanhopetodoiscompare formed; wecanonlysurmisethatcertainprocessescouldhave,orwerelikelyto processes attempttoinfersomethingabouttheofnature.Itfollows,then, compare naturalandhumandiamond-makingprocessesdirectly;theinaccessibilityof genesis ofdiamondsinnature.Tobeginwith,wenotethatcannoteverhopeto compare naturaldiamondswiththosemadebymen,perhapstoshedsomelightonthe Hall, H.M.Strong,andtheauthors,amongothers, havebeenmakingsuchobservations various conditionsofformationtothekinddiamond produced.F.P.Bundy,H.T. form diamondsarenowtolerablywellknown,and itisnaturaltoattemptrelatethe lumps composedofmanytinydiamondcrystals,eachperhaps0.01mmorlessinaverage hypotheses aboutthegenesisofnaturaldiamondsandconcentrateonmoreplau- diamonds arefoundmakestheduplicationinlaboratoryofnaturaldiamond- occurred innature.Indeed,theextremecomplexityofnaturalsystemswhich for mentoduplicateallthedifferentkindsofnaturaldiamonds.Nevertheless,com- growing processesimpossibleforallpracticalpurposes.Moreover,itisnotyetpossible for thepastseveralyears,andfollowingrough ruleshavebeenestablished(Boven- sible ones. siderably larger;itisrarelybelow0.02mm,evenin carbonado,andisoften5or10mm. diameter. Thecrystalsizeofterrestrialdiamonds,ontheotherhand,isusuallycon- semble anykindofterrestrialdiamond.Meteoriticdiamondsarefoundasfriableblack about 10gm(50carats),thesediamondsresembleeachotherfarmorethantheyre- and thetotalweightofalldiamondextractedfrommeteoritesuptonowisprobably that wecannotheredescribewithcertaintytheprocessesbywhichnaturaldiamonds the diamondsofmenandnatureand,fromourknowledgemen’s trial. Althoughrelativelyfewmeteoriteshavesofarbeenfoundtocontaindiamonds © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem So far,notallthedifferentkindsofdiamondsfoundinnaturecanbeduplicatedbymen,butenough Now thatithasbeendiscoveredhowtomakediamond,wouldbeinteresting The conditionsofpressure,temperature,chemical environment,andtimeusedto Natural diamondsatpresentcanbeclassifiedintotwogroups:meteoriticandterres- 1. Nodiamondformsunlessthepressureishighenough fordiamondtobestableat ON THEORIGINOFNATURALDIAMONDS R. N.Wentorf,Jr.,andH.P.Bovenkerk Received October3,1960;revisedJuly21,1961 General ElectricResearchLaboratory I. INTRODUCTION ABSTRACT 995 1961ApJ. . .134. .9 95W 996 mum pressureseemstobearound45000bars. molten metal-carbonmixture)whichactsasacatalystforthetransformationby the particulartemperatureemployed.Withcatalystsystemsnowknown,mini- perature attheinstantofitsformation.Onceformed,adiamondcrystalisloathto mechanisms thatmainlyresemblethesolutionanddepositionofcarbon.Thebest catalytic metalsareCr,Mn,Fe,Co,Ni,Ru,Rh,Pd,Os,Ir,andPt.Thedirectionof are theresultingdiamonds,althoughchemicalenvironmentmayhavesecondary change intype,size,orcrystalhabit,excepttographite. the temperaturegradientseemstobeunimportant. influences. Attheverylowesttemperaturessofarfoundeffective(about1200°C), librium, thegreaterrateofnucléationandgrowth ofdiamondcrystals,andthe diamonds areusuallyblack. made upoftinyoctahedralfaces. kerk etal.1959).Thepressuresquotedherearebased ontheabruptelectricalresistance resemble thatofnaturaldiamond(Bovenkerk1961; Tolansky1960).Theman-made smaller andmoreimperfecttheyare,regardlessof thetimetakenfortheirformation. i.e., atrelativelylowpressures, fromFeCorsomeiron-carbonmixture.So far,labora- which thediamond-growingregionisindicatedfor aparticularcatalystsystem(Boven- cube facestendtobesmoothorshowterraces; natural cubefacestendtoberough, sures theoctahedralhabitismorecommon.This “low-pressure”cubehabitdoesnot tory experiencehasshown thatatpressuresof43000barsorbelow,graphite isformed changes inbismuthmetalat25000barsandbarium metalat60000bars. 3 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 3. Thehabitofadiamondcrystaldependsprincipallyuponthepressureandtem- 2. Diamondformsfromnon-diamondcarboninthepresenceofamoltenmetal(or 4. Thehigherthetemperatureofformationforagivensystem,morecolorless 5. Atagiventemperature,themorepressureexceedsthatnecessaryforequi- 6. Atpressuresbelowabout50000barsthecubehabit predominates;athigherpres- Rules 1and2areconvenientlyexpressedinthephase diagramshowninFigure1, From timetothe hypothesis israisedthatdiamondsmayformmetastably, R. N.WENTORF,JR.,ANDH.P.BOVENKERK Fig 1.