Records of the Western Australian Museum 17: 51-59 (1995).

Metallography and thermo-mechanical treatment of the Veevers (IIAB)crater-forming iron meteorite

A.W.R. Bevanl, E.M. Shoemaker and C.S. Shoemaker

'Department of Earth and Planetary Sciences, Western Australian Museum, Francis Street, Perth,Western Australia 6000

2U. S. Geological Survey, Flagstaff, Arizona 86001, USA.

Abstract - Thirty six fragments of iron meteorite (group HAB Wasson etal. 1989) totalling 298.1 g found at Veevers crater in Western Australia(22°58'06"S, 125°22'07"E) represent the disrupted remnants of the crater­forming projectile, and confirm an origin for the ca. 75 m crater by meteoriteimpact. The morphology and metallography of the residual material showthat the impacting meteorite was a coarsest octahedrite with a kamacitebandwidth of > 8.6 mm. Disruption of the meteorite during impact probablyoccurred along the grain boundaries of a-kamacite crystals in the originaloctahedral structure, but may also have resulted from failure related tointense shear deformation. Further disintegration of the surviving fragmentsmay have occurred as the result of prolonged terrestrial weathering. Thermo­mechanical alteration of the micro-structure of the meteorite as a result ofimpact includes transient, localised re-heating to >800°C, shearing and plasticdeformation. Failure of parts of the meteorite along brittle-cracking paths,such as crystal boundaries, may have absorbed some of the energy ofterrestrial impact and allowed portions of the original micro-structure of themeteorite to be preserved. Veevers is the only known group HAB ironassociated with an impact crater.

INTRODUCTION

In Australia there are five meteorite impactcraters (Wolfe Creek, Dalgaranga, Henbury,Boxhole and Veevers) with associated meteoriticfragments representing the remnants of theimpacting projectiles. Of these, Veevers crater isthe most recently recognised and the least welldescribed. Veevers meteorite impact crater issituated between the Great Sandy and GibsonDeserts in Western Australia at co-ordinates22°58'06"5, 125°22'07"E. The bowl-shaped, circularstructure, measuring 70-80 m in diameter and 7 mdeep, was recognised as a possible impact crater inJuly 1975 (Yeates et al. 1976). Yeates et al. (1976)surveyed the crater but did not find any meteoriticmaterial that would have provided conclusiveevidence of an origin by meteorite impact.Subsequently, in August 1984, two of us (EMS andCSS) visited the locality and recovered severalsmall fragments of iron meteorite from twolocalities immediately to the north of the crater(Figure 1). The material (Table 1- WAM 13645-46)comprises several irregular, weathered fragments,the largest weighing 8.9 g. In July 1986, during afurther visit to carry out a detailed survey of thecrater (Shoemaker and Shoemaker 1988), anadditional 32 metallic slugs and fragments of

meteoritic iron were recovered, the largestweighing 36.3 g (Graham 1989) (see Table 1). Mostof this material was found just to the east of thecrater, on the flanks of the crater rim and adjacentplain (Figure 1). A precise age for the crater has notyet been published, although Shoemaker andShoemaker (1988) estimated that it was formedaround 4000 years ago.

More recently, Wasson et al. (1989) analysed themeteorite and have shown it to be a normalmember of chemical group IIAB containing 5.82wt% Ni, 57.7 pg/g Ga, 160 pg/g Ge and 0.028 pg/ gIr. Wasson et al. (1989) also suggested that the sizeof the recovered fragments reflects separation ofcm-thick kamacite lamellae as the result ofweathering or impact fragmentation. In this paper,a detailed metallographic description of themeteorite is provided, and the disruptive thermo­mechanical history of the meteorite during crater­forming impact is interpreted.

PHYSICAL DESCRIPTION AND SAMPLEPREPARATION

The weights and dimensions of the meteoritefragments recovered from the vicinity of Veeverscrater are listed in Table 1 and shown (WAM

52

20 m

A.W.R. Bevan, E.M. Shoemaker, C.S. Shoemaker

• WAM 13645

•

13761

•• •-WAM 13735•

•

WesternAustralia

Figure 1 Location of Veevers crater and schematic map showing the distribution of meteorite fragments found byE.M. and C.S. Shoemaker.

