Metallic Spheroids from Meteor Crater, Arizona'

Home , Cohenite, Octahedrite, Schreibersite, Troilite

TIIE AMERICAN MINERALOGIST, VOL 50, MAY_JUNE, 1965

METALLIC SPHEROIDS FROM METEOR CRATER, ARIZONA'

CvNrura W. Mnel, Jexnr LrrrrBn ANDE. C. T. Cneo, Li. S. GeologicalSuraey, Wash'ington,D. C.

Anstn.tct The mineralogy, texture, and chemical composition of metaliic spheroids from Meteor Crater, Arizona, were studied in detail. The spheroidal to elongateparticles studied average 0.5 mm across and characteristically are coated by siliceous glass containing numerous minute fragments of quartz. Some spheroids consist of a fine grained granular kamacite core, commonly intergrown and surrounded by a go.thiteJike iron oxide and maghemite. Associated with the kamacite is an interstitiai pink mineral optically similar to the pink schreibersite inclusions in the metallic spheruies in philippinites and indochinites. X-ray powder patterns of this pink mineral from the Meteor Crater spheroids are also iclentical to that of schreibersite. Eiectron microprobe analysis sholvs: Ni% FeTo Kamacite 2-24 74-97 Maghemite 5-6 59-62 Goethite-like mineral 21+ 45-60

The wide range of nickel content in kamacite, the fine-grained texture, and the siliceous glasscoating on the spheroidsindicate that the Meteor Crater spheroidsprobably condensed from a vapor or melt produced during impact of the meteorite. The range in nickel content in the kamacite of the Philippine and Indochina tektite spheruies is nearly identical to that of the kamacite in the spheroids from Meteor Crater, whose meteoritic origin is certain.

INrnotlucrroN As part of a systematicstudy of particles formed by fusion from the impact of a meteorite,the magnetic metallic spheroidscollected by Dr. H. H. Nininger from Meteor Crater, Lrizona were studied in detail. Thesespheroids were obtained with the kind permissionof Dr. Carleton Ioore, Director and Custodianof the Nininger Meteorite Collectionat the Arizona State University, Tempe, Arizona. Zaslow and Kellogg (1961)published a short note on the study of thesespheroids, but to our knowledge,no detailedstudl' [65 beenundertaken. In this studv, nearly 50 different spheroidsmost of them about half a millimeter but a few as large as 2 to 3 millimeters in diameter were studied in polished sections,by r-ray diffraction, and by the eiectron microprobe,or by a combinationof all three methods.The shapeof these sphereoidsvaries from irregular,to elongate,to spherical. Nearly all the spheroidswhich were examined under the binocular microscopewere coated on the outside by brown iron oxide, which can easilvbe removedb-v- treatment with diiute aceticacid and gentlerubbing

1 Publication authorized by the Director, ti. S. GeologicalSurvey.

667 C. IV. ML.AD. I LITTLDR AND E. C T. CHAO with a glassrod in a spot plate. Removal of this brown iron oxiderevealed that directl-vbelow this oxide,most if not all the spheroidshave a fragile thin white coatingof silicaor siliceousglass generally a fraction of a miili- meter thick (Fig. 1). This white isotropicglass coating, with an index of refraction f.rom 1.47 to 1.49 contains numerous powdery angular fragments of quartz. Inside the siliceousglass coating of the spheroids,commonly a layer of gray iron oxide of variable compositionand thickness wraps around a core of metallic nickel iron. That these spheroidsare

, 1,Tf., ,

Frc. 1. Photomicrograph of several spheroids shorving siliceousglass coating (light colored grains). Dark grains are uncoated results of impact induced fusion of the Canyon Diablo iron meteoriteis unmistakably indicated by their close association with the Meteor Crater of Arizona, the presenceof the glasscoating, the notable nickel content in both the gray oxide phase and the metallic core, the fine grainedtexture and the mineral assemblageof the core. Mrxpn.q.rocrc AND Pnrnocn.q.pnrcSrulrBs In polishedsections and under the reflectingmicroscope, most of the spheroidsconsist of the following mineral assemblage:a gray goethite- like iron oxide commonly intergrown with light gray maghemite,bright cream\i white kamacite and more rarely white schreibersite.In many spheroidsan intergrowth of troilite and pink schreibersiteoccurs interstitially among the fi.ne-grainedkamacite. In most spheroidsthe gray oxide phasesgrade into or interlock with the nickel-iron core. In a few spheroidshowever the order is reversed,the core being gray iron oxide rimmed by creamywhite nickel iron. METEORITIC MDTALLIC SPHEROIDS 669