—Phasediagramwithdiamond-growingregion 1961ApJ. . .134. .9 95W © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Fig. 2.—a,CanyonDiablometeoritediamond;b,NovoUreidiamond 1961ApJ. . .134. .9 95W from suchmixtureswhethertheybequenchedorcooledslowly.Inspiteoftheobvious fold, andtheactivationenergyincreasedtoabout250kcal/gmatomat20000atm. deserves toberegardedasextremelytentative. bly succeedsinmakingdiamondsmetastably,thishypothesisofdiamondformation diamond canbeformedatpressureswheregraphiteisstable.Untilsomeonereproduci- much laborontheproblem,noreproducibleprocesshasyetbeenfoundbywhich commercial advantagesofalow-pressureprocessforproducingdiamondandinspite Phinney (1954).found1-2percentgraphitizationofwell-formednatural at anypressurebelowequilibriumfordiamond. Diamonds containingmoreimpuritiesgraphitizeathigherrates.Inthepresenceofmol- of theprocessisabout170kcal/gmatom.Therategraphitizationdecreasedten- entirely graphiteinabout100seconds,ontheaverage.Theapparentactivationenergy 2250° Cshowedthat,at2000°inanear-vacuum,naturaldiamondchipsbecame in thislaboratoryweredoneathighertemperatures,wherethegraphitizationwasrapid diamond crystalsin48hoursat1400°C,thehighesttemperaturereported.Thestudies ten catalystalloys,thechangetographiteisquiterapid(limited,atmost,bydiffusion) portions andmightbeexpectedtographitizedifferently,inviewofthesensitivity surface. Itisknownthat,onaccountofpercussionorsubterraneanchemicalaction,the at thelowertemperatures,wherebygraphitizationproceedsslowlyandmainlyon and usuallyover25percentcomplete.Itispossiblethatadifferentmechanismoperates pockets inthemetal.Thematrixgenerallyconsistsoftroilite(iron-richFeS),with mm andareusuallyfoundinablack,non-metallicmatrixwhichoccurstheformof The firstisanickel-ironmeteorite,whereasthelattertwoarepredominantlystone. outer portionsofnaturaldiamondcrystalsarefrequentlydifferentfromtheinterior netite, andtracesofgraphite.Thefragmentswerestronglyferromagneticeasily meteorite. TheirDebye-Scherrerpatternsshoweddiffuselinesofdiamond,FeS,mag- Nininger 1952,1956;LipschutzandAnders1961). smaller amountsofgraphite,iron-nickelphosphides,andmagnetite(Moissan1904; the rateofgraphitizationtocrystalimperfectionsorchemicalpurity. tiny crystalsinafewmeteorites,includingCanyonDiablo,Goalpara,andNovoUrei. furic, nitric,andhydrochloricacidsinordertoremove superficialFeS,graphite,etc. Its Debye-Scherrerpatternshowedittoconsistof tinydiamondcrystalsmixedwitha lump isnotsosharpaswouldbeexpectedfrom0.01-mm crystallites. on edgearevisiblehereandtherethisfragment. However,theX-raypatternof appeared asshowninFigure2,a.Onlyafewtinyoctahedral crystalfacesabout0.01mm small amountofironandFeS.Afteracidtreatment itwasweaklyferromagneticand scratched aboroncarbidetestplate. affect diamond.)AnX-ray diffractionpatternofthesediamondsshowed themtobe oxidation inahotmixtureofH2SO4andKN0.(This treatmenthasnotbeenfoundto Figure 2,b.Thesefragmentswereisolated,together withsomegraphite,fromapartof octahedral facesabout0.01 mmlong. made oftinycrystals,and throughamicroscopeonecanseeoccasional portionsof this stony-ironureilitemeteoritebyG.O.S.Arrhenius. Theyweresenttotheauthors through thecourtesyofDr.H.C.Urey,andgraphite wasremovedbyafewminutes’ 3 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Some graphitizationstudiesinthislaboratoryattemperaturesrangingfrom1700to The abovegraphitization-ratemeasurementsdonotagreewellwiththeresultsof The diamondlumpsintheCanyonDiablometeoritemaybeaslarge1X1.5 Meteoritic diamondshaveablackcolorandareusuallyfoundasirregularlumpsof Some ofthediamondlumpswerepriedoutblackmatrixafragmentthis A numberoftinydiamondfragmentsfromthe Novo Ureimeteoriteareshownin One ofthelumps,about0.1mmindiameter,washeatedboilingmixturessul- II. METEORITICDIAMONDS—GENERAL ORIGIN OFDIAMONDS 997 1961ApJ. . .134. .9 95W paths ofapproachtotheseconditions. persists, accordingtotheroughrules.Thereare onlytwootheressentiallydifferent