13731-13762) in Figure 2. Fragments are weatheredand possess variably thick (up to 1mm) rinds ofterrestrial corrosion products around fresh metal.The total weight of the fragments is 298.1 g;individual fragments range in weight from < 0.1­36.3 g with a mass distribution skewed towardsfragments in the range 2 - 8 g (Figure 3b).Fragments weighing more than 3 g are generallyelongate and flattened in shape; the ratios (LIT) oftheir lengths (L) to maximum thicknesses (T)average 3 and range from 1.6 - 6.5. Fragmentsweighing more than 3 g have a mean thickness of8.6 mm (Figure 3a).

Two fragments, one found to the north (WAM

13645) and one found to the east (WAM 13761) ofthe crater, were cut along their long axes, thenpolished and micro-etched with 2% nital formetallographic examination. An additional,irregularly shaped fragment (WAM 13735), alsofound to the east of the crater, was cutperpendicular to a suspected grain boundary.

METALLOGRAPHY AND STRUCTURALCLASSIFICATION

Microscopically, the fragments examined arecomposed essentially of single crystals of a-FeNi(kamacite) that have been partially or completely

Veevers crater-forming iron meteorite

Table 1 Numbers, weights and dimensions of iron meteorite fragments found at Veevers impact crater

Field No. WAM No. weight (g) length L (mm) max.thickness T (mm)

VC-1-84 13645 8.9 17.9 11.4YC-4-84 13646.1 4 32.9 5.05YC-4-84 13646.2 2.2 23.9 4.6YC-4-84 13646.3 <0.1 (fragments)YC-1-86 13731 6.9 26.75 8.05YC-2-86 13732 2.5 20.4 5.75YC-3-86 13733 1.8 15.7 4.9YC-4-86 13734 12.4 20.3 9.2YC-5-86 13735 13.9 33.1 9.3YC-6-86 13736 4.5 27.3 6.5YC-7-86 13737 9.3 25.5 8.9YC-8-86 13738 3.9 16.05 8.25YC-9-86 13739 6 20.6 8.55YC-1O-86 13740 1.3 21.15 4.6YC-11-86 13741 8.5 24.8 8.9YC-12-86 13742 4.7 20.35 8.45YC-13-86 13743 13.8 30.35 11.1YC-14--86 13744 18 26.95 12.95YC-15--86 13745 12.2 34.7 7.3VC-16-86 13746 1.9 15.85 4.4YC-17-86 13747 17.8 24.35 12.95YC-18-86 13748 2.8 17.75 5.2YC-19-86 13749 7.7 19.45 7.8YC-20--86 13750 2.3 15.85 5.6YC-21-86 13751 1.3 (small fragments)YC-22--86 13752 26.6 (specimen used for terrestrial age determination)YC-23-86 13753 4.7 15.65 7.0YC-24--86 13754 4.9 17.5 8.75YC-25--86 13755 6.2 21.75 7.8YC-26--86 13756 5 19.5 7.2YC-27-86 13757 4.8 19.0 6.75YC-28--86 13758 4.6 17.1 8.1YC-29-86 13759 6.6 22.0 6.05YC-3D-86 13760 19.1 36.0 11.3YC-31-86 13761 36.3 31.35 15.4YC-32-86 13762 10.6 26.8 8.3

53

transformed to unequilibrated, ragged (t,2-kamacitesimilar to that found in the heat affected zones offreshly fallen irons. Metal in the fragments WAM13761 (36.3 g) and WAM 13735 (13.9 g) found tothe east of the crater shows completetransformation to coarse (10-200 pm units), ragged(t,2' whereas the largest fragment (WAM 13645 - 8.9g) found to the north of the crater is only partiallytransformed to finer grained (5-50 pm) (t,2 units,mainly along zones of intense shear deformation.