Goethite-li,kemineral. This is a gray iron oxide phase which generally occurs around the creamy white nickel-iron core; sometimes intimately intergrown with it in a fingerprint or pearlitic texture. It also occurs as alternating bands in maghemite and is referred to as a goethite-like mineral on the basis of its coior (Figs. 2, 3, 4, and 10). Strong lines of goethite were detectedon the powder pattern of only 2 of the 15 spher-

Frc. 2. Photomicrograph of grain 1 in polarized reflected light showing lorv nickel kamacite (K), maghemite (M), and goethite-like iron oxide (G). oids which were r-rayed. The goethite-likemineral however,is observed optically in all the polished sections of the spheroids. The identification of this phaseis uncertain becauseof the poor r-ray data owing principally to the poor crystallinity of this phase.Furthermore, as shown by microprobe analysis, this gray iron oxide is also defi.cientin iron as compared to the compositionof goethite.

Maghemite. Maghemite is light gray in reflected light and generally occursin elongateisotropic grains alternating in bands with the goethite- Iike iron oxide (Figs.2 and 4) . Tracesof maghemitebased on the presence of only the strong reflections were detected by o-ray in 8 spheroids (Table 1). 670 C. W. MEAD, J. LITTLER AND E C. T. CHAO

Frc 3 Photomicrograph cf grain 2 in polarized reflected light showing contamination spots left in a point-by-point traverse made by the electron probe on kamacite (K) and goethite-like iron oxide (G)

Frc. 4. Photomicrograph of grain 3 in polarized reflected light showing elongated bodies of schreibersite(S), goethite-like iron oxide (G), and minor maghemite (M). M ET EORIT I C M L,TA LLI C SP II EROI DS 671

Tasrn 1. Mrxrnar, Assrnerecps ol UNrnn'lrr'n Spnnnotos Ex.lrrrNno sv X-Rav DrrnnncrroN

Relative Abundancel Film Spindle No. No. Kamacite Taenite Maghemite Quartz

17855 118 M m tr tr tr 17860-a r28 M m m tr 17861-a t29 M tr tr tr 17862-a 130 M m m m 17863-a lJt M m m

t7864 t32 M m tIl m l / ItO.) a t.t.t M m m 17866 t34 M m tr r7867 IJJ m M 17883 r47 M tr m m r7884 148 M m m M 17886 150 m tr M 17887 151 M m m M 17888 IJZ M m M r7890 154 M M

I M -major constituent. m minor constituent tr-trace constituent.

Kamac'ite.This mineral is commonly the major constituent in the core of the spheroids.It is creamy white, highly reflecting,and extremely fine- grained. Being isotropic, the fine-grainednature was revealedby interstitial pink schreibersite which marks the grain boundaries, and by its excellent homogeneous,nonspotty tr-ray powder pattern. Because of extremely fine grain size and random orientation of kamacite, the texture (Figs. 5 and 6) is interpreted as one of rapid quenching. There is no widmanstiitten stru cture.

Taenile. Taenite is difficult to detect in polished sections.Trace amounts are commonly detected in most of the spheroidsby *-ray. Taenite be- comesa major phaseof the residue,after the spheroidshave beentreated with hydrochloric acid and the kamacite and iron oxide phases are removed.

Whi.teschreibersite. These white grains are believed to be relict grains, not completely destroyed or transformed by the impact-induced fusion (Fig. a). Large schreibersitegrains are however rarely found in the smallerspheroids which we have studiedand in only oneof the grainswas 672 C. W. MEAD, J. LITTL]JR AND D, C. T. CHAO

Frc.5. Photomicrograph of a grain in polarized reflected light showing interstitial pink phase (light gray) and troilite (dark gray) outlining the boundaries of the kamacite grains.

schreibersiteobservedl here, as a white elongatedphase surrounded by an intergrowth of nickeliferousmaghemite and goethite-likeiron oxide. It is identified chieflv on the basis of its chemical composition and optical properties.