metal. InFigure4,£,isshownasimilarkindofdiamondwhichwasproducedoniron- obtained. Thediamondisthethin,blackdepositoftinycrystalsonnickel then nodiamondforms. If thetemperatureisaboveappropriatemelting tempera- metal catalystundersimilarconditions.Boththesedepositswereformedinabouthalfa loses itsresemblancetometeoriticdiamond.If the temperatureispermittedtoin- appear. Moreover,diamondcubesareformedinstead ofoctahedra.Thustheproduct mum requiredtemperatureat67000barswithnickel-metalcatalystandspectroscopic patterns areformedinthelaboratoryunderconditionsoflowtemperature(toproduce carbon, inareactioncellasshownFigure3,thediamond4,a,was described byH.T.Hall(1960)wasused.Bytakingspecialcaretooperateatthemini- many tinycrystals).Ahigh-pressure,high-temperatureapparatussimilartotheone size wasestimatedtobeabout100AfromtheirX-raydiffractionpatterns. et al.1957)arereportedtobesmallandmixedwithgraphite.Theiraveragecrystallite 998 some otherpathessentiallydifferentfromincreasing thetemperatureathighpressures, come moreorlesstransparentinsteadofblack. the blackpolycrystallinediamonddoesnotform.Some otherkindofdiamondformsand crease slightly,therateofgrowthandcrystal sizeincrease,andthediamondsbe- minute. Ifthereactionisallowedtocontinue,i.e.,ifzonekepthotfor iron andcarbon.Ifthesynthesispressureisreducedbelow50000barsorso,nucléa- about 10minutes,diamondlumps,suchasthoseshowninFigure4,c,areobtainedfrom the blackcolor)andmoderatelyhighpressure(toproduceanucléationrate tion ratefordiamondcrystalsisreducedsothatsignificantly largercrystalsizesbeginto © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem If thetemperatureisbelow themeltingtemperatureofcatalyst-carbon system, Diamonds similartothesemeteoriticdiamondsinsize,color,andX-raydiffraction The diamondsextractedfromtheGoalparameteoritebyUreyandco-workers(Urey If thereactionconditionsofhighpressureandlow temperatureareapproachedby R. N.WENTORF,JR.,ANDH.P.BOVENKERK a) IncreasingthePressureatConstantTemperature 1961ApJ. . .134. .9 95W © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem diamond lumpsgrownwithiron;d,whitepolycrystalline mass. Fig. 4.—a,Diamondlayeronnickel;b,diamondiron(insidewhiteboundary);c,black 1961ApJ. . .134. .9 95W o iron, nickel,etc.(Boydand England1959;StrongandBundy1959).Thefact thatmetal melting temperaturesofsuchsilicatesincreasewith pressuremorerapidlythanthoseor have permittedfusionofthesilicatecomponents themeteorite,particularlysince is dispersedthroughoutthe fragmentsofthemeteoriteswouldhelppreserve fromsolu- large body.Temperaturesjusthighenoughtoproduce suchblackdiamondswouldnot in theneighborhoodof1200-1300°C.Suchconditions arelikelytooccurdeepinsidea very littleofthematternowmakingupthesemeteorites istheactualmothermatrixof views, whichwillbediscussedlater. processes ordeepinsidealargebody.Goalpara appearstohavebeenwellannealed it appearsthattheirdiamondsweresynthesizedextra-terrestrially,eitherbyimpact relatively lowpressuresweredevelopedinthemuponcollisionwiththeearth.Hence conditions ofpressureexceedingabout55000bars andslowlyincreasingtemperatures since itslastcollision.Thesemeteoritesalsocontain somegraphite.Itispossiblethat by collisionprocesses.Therearecertainobstaclesinthewayofadoptingeitherthese pressures andtemperaturescouldhavebeenproducedeitherdeepinsidealargebodyor monds havebeenfoundonlyinmeteoriteswhichcontainsomeiron.Thenecessaryhigh formation islimitedbydiffusionofcarbonorcatalyst.Thefastestgrowthratesfor diamond byvirtueofanincreasedratenucléation,butultimatelythetrans- Higher pressurestendtoacceleratethelinearrateoftransformationgraphite tion inmetalanydiamonds whichwereformed.Arelativelysmallamount ofiron- these diamondsformedinamatrixsimilartothat inwhichtheyarenowfound,under as wellthefactthattheyarenotassociatedwithlargeterrestrialcraters,suggest formation ofthesediamonds.Sofar,laboratorystudiesondiamondalonedonotpermit If themeteoriticdiamondsofCanyonDiabloformedbyprocessesnowknowntoexist, diamond lumpssofarobservedinlaboratorysynthesesdonotexceed1mmpersecond. monds form,buttheactualentryintodiamond-formingregionshouldbeatcon- perature ishigherthanthis,green,yellow,orcolorlessdiamondcrystalswillform. by