One fragment (WAM 13761) displays severalmm-sized inclusions of schreibersite [(FeNi)ll.and both fragments examined contain abundantrhabdites (prismatic crystals of schreibersite). In allfragments examined, schreibersite crystals andrhabdites are kinked, kneaded and deformed, andtheir grain boundaries show incipient reactionhaloes with the surrounding metal (Figure 4a).Evidence of shock-melting in Veevers is sparse.However, where shear-zones intersect large

crystals of schreibersite, small « 20 pm) cloudy,wedge-shaped pools of shock-melted phosphidehave been generated that penetrate the phosphidefrom the phosphide-metal interface. Additionally,some rhabdites have been smeared out and meltedalong zones of shear deformation.

Portions of fragment WAM 13645 that have notbeen transformed to (t,2 show deep-seatedNeumann bands (mechanical shock-twins) that aredegenerated (Figure 4b), and sub-grain boundariesthat are decorated with rhabdites. The fragmentshave been plastically deformed and the metal istraversed by a few intense zones ( <1 mm wide) ofshear deformation characterized by sub­microscopic recrystallization. Locally, terrestrialcorrosion has penetrated along cracks developedin the shear-zones making them visible to thenaked eye. Several shear-zones run parallel with,and close to, the outer surfaces of the fragments. Inthe fragment from north of the crater, three sets of

54 A.W.R. Bevan, E.M. Shoemaker, C.S. Shoemaker

Figure 2 Fragments of the Veevers meteorite (WAM 13731-13762) recovered from the area to the east of the crater(see Figure 1).

shear-zones are evident that intersect at ca. 60° andsome shear-zones have been propagated along pre­existing Neumann bands. In the fragment WAM13761, in addition to zones of shear deformation,there are occasional diffuse, sinuous lines ofstained metal that do not appear to be associatedwith plastic deformation. One such sinuous line iscut and displaced by a shear-zone indicating thatthe structure from which it was formed pre-datesplastic deformation of the metal (Figure 4c). Thedisplacement of the structure indicates that thethrow of the shear zone is ca. 400 ~.

The suspected grain boundary in fragment WAM13735 is heavily invaded with terrestrial corrosionproducts that have masked the original structure.Vestiges of troilite occur and, locally, terrestrialoxides pseudomorph a pre-existing eutectic-likestructure. Other minerals observed in the unalteredportions of Veevers fragments include carlsbergite(erN) and an unidentified, partially resorpedmineral (Figure 4b), probably roaldite ([Fe,Nil4N)(Nielsen and Buchwald 1981), the latter occurring

rarely as narrow (1-2 pm), long (up to 1.2 mm)lamellae. Neither y-FeNi (taenite), nor shock­hardened e-kamacite transformations wereobserved.

Structural classificationAll previously described irons belonging to

chemical sub-group lIB with Ni contents in therange 5.5-6.9 wt % are structurally coarsestoctahedrites (Ogg) with a-kamacite bandwidths>3.3 mm (Buchwald 1975). Disruption of theVeevers projectile during impact has destroyed theoriginal macro-structure of the meteorite.However, the nature of the surviving fragmentsgives some indication of the original structure ofthe meteorite. The shape and mineralogy of thefragments show that, predominantly, the structureof the meteorite comprised irregular, stubbylamellae of kamacite (LIT c.1.6-6.5). From theirplate-like morphology, it follows that themaximum thicknesses of the surviving fragmentsare likely to approximate to the kamacite

55

k

a

or plessite was encountered. However, it is likelythat the undisrupted Veevers meteorite containedresidual taenite.

Veevers crater-forming iron meteorite

Ca)

mean

V) ~Q)10V)

~u.....0 80z

6

4

2

0.2 004 0.6 0.8 1.0 1.2 lA 1.6

Max. thickness (cm)

6

Cb) .'..

.' ..•• 'f "..... ~

'". ,," "

-..\ '\

Details of the microstructure of Veeversmeteorite fragments. (a) Schreibersite [s]showing incipient reaction haloes withsurrounding kamacite [k]. (b) Kamacitecontaining abundant rhabdites, degeneratedNeumann bands and lamellae of anunidentified mineral [x], probably roaldite.(c) Sinuous line of stained metal (probablydecomposed roaldite) cut and translated by azone of shear deformation (arrowed).Surrounding metal transformed to ragged u

2­

kamacite. Scale bars 100 pm (2% nital etch).

, ...