Pink schreibers.ite.Thls pale pink phase occurs interstitially alone or intergrown with troilite along the grain boundaries of the kamacite (Figs.5, 6,7 and 8). It is distinctly anisotropicand appearsin strong relief when etchedfor 10 secondswith 5 per cent niLal(57a nitric acid in alcohol) (Fig. 7). ft has a completelydifferent habit from the rhabditel that occursas rhombs throughout the Canyon Diablo iron and which is not present in the spheroids.It is identified principallv by its r-ray difiraction powder pattern obtained from material in the residuesample after the spheroidswere digested with cold 1 : 1 HCl.

Troilite. Troilite is brownish-pink in color and is observedat high mag-

1 Rhabdite is needle- or rod-shaped schreibersite It is observed in cross section in the form of squaresor rhombs- M ET LORI TI C M ETA LLI C SPH EROI DS 673

Frc. 6. Photomicrograph of the same grain as in Fig. 5 under crossed nicols illustrat- ing the anisotropism of the schreibersite and troilite.

Fro. 7. Photomicrograph of an etched kamacite in polarized reflected light showing a network of the interstitial oink mineral in relief 674 C. W. MDAD, J. LITTLER ANI) D. C. T. CHAO

Frc. 8. Photomicrograph showing fine intergrowth of troilite (dark) and schreibersite (light) in a matrir of kamacite. Width of the fine schreibersitefilaments is about 1 micron nification as an intergrowth with pink schreibersite(Fig.8). Its identification is basedon its optical propertiesand the presenceof major iron and sulfur in the microprobe analysis. Becauseof the small amount present,it was not detectedb,v the f-ra)'method.

X-nav hqvBstrcl'rroN In order to check the optical identification and to correlatethe min- eralogicaland chemicaldata, 15 spheroidswere r-rayed. The spheroids were gently rubbed by a giassrod in a spot plate, removing essentially the outer brown iron oxide and silica glasscoating. Each spheroid was then mounted on top of an ethyl cellulosespindle and :r-rayed in a Debye-Scherrerpowder camera of 114.49mm diameter using Fe Ka radiation and Mn filter. The major phaseof all the spheroidsr-rayed was the body-centeredcubic a-iron with a unit-cell size of approximatell' 2.86 A. It is not the distorted body-centeredcubic a2. Tracesof qtattz and the strong lines of maghemiteand taenite were generailydetected. Troiiite and schreibersitewere not detected in any of the untreated spheroids. Results of the r-ray analysis which show the relative abun- dance of the mineral phases present are given in Table 1. Wiistite, lepidocrocite,as weil as hematite and magnetite were not detected by the r-ray studiesof thesespheroids. M ET EORI T I C M ETA LLI C S P II EROI DS

'Ienr,n 2. MrNrner Asseurr,,tcns ol rHB Rnsroua ol 5 Tnrernl Slnpnorls AS DETERMTNEDBy X-RAv Drrlnecrron

Relative abundancer I,-ilm No. SpinclleNo. Schreibersite Taenite Quartz

17850 7t7 M 17857 r20 m M 17900 155 M r7901 156 M r7902 l)/ M M

I M major constituent rn minor constituent.