thesamesingleprocess.Sofar,ofallcomponentsmeteorites,onlyiron- establish theformation-timeintervalsmoreexactly. found inGoalparaandNovoUreiimplythatshortertimeswouldhavesufficedforthe since someofthelumpsareatleast1mmindiameter.Thesmallersizesdiamonds one wouldaccordinglyplacealowerlimitofabout1secondontheirformationtime, catalyst, i.e.,eitherthereactionoccursinafewsecondsoritfailstooccurhours. rate isaverysteepfunctionofthetemperaturenearmelting ditions ofhighpressureandlowtemperaturetoproducesmall,blackoctahedra. first formistherebyhighenoughtoproducegreen,yellow,orcolorlesscrystals.Of the diamonds. an accurateestimateoftheupperlimitforformation-timeinterval.Perhapsother course, thetemperatureandpressuremayfollowawidevarietyofpathsbeforedia- studies ofmeteoritesorthediscoverynewdiamond-makingprocesseswillhelpto will form,butthecrystalsbelarge(0.1mm)andhavecubehabit.Iftem- is inthenarrowintervalsuitableforproductionofblackdiamond,thendiamond ture, eitheroftwothingswilloccur,dependingonthetemperature.Iftemperature (and nickel-)richphasesareknowntocatalyzediamondformation,and,sofar,dia- The laboratoryexperimentsondiamondformation indicatethatitispossible © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem The relativelyintactappearancesofthestonyNovoUreiandGoalparameteorites, As forthetimevariable,laboratoryexperienceisthatdiamond-formation Perhaps notallmeteoriticdiamonds,despitetheirsimilarappearances,wereformed If thepressureishighenoughforoctahedratoform,temperatureatwhichthey III. METEORITICDIAMONDS—GOALPARAANDNOVOUREI b) DecreasingtheTemperatureatConstantPressure ORIGIN OFDIAMONDS 999 1961ApJ. . .134. .9 95W mond. nickel nexttoalumpofcarbonwouldhavesufficedtransformtheintodia- distributed inthemandwithwhatphasestheyarenowcontact. mondiferous meteoritefragmentshaveonlytracesofWidmanstattenstructure;these fragments consistsofiron-nickelwithsomecohenite(impureFesC).Usuallythedia- investigators (Moissan1904;Nininger1952,1956;LipschutzandAnders1961).The been foundnearthecraterrim.Onplainsurroundingarefragments fragments appeartohavebeenheated(LipschutzandAnders1961)always and areoftenassociatedwithschreibersite(Fe,Co,Niphosphides).Thebulkofthe diamonds inthesefragmentsareusuallyfoundembeddedmassesoftroilite(FeS) 1000 R.N.WENTORF,JR.,ANDH.P.BOVENKERK which wereprobablycondensedfromvapor,arealsofoundthere.Afewfragments which havewelldevelopedWidmanstattenstructures;manytinymetallicspheres, from nearbymassescontainingtheseelements(alloysorcertaincompoundsof used graphitewhichextractedthenecessaryironcatalystfromsomenearbysulfide melt froze.Evidentlythecooledrapidlyenoughthattherewasnotmuchreaction unheated fragmentsofthismeteoritecontainsomegraphite.However,onlyrarelydothe found whichareunlikethemainmass(Nininger1952,1956).Bothheatedand elements, suchassulfides,oxides,etc.)tocatalyzetheformationofdiamondin suitable fordiamondformation,graphitemayabsorbenoughcatalystiron,nickel,etc. between thediamondandiron,norwastheremuchgrossseparationofphases bars. Thustheironnowdispersedinmeteoritictroiliteprobablyseparatedas main featuresofthe1atm.iron-sulfurequilibriumdiagramexistalsoatabout65000 mond whileanearbymassofirondoesnotbecomesaturatedwithcarbon.The may sufficetotransformarelativelylargeamountofgraphitecarbonintodiamond. graphite. Infact,severaloftheearliestpreparationsdiamondinourlaboratory ratory experimentsathighpressureswithiron-ironsulfidemixturesindicatethatthe diamonds lieincontactwithgraphite;mostoftentheyareembeddedtroiliteor were everappreciablyabovethecarburizingtemperature ormeltingtemperatureofthe the solidifyingmelt. schreibersite. fragments couldhavebeenproducedfromgraphite athighpressuresinthepresenceof iron-nickel-carbon eutecticformorethanafewminutes. and allowdiamondformationbutwouldnotnecessarily permitthecarbonordiamond forming conditions.ThusalumpofgraphiteenclosedbyFeSmaybeconvertedtodia- found incontactwithgrowing diamondcrystalsafterquenchingathigh pressuresor ite doesnotseemimpossible;infact,thismightalmost benecessarytopermitdiamond to diffuseveryrapidlyintotheiron.Asimilaroccurrence intheCanyonDiablometeor- sulfide