..

Figure 4

6 10 14 18 22 26 30 34 38

Weight (grams)

2

4

2

bandwidth of the original octahedral structure. Theaverage thickness of fragments > 3 g is 8.6 mm(Figure 3a), whereas the average thickness offragments> 5 g is 9.6 mm. These dimensionsindicate a coarsest octahedral (Ogg) structure, andcorrespond well with the bandwidths (9 - 10 mm)of other group nAB irons with similar Ni contents(ca. 5.8 wt%) that have been described (Buchwald1975).

Group nAB irons with bulk Ni contents greaterthan 5.5 wt% frequently contain some residual y­taenite or plessite (a+y) (Buchwald, 1975). Thefrequency of taenite or plessite fields in thesemeteorites rarely exceeds one in sectional areasranging from 10 - 25 cm2• The bulk Ni content (5.82wt%) of Veevers (Wasson et al. 1989) suggests thatsome residual y-taenite should be present in themeteorite. In the small sectional area of the residualfragments ofVeevers examined « 4 cm2) no taenite

Figure 3 Dimensional and mass analysis of meteoritefragments found at Veevers crater; (a)fragment thickness frequency, (b) massfrequency.

V)

~ 10~u.....o 8oz

56

DISCUSSION

In Veevers fragments, transformation of kamaciteto ragged u

2indicates a transient, but severe re­

heating to temperatures above the u to ytransformation temperature, followed by rapidcooling (Brentnall and Axon 1962; Axon et al. 1968;Lipschutz 1968). Reaction haloes betweenschreibersite and metal indicate incipientresorption of the phosphide and are also consistentwith a brief, but severe re-heating event. This re­heating event is superimposed on earlier formedstructures in Veevers and appears to be the mostrecent thermal event in the history of the meteorite.There are several possible explanations for thecause of the re-heating in Veevers that include; pre­terrestrial cosmic shock reheating, frictionalheating during atmospheric passage, shockreheating on impact, and contact with hot impactejecta. However, there are a number of significantfeatures of the microstructure of Veevers indicatingthat the observed effects of transient re-heatingwere associated with the impact event.

As shown for Canyon Diablo by Heymann et al.(1966), the localised occurrence of severe thermaleffects observed in the interiors of some of theVeevers fragments indicates steep temperaturegradients that exclude conductive heating (e.g.,atmospheric passage and contact with hot ejecta)as a mechanism for re-heating. In Veevers, thesuperimposition of thermal alteration on earlierstructures, and its clear association in at least onefragment with intense shear deformation, suggeststhat heating in that case was caused by shock­loading during the formation of the crater and thedisruption of the impacting projectile. However, inthe largest fragment recovered (WAM 13761) thegeneral transformation of metal to u 2-kamacite thatis not obviously associated with mechanicaldeformation does not exclude the possibility ofconductive re-heating. Notwithstanding, thesimilarity of the condition of the schreibersite andrhabdites in all three fragments examined isconsistent with re-heating for short (seconds)duration rather than prolonged (minutes/hours)heat-treatment in a hot ejecta blanket.

Comparison with experimentally heated andshock re-heated samples of iron meteorite allowsthe magnitude and duration of thermo-mechanicaltreatment of Veevers to be determined moreaccurately. The thermal effects observed inkamacite and schreibersite in lightly shockedsamples of Canyon Diablo experimentally heat­treated in air for 10 and 100 seconds at 800 - 850°C (Figure Sa and b) and allowed to cool byradiation are very similar to the range of structuresobserved in Veevers fragments. In these samples,metal is partially or wholly transformed to u

2­

kamacite comprising 10 - 200 !lm units, and

A.W.R. Bevan, E.M. Shoemaker, C.S. Shoemaker

aI

s..-

Figure 5 Samples of the lAB iron, Canyon Diablo,experimentally heated in air for (a) 100 secsat 800°C and (b) 10 secs at 850°C. Noteincipient reaction schreibersite [s] withsurrounding metal, degenerated Neumannbands and u

2transformations. Scale bars 100

Jlm (2% nital etch).

phosphides show incipient, ragged reaction haloeswith surrounding metal. In samples heated to thesame temperatures but for longer (100 - 1000 secs)periods, the reaction haloes around phosphidesbecome more prominent, and thorn-likeprotuberances penetrate the metal from thephosphide metal interface. These features were notobserved in the Veevers fragments examined.