Five spheroidswere treated with cold 1:1 HCI which dissolvedawav the kamacite and iron oxide phasesleaving a dark colored,soft, friable spongy massas a residue.X-rav powder data show that the residuecon- sisted essentiallyof schreibersite(Table 2). From 2 of the 5 residues,a pattern of taenite was obtained.The residueof two additional spheroids treated this wa1- were prepared for polished section and microprobe studl-. Becauseof the friable and spong)'nature,the resultingmount was not satisfactoryeither for optical study or for quantitative microprobe analysis. ElncrnoN Vlrcnopnoen AN.+r,vsrs The electron microprobe of the same type as the one describedby Castaing (1951,1960) built at the U. S. GeologicalSurvey was usedfor the chemical analysis of the spheroids. In this study, three crystals and three detectorswere used to measurethree elements simultaneously. Two lithium fluoride crystals and two geiger counters were used to measure Fe and Ni concentrations. A mica crystal and a proportional counter which were placedin a vacuum were usedfor the measurementof either phosphorus or sulfur. Prior to quantitative analysis, spectrometer traces for each mineral were made. Thesetraces gave qualitative information as to the elements present and their relative amounts, which helped in the selectionof standards used.An exampleof the kind of information obtainedfrom qualitative scansis presentedin Fig. 9. This figure showsthe relative intensities of the Fe K0 and the Ni KB peaks obtained from spectrometer traces madeon the white areasof grainsl, 2, and3 (Figs.2, 3, 4) . Optically these three areaslook similar but the qualitative spectrometerscans with a Iithium fluoride crystal quickly indicated that all three white areas are different in chemical composition. Spectrometerscans using the mica 676 C. W. MI,:.AD,J, LITTLER AND E. C. T. CHAO

424450?54 2e+ I I I I I

t5 37 59

2S- 2e+

4?44S5?y3557:3

2e- 20 .. xB r.(B Pxd

Frc. 9. Relative intensities of Fe KB and Ni KB peaks obtained from spectrometer traces of the kamacite in Figs. 2, 3, and schreibersiteof Fig. 4, crystal showedthe presenceof phosphorusin grain 3. There is no detect- able phosphorusin grain 1 or 2. A slight suggestionof a P peak at 36" in thesegrains occurs on the descentof the largeFeKa peak at 33.9". Pure ironandpure nickel, analvzedkamacitefrom the Canyon Diablo iron meteorite(Fe:93; Ni:7), goethite (Fe:61.1), hematite (Fe :69.9), pyrite (Fe:46.2, 3:53.6), and rhabdite from the Canyon Diablo iron meteorite(Fe:42, Ni:44, P:15) wereused as standards in the study. The precisionfor the quantitative analysis of Fe in nickei-iron and iron oxides using the above standards is generaliy in the order oI i 5/6 ol the amount present,and for the nickel analysis,for the range of concentra- tion lessthan 3O/6,is in the order of + l07o of the amount present. Results of the microprobe analysisfor the spheroidswhich were not MLTEORITIC METALLIC SPil]'ROIDS 677

Tesra 3. Er,ncrnoN Pnonp D,rra loR THESpnrnoos Wntcn Wnnn Nor X-RnvBo

Sample No. Mineral No. of Analyses a/6 Fe %Ni Total

MS-1 Grain 1 Kamacite 8 98.3 2.3 100.6 Light gray oxide 6 61.6 s.7 67.3 Dark gray oxide 5t.7 3.0 54.7 MS-1 Grain 2 Kamacite 9 for Fe 74.1 23.5 97.6 7 for Ni Dark gray oxide 33.3 5.1 38.4 MS-l Grain 3 Schreibersite 8 4r.3 40.7 101.101 Dark gray oxide 16for Fe 45.3 2.2 47.5 12for Ni MS-l Grain 4 Pink 10for Fe 6.4 7+.e 9forS 6 for Ni MS-1 Grain 5 Kamacite 3 82.5 12 8 95.3 Dark gray oxide 4 42.r 8.6 50.7 MS-l Grain 6 Light gray oxide 5 s9.0 5.1 64.r MS-1 Grain 7 Pink 10 53.4 16.0 98 03

l Contains 19.OTaP. 2 Contains 24.67a S. 3 Contains 28.6% S. r-rayed are given in Table 3 and results of the microprobe analysisfor the r-rayed spheroidsare given in Table 4. The nickel values, obtained by both random spot analysis and by point by point traversefor the kamacitein grain 1 (Fig. 2) are consistentlylow,

Frc. 10. Photomicrograph of 2 of the :r-rayed spheroids in polarized reflected light showing typical sizes, shapes and textures. The dark gray mineral is goethiteJike iron oxide, white is kamacite and the light gray inclusions in the kamacite are schreibersite. A-spheroid 129,B-spheroid 131. 678 C. W. MNAD, J. LITTL]iR ANI) I], C 7-. CIIAO