canpermitenoughirontoreachthecarbon“saturate”with to existinthismeteorite,whoseaveragecarboncontent isbelowthatrequiredtosaturate even afterseveralhoursof slowcoolingathighpressure(65000bars)inthe iron-carbon decomposition ofFeCathighpressuresbecause inlaboratorytestsFeCisusually eutectoid region.ThusFe C isstableundertheseconditions.Thisnottoo surprising small amountsofironandtroilite.Theevidence is notgoodthattheywereformedby (Bovenkerk etal.1959).Onlyarelativelysmallamountofcatalyst,perhaps5percent, the iron-nickelalloyofmeteorite,particularly ifthetemperatureoffragment 3 3 3 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem These meteoritesdeservefurtherstudytodeterminejusthowtheirdiamondsare The diamondiferousfragmentsofthismeteoritehavebeenstudiedcarefullybymany Usually thetroilitewhichsurroundsdiamondscontainssomeexcessiron.Labo- There isthusampleevidencethatthediamonds foundinsomeCanyonDiablo It hasbeenobservedinlaboratoryteststhat,athightemperaturesandpressures It hasalsobeenfoundthatcarbonisnotverysolubleinironsulfideatdiamond- IV. METEORITICDIAMONDS—CANYONDIABLO 1961ApJ. . .134. .9 95W 9 9 because thedecompositionofcementitetographiteordiamondandironinvolvesan momentum considerations.Thecraterisabout170metersdeepand1.3kmin more detailinafuturepaper.Abriefsummaryofthetwoalternativesisgivenbelow. pact withtheearthordeepinsidealargebody.Bothhypotheseswillbediscussedin more orlessseparatefromeachother,embeddedinamassofiron,forthevolume products ofthedecompositionalumpFegCtoconsistmanytinydiamondgrains, increase involume.Furthermore,theCanyonDiablodiamondsarelumpsaslarge has consideredmanyofthedetailsimpact. diameter. Themeteoriteprobablystruckwithavelocitynotmuchbelow11km/sec, sary fortheformationofCanyonDiablodiamondswereproduced,whetherbyim- ratio ofirontodiamondwouldbeabout7. kilometers indiameter(AndersandGoles1961). where pisinbars,grams/cc,andRcentimeters. diamond-forming pressuresintheirinteriors.Thegravitationalpressure,patthecenter necessary topostulatetheexistenceofmeteoriticparentbodieslargeenoughproduce expected tolastformorethanabout0.1second. be favorableforproducingsomecoesite,eventhoughsuchconditionswouldnot rock underthecollidingbody.Thisviewisreinforcedbydiscoveryofcoesite,a duced inthemeteoriteforasmallfractionofsecondduringimpact.Shoemaker(1960) 2 X10tons(diameter60meters,ifiron)andhadavelocityof15km/sec.Thecalcula- the earth’sescapevelocity.Öpik(1958)estimatedthatmeteoriteweighedabout depth toprobableimpact velocityisabout0.01second.)Thecatalyzedgrowth rateof of laboratoryexperience, calls forgrowthtimesofatleast1second.However, thedura- of abodyuniformdensitypandradiusRisgivenby in silicatesystems.Thecombinationofimpactcompression,heating,andshearingwould found thataddedshearingstressesmaygreatlyacceleratetheapproachtoequilibrium necessary toproducethisformofsilica(Coes1953).DachilleandRoy(1960)have dense formofsilica,insometheshearedandpartiallymeltedsandstonecrater tively highpressures,welloverseveralhundredthousandatmospheres,wouldbepro- tions ofGilvarryandHill(1956)ontheimpactlargemeteoritesindicatethatrela- 2000° C;toformcoesiteatsuchtemperaturesrequiresstaticpressuresoftheorder of theseisthatthediamondlumpsaresometimes larger than1mm,which,onthebasis ite-sized fragments.Manyoftheobservedproperties ofmeteoritesmaybemoreeasily Lastly, therewouldremaintheproblemofbreaking upsuchalargebodyintometeor- explained iftheirparentbodiesareassumedto have beenonlytensorhundredsof are believedtohaveformedattemperaturesbetween 350°and700°C(Uhlig1954; body tohavearadiusofabout1540km,theourmoon.According 35000 barsorhigher.Staticpressuresoftheorder25000at600°Carenormally tion ofhighimpactpressures couldnothaveexceeded0.1second.(Theratio ofcrater 1.0 mminwhichthediamondcrystalsarebondedtoeachother.Onewouldexpect as 4X10years;suchanageiscomparablewith theagesofolderterrestrialrocks.) Urey 1956).