The peak temperature to which Veeversfragments were subjected during terrestrial impactis more difficult to determine. Samples of CanyonDiablo experimentally heated to 1000°C for shortduration (300 secs) by Brentnall and Axon (1962)showed extensive eutectic melting and resorptionof schreibersite. Widespread melting ofschreibersite was not observed in the Veeversfragments examined, suggesting that overall re­heating of the surviving fragments wasconsiderably less than 1000°C. However, to

Veevers crater-forming iron meteorite

account for the isolated areas of incipiently meltedschreibersite, attenuation of shock waves at grainboundaries and shearing could have generatedlocalised 'hot spots' where temperatures may haveapproached 1000°C. Incipient melting of schrei­bersite at grain boundaries as the result of intenseshear deformation observed in Veevers is identicalto that described by Axon et al. (1977) fromsimilarly deformed phosphide inclusions in thecrater-forming iron Canyon Diablo. Evidence fromfragment WAM 13735 indicates that extensiveshock-melting in Veevers may have occurred alongthose grain boundaries containing abundanttroilite. Troilite has a low shock impedence relativeto Fe and is known to induce high shock­temperatures even under conditions of moderateshock-loading.

The shock pressures required to produce thestructural changes in Veevers fragments can beinferred from the estimated residual temperaturesindicated by the thermal alteration of minerals inthe meteorite. The relationship between shockpressure and residual temperature for pure ironhas been determined experimentally by McQueenet al. (1962) and for iron meteorite material byHeymann et al. (1966). In shocked iron meteorites,extensive or complete transformation to (X2­

kamacite occurs at applied pressures in the range0.8 - 1Mb (Heymann et al. 1966). In specimens ofthe Odessa meteorite shocked to pressures in thisrange, Heymann et al. (1966) also noted that smallpatches of shock-hardened £-kamacite may occuronce in sectional areas up to 20 cm2• The apparentabsence of £-kamacite in Veevers may simply be afunction of the small sectional area of the meteoriteexamined.

As suggested by Wasson et al. (1989), the plate­like morphology of the surviving fragments ofVeevers indicates that the meteorite broke upduring impact predominantly along (X-(X crystalboundaries in the original octahedral structure.Large crystals of schreibersite, such as thatobserved in Veevers fragment WAM 13761, alsoprovide brittle-cracking paths that could havefacilitated break-up. From a study of shrapnel-likefragments of the Henbury crater-forming iron,Axon and Steele-Perkins (1975) have suggested thatfracturing of that meteorite took place alongsurfaces of shear-faulting generated during impact.Parting along zones of shear displacement mayhave provided an additional mechanism of failurein the Veevers meteorite, and this is supported bythe occurrence of shear-zones that parallel theouter surfaces of the fragments. The angles ofintersection (ca. 60°) of some of the shear-zones inVeevers coincides with the angles between the(111) directions of kamacite in the octahedralstructure of iron meteorites. It is possible that thehabit planes of the octahedral structure and other

57

cubic planes in Veevers influenced the shearingforces generated by impact. Subsequently, thepenetration of terrestrial oxidation along grainboundaries and shear-zones may have led tofurther disintegration of the surviving fragments,as suggested by Wasson et al. (1989).

Much of the pre-terrestrial micro-structure ofVeevers has been destroyed or modified as theresult of thermo-mechanical alteration duringcrater-forming impact. Nevertheless, portions ofthe original micro-structure of the meteorite havebeen preserved. The observed Neumann bandsthat have been plastically deformed and partiallydegenerated by re-heating appear to pre-dateterrestrial impact. The sinuous lines of stainedmetal observed in fragment WAM 13761 areinterpreted as the resorped lamellae of an as yetunidentified mineral, probably roaldite.