Tegr,r 4 Elrcrnox Pnoer Dara lon X-Ravro Spuptons

Sample No. of '1;T* Mineral c/arte % Ni No. Analyses

Kamacite 13for Fe,14 for Ni 83 17 100 Predominantly dark gray oxide 74 45752 128 Kamacite 3 78 18 96 Predominantly dark gray oxide 2 45550 129 Kamacite 3 83 r7 100 Predominantll' dark gra)' oxicle 3 4785s 130 Kamacite 3 88 13 101 Predominantly clark gray oride 5 563s9 131 Kamacite 3 83 17 100 Predominantl-v clark gray oxide 3 +6854 132 Kamacite 3 81 t7 98 Predominantly dark gray oxide 3 53962

I J.) Kamacite 5 82 18 100 Predominantll' dark gray oxide 3 48553 134 Kamacite 3 83 15 98 Predominantly dark gray oxide 3 for Fe,2 for Ni .)o / o.t 135 Kamacite 3 83 r7 100 Predominantly dark gray oxide 3 53 14 67 t47 Kamacite 3 82 17 99 Predominantly dark gray oxide + 47 10 57 148 Kamacite 3 86 12 98 Predominantly dark gray oxide 3 53760 150 Kamacite 5 83 t7 100 Predominantly dark gray oxide 3 44 12 56 t-)l Kamacite 5 81 15 96 Predominantly dark gray oxide l1 52456 152 Kamacite 8 81 15 96 Predominantly dark gray oxide 7 46551 Kamacite 4 84 18 102 Predominantll' dark gray oxide 3 35540 never exceeding3 per cent. The kamacite in grains 2,5, and in all the tr-rayedspheroids which appear to be optically very homogeneous,contains an unusually high nickel content for kamacite. The n-ray data confirm that a iron or kamaciteis the major phasein all the r-rayed spheroids. The high nickel content could be partially causedby a fine intergrowth of taenitewith the kamacitewhich is not resolvedoptically. The dark gray mineral which occursin all the spheroidsstudied and is referredto as a goethite-likemineral was deficientin iron as comparedto goethite(Table 3,4). In addition to the iron deficiency,the nickel content LIETEORITIC METALLIC SPilI'ROII)S 679 of this goethite-likemineral in someof the spheroidswas as much as 14 per cent. Becauseof the fine-grainedand poorly c1'rstallinenature of this dark gray phase,it can not be certain whether this dark phaseconsists of a singlephase or a mixture of more than onephase. The light gray iron oxide or maghemite is usually finelv intergror'vn with the dark gray iron oxide and the analysisof a pure areaol it is difficult. It generallycontains more iron than the dark gray iron oxide. The white mineral in grain 3 (Fig. 3) is describedas schreibersite.In addition to a higherreflectivity than normal schreibersite,analysis of this mineral indicatedthat it is enrichedin phosphorusb1- about 4 weight per cent as compared to the stoichiometric composition of schreibersite (Table 3). This enrichmentin phosphorussuggests a possibility that this phasecould be (Fe, Ni)2P rather than (Fe, Ni)BP.I{owever, owing to the small sizeof the sample,identification of this phaseb1' r-ray diffraction was not possible. The pink interstitial mineral identified by r-ray as schriebersiteis commonly lessthan 5 p wide and occursin nearly every spheroidstudied (Figs. 5 and 6). It is intimately associatedwith troilite and kamacite, so that generally Fe, Ni, and S are detected over such an area. In some spheroids,the pink schreibersiteand the brown troilite ale clearly re- solved (FiS.8). It is observedthat the pink schreibersitealso occurs interstitially as spongyfilaments between the kamacite grains.The hoies arefilled with kamaciteand becausethe width of the filamentsis lessthan a micron, it is extremely difficult to obtain an anaiysis of only this nrate- rial (Fig.8) . Very smali quantities of phosphorushave been detected with the microprobe.In grains 4 and 7 of Sample MS-1, this "impure" pink phasewas analyzedand the resultsshow major Fe and S and minor nickel, and little or no phosphorus(Table 3). In an attempt to locate Iarger areasof the pink mineral, severallarger spheroidswere prepared for microprobeanalysis but it was found that the grain size of the pink mineral seemedto remain constantregardless of the sizeof the spheroid. Attempted microprobeanalysis of the pink phasein the r-rayed spheroids proved inconclusivebecause it was not possibleto obtain an analysis on the pure material.