(Potassium-argondatingmethodsindicate thatmanymeteoritesareasold calculations ofAllanandJacobs(1956),suchabody wouldbeexpectedtotoolarge to cooloffsuffcientlyin4X10yearsproduce theWidmanstattenfigures,which (Chao etal.1960).Evidentlytherockreachedtemperaturesoforder1500°- 1 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem An impacthypothesiscertainlydoesnotseemimplausibleonthebasisofenergyor It isnotyetpossibletosaywithcertaintyhowthepressuresandtemperaturesneces- An impacthypothesisisalsofavoredbysomeauthorsbecauseitwouldnotthenbe Some argumentscanbemadeagainstanimpactorigin ofthediamonds.Thestrongest To produce55000barsinapredominantlystonymaterialofdensity4requiresthe Similar highpressuresandconcomitanttemperatureswouldbeexpectedinthe -142 ÿ =6.67X1017rpR, ORIGIN OFDIAMONDS 1001 1961ApJ. . .134. .9 95W 9 o 9 ble withthelaboratoryexperienceondiamondformation.Itisalsocompatible mentation, andbecauseoftheweaknessmeteoriticmaterialsatdiamond-forming mechanically weakstructuresofmostmeteorites,whichwouldpermittheireasyfrag- pressures foraslongseveralsecondsinsidebodiesmorethanmetersindiam- was dependentoncatalyticaction. The decompositionofFeCtoironandgraphiteinvolvesanover-alldecreaseindensity, produce diamondfromgraphite,withoutbenefitofcatalystaction,thenonewouldex- diamond islimitedultimatelybydiffusionofcarbonormetal,andonewouldnotexpect average. Theprobability offormingsuchbodieswouldappeartobelow, since special pure nickel-ironalloys. or so.Higherpressureswouldtendtoreducethetemperatureatwhichy-atran- of coolingalargebodysoastoproducetheWidmanstättenfiguresin4X10years parently notsohighasthoserequiredfordiamondformation,andthereisthepossibility patterns (Uhlig1954).However,thepressuresrequiredforstabilityofFeCareap- about 900-1100°Kinthemanufactureofmalleableiron.Theseprocessingtemperatures Laboratory testsathighpressurehaveindicatedthatFeCisstablewithrespectto namic dataevidentlydonotpermitanaccurateestimateoftheequilibriumpressures so thatpressureshouldenhancethestabilityofFeC.Presentlyavailablethermody- observation thatmuchcohenite(impureFeC)isfoundinthemeteoritefragments. eter. However,theprobabilityofsuchaneventwouldseemtobelowbecause is notalwayssufficientfordiamondproductionandthattheformationofdiamonds However, muchgraphiteisfoundinthem,whichindicatesthatstrongshockcompression the compressionsduringimpacttoaffectthisratematerially.Ifextremelyhigh pect nearlyalldiamondandverylittlegraphiteinmostofthefragmentsmeteorite. segregative effectswould be necessary.Possibleeffectswouldincludethegreater tough- for itwouldhaveasmall size, ahighthermaldiffusivity,andradioactivity lowerthan pressures wouldbeproducedinsuchabodyifithad aradiusofperhaps900km.Such body. Urey(1956)hasproposedthatseveralgenerations ofparentbodiesmayhave formation oftheWidmanstättenpatternsathigher temperaturesthanareobservedfor and thusprolongorblocktheprocess(Kaufmann et al.1961).Ontheotherhand,Vogel sition (believedtobethebasisforformationof theWidmanstättenpatterns)occurs pose, andillustratetheneedformoreaccuratethermodynamicdatathissystem. a bodymightcoolrapidlyenoughtoformWidmanstätten figuresin4X10years, that presentlymakethelarge-bodyhypothesisdifficult.Chiefamongtheseisproblem are comparabletothosebelievedbeinvolvedintheformationofWidmanstatten diamond orgraphiteatpressuresofabout50000barsinthetemperaturerange500°- should bestableat1100°Kand1atm.,whereitisobservedexperimentallytodecom- sure tobeabout18kbat800°K.BothcalculationsunfortunatelyindicatethatFeC wood (1960),usingdifferentbutpresumablyequallyreliabledata,estimatedthepres- required topreserveFeCatvarioustemperatures.Forexample,LipschutzandAnders temperatures. that thereliefofthesestressesmightbeslowenoughtomaintaindiamond-forming 1002 existed. that certainimpuritiesinthecohenitemayservetostabilizeitatlowpressures. transient pressuresoftheinitialstagesimpacthadbeen,bythemselves,sufficientto 1700° K.Bycomparison,at1atm.,FeCcommonlydecomposestographiteandiron (1951) hassuggestedthatthepresenceofphosphorous inmeteoritesmightpermitthe (1961) estimatedthattheequilibriumpressureat800°Kwasabout0.7kb,whileRing- 3 3 3 3 3 3 3 3 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem A large-bodyoriginofthepressuresnecessaryfordiamondsynthesisisquitecompati- It ispossiblethatanimpactmightcreatecomplexhighstressesinsideabodyand It