Comparison with other crater-fonning ironsOut of the twelve other crater-forming irons

known (Grieve 1991), the most extensively studiedare Canyon Diablo (lAB), Odessa (lAB) andHenbury (IIIAB) (Buchwald 1975). Canyon Diabloand Odessa are both coarse octahedrites, whereasHenbury is a medium octahedrite. All threemeteorites are associated with craters that are verymuch larger than Veevers. In the case of Henbury,the impact resulted in some thirteen cratersincluding several that are of similar size toVeevers. Structural variations between the crater­forming irons and differences in the magnitude oftheir impacting events have resulted in anenormous range of shock-induced features in thesurviving fragments. Notwithstanding, there arestrong similarities in the overall nature of thenno­mechanical alteration and mechanism of disruptionsuffered by many crater-forming irons.

In terms of crater size, the closest analogue toVeevers is the largest of a group of nine craters atKaalijarv, located on Saaremaa Island, Estonia(Buchwald 1975). The largest crater measures 110m in diameter and eight smaller craters vary from12 - 40 m in diameter (Tiirmaa 1992). Kaalijarv is acoarse octahedrite (lAB) and the material recoveredfrom the craters comprises small slugs of metalgenerally less than 20 g in weight (Buchwald 1975).Metallographically, the Kaalijarv meteorite showsshock hardening, shear deformation and localisedrecrystallization of metal. Overall, the thermalalteration of Kaalijarv fragments is less than thoseobserved in Veevers fragments and is generallyconfined to zones of shear deformation.

The IIAB iron Sikhote-Alin, that fell in easternSiberia in 1947, is the largest known shower inhistorical times and is structurally and chemicallysimilar to Veevers. Some 23.2 tons of fragmentalmaterial were recovered from a large strewnfieldcovering 1.6 sq. km and including 122 impact

58

holes (Krinov 1963; Buchwald 1975 - andreferences therein). Many of the masses of Sikhote­Alin broke up during impact and some of therecovered fragments display the earliest stages ofshock-metamorphism. Buchwald (1975) noted thatmany of the disrupted fragments showedoctahedral parting, and that fissures are welldeveloped along kamacite grain boundaries thatare loaded with schreibersite. However, moreintensely deformed fragments also show sheardeformation and visibly distorted zones near theirsurfaces. In addition, Buchwald (1975) noted thatfissures had also developed along the cubiccleavage planes of kamacite lamellae in Sikhote­Alin, although these appeared to have played aminor role during the fragmentation of theimpacting masses.

Although of much less intensity, the style ofthermo-mechanical impact alteration displayed byboth the Sikhote-Alin and Kaalijarv meteorites isvery similar to that observed in Veevers fragments.

SUMMARY AND CONCLUSIONS

Veevers is the only known crater-forming iron ofchemical group lIAB. SUb-group lIB irons are rare,accounting for only 4% of all iron meteorites, andless than 0.5% of all meteorites. The association ofan unusual meteorite type with an impact crater isespecially significant. Dry conditions andextremely low erosion rates in the arid zone ofAustralia over at least the last 4000 years accountfor the excellent state of preservation of both thecrater and the surviving fragments of meteorite.

The metallography of the surviving fragments ofVeevers shows that the meteorite was subjected toa pre-terrestrial history of mild shock-loading of athoroughly annealed structure that resulted in atleast one generation of Neumann banddeformation. Subsequently, terrestrial impactcaused intense shock-loading of the meteorite thatresulted in shearing and plastic deformation withattendant localised heating to >800°C. Inaccommodating the complex pattern of shockwaves generated by high-velocity impact with theEarth, a portion of the meteorite was disruptedmainly along kamacite crystal boundaries in theoriginal coarsest (> 8.6 mm) octahedral structure.Failure may also have occurred as the result offracturing along zones of intense shear­deformation that may, in turn, have beeninfluenced by the octahedral structure of themeteorite. As a result of the disruption of a portionof the projectile along brittle-cracking paths someof the energy of the impact event may have beenabsorbed, allowing portions of the originalmicrostructure of the meteorite to be preserved.