CoNcr,usrorqs As a result of meteorite impact-fusion, the spheroids which were originally part of the Canyon Diablo iron meteoritel are completely

I The Canyon Diablo meteorite is classified as a coarse octahedrite. It has the usual widmanstdtten structure consisting or oriented bands of kamacite and taenite. Numerous rhombs of rhabdite occur within the kamacite Minor amounts of other minerals such as graphite, troilite, diamond, cohenite, and olivine are present as inclusions' C. W. A4EAD, J. LITTLER ANl) E. C, T. CHAO

changed in texture. Except for the addition of iron oxide phases the mineral assemblageswhile differing in composition remain essentiaily the samein both the iron meteoriteand the spheroids. The widmanstd,ttenstructure present in the Canyon Diablo iron meteorite is completely destroyed in the spheroids.High temperature heating and quenchingcaused the destructionbut at the sametime gave rise to the fine-grainedquenched texture which is partly pearlitic in nature. The numerousrhabdite rhombspresent throughout the kamacite in the unshockedmeteorite are no longerpresent in the spheroidsand the shreibersite,which is generally present, occursas a fine network outlining the kamacitegrain boundaries. The siliceousglass coating on the spheroidsindicates that they must have consolidated and cooled in an environment where a great deai of silicate vapor was also condensing. The sharp angular fragments of quartz which they contain,probably camefrom the sandstonein the area of Meteor crater which was pulverizedby the impacting canyon Diabro iron meteorite. The compositionalvariation among the spheroidscan best be seenin the variability of the Ni content in the body-centered a-iron and the various iron oxide phases.Both the low nickel (up to 3 per cent) and the high nickel (up to 24 per cent) content in the kamaciteindicate the dras- tic changein the compositionof phasein the canyon Diablo iron meteorite as the resuit of impact. These spheroids are notably different from all the alleged cosmic magnetic spheruleswhich have been examinedat the U. S. Geological Survey. Many of the latter are hollow and none of them contain derect- ablenickel or any phaseresembling kamacite in composition. By comparison,the texture and mineral assemblagesof the Meteor Crater spheroidsare nearly identical to those of the philippine and Indochina tektite spherules(Chao et al.,1962,1964). In all thesespheroids or spherules,anisotropic schreibersite and troilite occurinterstitially as a network within a fine-grainedmatrix of kamacite. Kamacite, the major phase in all these particles, is isotropic, body-centered cubic a-iron with a unit-cell sizeof approximately2.86 A. |Itre grain sizeof the mineralsis not dependenton the sizeof the spheroidsor spherules.The extremesof the high and the low nickel content of kamaciteis alsocharac- teristic of the spherulesfrom the Philippine and rndochinatektites. Thus, the results of this investigation indicate that the metallic spherulesin the Philippine and fndochina tektites, when compared to the spheroidsfrom Meteor Crater whosemeteoritic origin is certain, must also have a simi. Iar, i.e. meteoritic,origin. M ETEORITIC M ETALLI C SPII EROI DS 681

RnlBnnxcrs

C,r.srnwc, R. (1951) Application of electron probes to local chemical and crystallographic analysis. Thesis. University of Paris. Cas:rtxc, R. (1960) Electron probe microanalysis. Advances in Electronics and Electron Physics13,360. Cnao, E. C. T., Isrnonu Anr.nn, Enwnno J. DworNrx, Jarnr Ltrrlnn (1962) Metallic spherules in tektites from Isabella, Philippine Islands. Science 735,97-89. Cneo, E. C. T., Enw,*o J. Dworwrx, JaNrr Lrrrlen, (1964). New data on the nickel-iron spherules from Southeast Asian tektites and their implications. Geochim. et Cosmo- chim. Acta 28,97t-98O. Ztst-ow, B.,c.No L. M. Krr,r,occ, (1961) The analysis of metallic spheroids from Meteor Crater, Arizona. Geochim. et Cosmochim. Acta24,315-316.

M anuscript receiled, Oetober3, 1964; acceptedfor publication, December2, 1964.