isgenerallyagreedthatthemeteoriteswerederived frommorethanoneparent It isnotthepresenceofdiamondsorcohenitebutratherotheraspectsmeteorites As anextremecase,onemightconsiderabodyquite richiniron.Diamond-forming R. N.WENTORF,JR.,ANDH.P.BOVENKERK 1961ApJ. . .134. .9 95W ness anddensityofironcomparedwithrock,sothatthefragmentsdispersedfroma found (Grodzinski1944).Theycanbeputintotwomainclasses:(1)smallcrystals, e.g., carbonado,frames!te,ballas,andotherpolycrystallineforms,(2)largecrystals, collision wouldtendtobemorestony,andthepossiblepresenceofappreciablemagnetic body hypothesesingreaterdetail,payingspecialattentiontotheimpactprocesses, 4, d.Suchaclustercontainsonlyrelativelysmallamountofcatalystmetalandgraphite rium, sothatnucléationisrapid.Manytinycrystalsform.SomeareshowninFigure e.g., thebulköfnaturaldiamondstones,whichusuallyconsistofonecrystal,orperhaps the environmentofdiamonds,andbehaviorpossibleparentbodies. mond formation.Itisintendedthatafuturepaperwillexaminetheimpactandlarge- moderate synthesistemperatures,1700°Corso. in theintersticesbetweencrystals.Thecolorofmassbecomessubstantiallywhiteat two, three,orfourcrystalsgrowntogether.Consideringsmalldiamondsfirst,onefinds fields, whichwouldhaveenhancedtheattractionofironparticlesforeachother. various formsofcarbonado,framesite,etc.thatarefoundtoday.Surely,millions that suchdiamondsformatpressuressubstantiallyabovethoserequiredforequilib- years wereavailableforsuchprocesses,andnogreatstretchoftheimaginationisre- process athighpressuresorbyasuddenincreaseinpressuremoderatelytempera- natural polycrystallinediamondswereformedbyarapidheating-uporcooling-off of relativelyhighpressuresandmoderatetemperatures.Thevarioussubsequentplu- between graphiteanddiamond.Thehigherthetemperature,morenearlycolorless pressures suitableforthestabilityofgraphite,viaeitherorbothtwoprocesses:(a) ture. quired tosupposethem.Thereisyetnoexperimentalevidencecontradictaviewthat tonio mineralenvironmentstowhichsuchlumpswereexposedcouldhaveproducedthe out ofamixturegraphiteandcatalyst,whichwasoriginallyathightemperatures are formedatpressuresandtemperaturesnotfarremovedfromtheequilibriumline time: thegenesisoflargediamondcrystals. implies thattheywereformedunderpressuresthermodynamically requiredfordiamond graphite-diamond equilibriumatatemperatureof atleast1600°Cforseveraldays.The case theslownessofchangeswouldhavepermitted thesystemtoremainclose a slowtemperaturedecreaseintothegrowingregion(Neuhaus1954,1957).Ineither a slowpressureincreaseintothediamond-formingpressure-temperatureregionor(b) the diamond,althoughchemicalenvironmentmayhavesecondaryeffects. bars morelikely.Ofcourse, thehigherpressure,deeperdiamonds wereinthe natural diamondcrystalsformedatpressuresof at least60000barswith70000-80000 high formationpressure.Takentogether,thelaboratory experiencesuggeststhatlarge existence ofmanystonesinwhichasecondcrystal hasgrownaroundafirstsuggests could beformedthere.Theoctahedralhabitofmany diamondcrystalsalsoimpliesa stability. Fortherateofchangediamondtographite istoorapidat1600°Candhigher that thegrowthprocessmayhavebeeninterrupted (Williams1932). surface. earth, andthegreater difficultywouldhavebeenforthediamonds to reachthe to permitanydiamondpersistforlongatalow, graphite-stablepressure,evenifit © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem In contrasttometeoriticdiamonds,manyformsofterrestialdiamondshavebeen At themomentthereappeartobedrawbackseitherhypothesisofmeteoriticdia- It ispossiblethatnaturalpolycrystallinediamondlumpswereformedatconditions We turn,atlast,toaproblemwhichhasfascinatedmen,andladiestoo,forlong These findingssuggestthatlarge,pale,naturaldiamondcrystalscouldhaveformed All laboratoryexperiencesofarindicatesthatthemostperfectdiamondcrystals The highformationtemperatureimpliedbythe pale colorofthesediamondsalso V. TERRESTRIALDIAMONDS ORIGIN OFDIAMONDS 1003 1961ApJ. . .134. .9 95W u iron, nickel,andgrowingdiamond.Theinclusionofpyrope-almandinegarnetinadia- etc. Mostsuchmineralsarefoundinthelaboratorytobestablepresenceofmolten Wentorf 1959). mond implies,inaddition,thatthediamondwasformedatapressureleasthigh 1004 R.N.WENTORF,JR.,ANDH.P.BOVENKERK laboratory testsshowthatonlygraphite,neverdiamond,seemstoprecipitatefromthis this materialwilldissolvecarbon,evenathighpressuresandtemperature.But,sofar, enough forthisgarnettobestable,about20000-25000bars(BoydandEngland1959; more easilycaughtupandcarriedaway.Obviously,thereareanynumberofsub- diamonds fromgreatdepths(over100miles)totheearth’ssurface.Themediumin of SouthAfricawasthesolventormediuminwhichdiamondsformed.Itistruethat Lipschutz, M.E.,andAnders, E.1961.