Most of the remains of the meteorite may bemixed widely with the breccia under the crater

A.W.R. Bevan, E.M. Shoemaker, C.S. Shoemaker

floor. A subordinate fraction of the meteorite wasbroadly sprayed out of the crater and is nowburied beneath the surrounding sand sheet.Subsequently, terrestrial weathering has corrodedthe fragments and may also have contributed tofurther disintegration. However, because ofprolonged aridity in the desert region where thecrater occurs, rusting is not extensive and thefragments retain large cores of fresh metal. Theclose approximation of the average thickness offragments (8.6 mm) to the bandwidths (9 - 10 mm)of other IIAB irons of similar Ni content suggeststhat corrosional losses due to weathering wereunlikely to have been greater than 1-2 mm. Athorough search of a wide area around the cratermay yield larger fragments of Veevers that becamedetached from the projectile during atmosphericflight, and that were not involved in the crateringevent.

ACKNOWLEDGEMENTS

The authors thank the late Or H. J. Axon forproviding experimentally heat-treated samples ofCanyon Oiablo for study. Assistance in thepreparation of the Veevers fragments wasgenerously provided by the Electron MicroscopeCentre of the University of Western Australia,through Or B. J. Griffin. Professor J. T. Wasson andOr V. F. Buchwald are thanked for their helpfulreviews.

REFERENCESAxon, H. J., Boustead, J. and Yardley, E. D. (1968). The

mechanical and thermal alteration of iron meteoritestructures. In B. M. French and N. M. Short (eds)Shock Metamorphism of Natural Materials, pp.585-599,Mono Book Corp., Baltimore.

Axon, H. J., Couper, W. R. D. and Kinder, J. (1977). Newshock effects in the phosphide and carbide phases ofCanon Diablo iron meteorites. Nature 267: 414-415.

Axon, H. J. and Steele-Perkins, E. M. (1975). Fracturemechanism of Henbury meteorite by separationalong surfaces of shear faulting. Nature 256: 635.

Brentnall, W. D. and Axon, H. J. (1962). The response ofCanyon Diablo meteorite to heat treatment. Journal ofthe Iron and Steel Institute 200: 947-955.

Buchwald, V. F. (1975). Handbook of Iron Meteorites,University of California Press, Los Angeles.

Graham, A. L. (ed.) (1989). Meteoritical Bulletin no.67.Meteoritics 24: 57-60.

Grieve, R. A. F. (1991). Terrestrial impact: The record inthe rocks. Meteoritics 26: 175-194.

Heymann, D., Lipschutz, M. E., Nielsen, B. and Anders,E. (1966). Canyon Diablo meteorite: Metallographicand mass spectrometric study of 56 fragments.Journal ofGeophysical Research 71: 619-641.

Krinov, E. L. (1963). In B. Middlehurst and G. P. Kuiper

Veevers crater-forming iron meteorite

(eds) The Solar System, Vol IV: Maon, Meteorites andCraters. pp.208-234, University of Chicago Press,Chicago.

Lipschutz, M. E. (1968). Shock effects in iron meteorites:a review. In B. M. French and N. M. Short (eds) ShockMetamorphism of Natural Materials, Mono Book Corp.,Baltimore.

McQueen, R. G., Zukas, E. and Marsh, S. (1962).Residual temperatures of shock-loaded iron. ASTMSpecial Publication 336: 306-316.

Nielsen, H. P. and Buchwald, V. F. (1981). Roaldite, anew nitride in iron meteorites. Proceedings of theLunar and Planetary Science Conference. 12B: 1343­1348.

Shoemaker, E. M. and Shoemaker, C S. (1988). Impact

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structures of Australia. Lunar and Planetary Science19: 1079-1080.

Tiirmaa, R. (1992). Kaali craters of Estonia and theirmeteoritic material. Metearitics 27: 297.

Wasson, J. T., Ouyang, X., Wang, J., and Jerde E. (1989).Chemical classification of iron meteorites:XI. Multi­element studies of 38 new irons and the highabundance of ungrouped irons from Antarctica.Geochimica et Cosmochimica Acta 53: 735-744.

Yeates, A. N., Crowe, R. W. A. and Towner, R. R. (1976).The Veevers Crater: a possible meteoritic feature.BMR Journal of Australian Geology and Geophysics 1:77-78.

Manuscript received 21 February 1994; accepted 8 July 1994.