“OriginofDiamondsinIronMeteorites,”Geochim. etcosmochim. which thediamondsformedwasprobablyanironalloy(saturatedwithcarbon)be- solution. Itismorelikelythatthebluegroundservedmerelyasavehicletocarry terranean processeswhichcouldbeexpectedtoproducethevariouschangesinlocal Allan, D.W.,andJacobs,J.A.1956,Geochim.etcosmochim.acta9,256. temperature orpressuresuitableforgrowingdiamond. cause ironissuchanabundantelement;diamondswouldfloatonalloyandbe Kaufmann, L.,Leyenaar,A,andHarvey,J.S.1961“The Effect ofHydrostaticPressureontheFC.C.- Hall, H.T.1960,Rev.SeiInstr.,31,125. Bovenkerk, H.P.1961,“Man-madeDiamondMorphology,”paperpresentedattheBoltonLanding Anders, E.,andGoles,G.1961,J.Chem.Ed.,38,58. Bovenkerk, H.P.,Bundy,F.Hall,T.,Strong,M., andWentorf,R.H.1959,Nature,184,1094. Moissan, H.1904,C.R.,139, 773. us tounderstandmoreaboutourworld. Boyd, F.R.,andEngland,J.L.1959,Experimentation at HighPressuresandTemperatures,”Car- Fish, R.A.,Goles,G.G.,andAnders,E.1960,Ap./.,132,243. Dachille, F,andRoy,R.1960,Nature,186,34. Chao, E.C.T,Shoemaker,M.,andMadsen,B.M1960, Science,132,220. garding meteorites. diamond mayhavebeenhighpressure(aboveabout55000bars)andlowtemperature As timegoesby,wehopethatourincreasingknowledgeofdiamondsynthesiswillhelp Grodzdnski, P.1944,DiamondTools(NewYork:AntonSmit &Co.). Coes, L.1953,Science,118,131. Gilvarry, J.J.,andHill,E.1956Ap./.,124,610. bars orhigher,byprocessesofpressureincreaseand/ortemperaturedecrease.When catalyst. Inthiswayblackpolycrystallinediamondlumpswouldbeformed. these processesoccurredrapidly,polycrystallinediamondswouldhaveformed;when at somewhathighertemperatures,probablyleast1600°C,andpressuresof60000 they occurredslowly,thelarger,moreperfectdiamondcrystalscouldhaveformed. y (about 1200°C),withgraphiteasthesourceofcarbonandironoranalloy © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Many naturaldiamondscontainmineralinclusions,suchasgarnet,olivine,diopside, There is,sofar,noexperimentalevidencetosupporttheviewthat“blueground” on VeryHighPressure(publishedbyJohnWileyin1961as ProgressinVeryHighPressureResearch). B.C.C. ReactionsinIron-BaseAlloys,”paperpresented at the1960BoltonLandingConference sure Research). negie InstitutionofWashingtonYearBook,58,82-89 Laboratory testsindicatethatthemostlikelyconditionsofformationmeteoritic acta, 24,83. Conference onVeryHighPressure(publishedbyJohnWileyin1961asProgressPres- Not allthetypesofterrestrialdiamondhaveyetbeenduplicatedinlaboratory. The authorsareindebtedtoDr.EdwardAndersforstimulatingcorrespondencere- On theotherhand,terrestrialdiamonds,formostpart,couldhavebeenformed REFERENCES VI. SUMMARY 1961ApJ. . .134. .9 95W .1957,“DieSynthesedesDiamanten.II,ibid,69,551. Shoemaker, EM1960,PenetrationMechanicsofHighVelocityMeteorites,IllustratedbyMeteorCrater, —■—^—. 1956,TheArizonaMeteoriteCrater(Sedona,Ariz.:AmericanMuseum) Neuhaus, A.1954,“DieSynthesedesDiamanten,”Angew.Chem,66,525 Nininger, H.1952,OutoftheSky(NewYork:DoverPublications) Phinney, F.S1954,Science,120,393. Vogel, R.1951,NeusJahrbMineralAbh,83,23 Tolansky, S,andSunagawa,I1960,Nature,185,203. Ringwood, A.E1960,Geochim.etcosmochimacta,20,155. Öpik, E.J.1958,IrishAstr./,5,15. Urey, H.C1956,ApJ,124,623 Uhlig, HH.1954,Geochim.etcosmochin.acta,6,282. Williams, AF.1932,TheGenesisoftheDiamond(London:ErnestBenn,Ltd). Wentorf, R.H.1959,J.PhysChem,63,1934. Urey, HC.,Mele,A,andMayeda,T.1957,Geochimetcosmochimacta,13,1-4 Strong, H.M.andBundy,FP.1959,Phys.Rev,115,278. © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Arizona (Copenhagen:InternationalGeologicalCongress,BerlingskeBogtrykkeri). ORIGIN OFDIAMONDS 1005