THE AMERICAN MINERALOGIST, VOL. 51, JANUARY-FEBRUARY, 1966

KAMACITE AND TAENITE SUPERSTRUCTURESAND A METASTABLE TETRAGONAL PHASE

IN IRON METEORITES

A. R. ReuslEN AND E. N. C-q.uexoN, Geology Department,fl niv er sity oJ W,i s c o n sin, M adi s o n, W'i s c o n s in.

Assrnacr

Kamacite and taenite in iron meteorites have cubic superstructures in which the unitcell edges are respectively three times and two times those of the disordered phases. Certainiron meterorites also contain a tetragonal phase that seemingly has the same chemicalcomposition as kamacite. This is considered to be a transitional state in the transformationof -y-phase alloy into a-phase alloy (kamacite).

INrnooucrroN

X-ray powder photographs have been obtained from the iron-nickelphases in a number of iron meteorites. The meteorites examined includehexahedrites, octahedrites and nickel-rich ataxites (Table 1) in the formof polished slices that are currentlv the subject of microscopic investiga-tions. For tr-ray analvsis each specimen has been sampled in two wa.vs:

1. by gently scratching the surface i,vith a biological needle;2. by drilling powder from the surface with a tungsten carbide bur.

The first method yields a minute sample that can be mounted on thetip of a gelatin fiber, following the method of Sorem (1960). Samples ofthis type have been tr-rayed in a 114.6 mm diameter camera using FeKaradiation and exposures of about 72 hours. The second method yields aconsiderabll' larger sample that can be mounted on a glass fiber usingcollodion as adhesive. Samples of this type have been r-rayed in a 114.6mm diameter camera using FeKa radiation and exposures of about 8hours. In both cases the r-ray fi lm has been mounted according to theIevins-Straumanis (asvmmetric) method.

The powder patterns obtained from scratch samples differ considerablyfrom those obtained from dril led samples of the same meteorite. Whereasthe drilled samples yielded the simple cubic patterns of kamacite (a Fe-Niailoy) and/or taenite (7 Fe-Ni alloy), the scratch samples yielded com-plex patterns containing many lines.

Analysis of the complex patterns has indicated

(1) the existence of a metastable tetragonal phase having the same composition as kamaciteand (2) the presence of kamacite and taenite superstructures in which the unit cell edgesare respectiveiy three times and trvo times larger than those of the driiled samples.

These superstructures and the tetragonal phase are lost when powder isdril led from the meteorites.

A. R. RAMSDEN AND E. N. CAMERON

PnBvrous Wonr

Relatively l i tt le attention has been devoted to r-rav analysis of iron

meteorites. Young (1926) r-rayed polished areas of kamacite and taenitein the Carlton meteorite and established that kamacite is body-centeredcubic (l ike a-iron) and that taenite is face-centered cubic. He also estab-lished that the Widmanstdtten orientation of kamacite in taenite is due

to the development of the (110) planes of kamacite parallel to the (111)

planes of the taenite. Derge and Kommel (1937) extended these studies of

Tasra 1. Inor- Mnrponrtn's Exeutxnp rN trrts Srt'ov

Bulk NickelbvmDol- Lontent 70

Reference

Union HexahedriteHex River

Mountain HexahedriteHorseCreek Hexahedrite

Linwood Coarsest octahedrite (Ogg)

Henderson (1941)

Cohen (1905)

Goldberg, Uchiyama and

Brown (1951)

Henderson and Perry(re4e)

Goldberg, Uchiyama and

(H)(H)

5 .63

5 .685 . 8 6

5 .98

o. o{l

BischtttbeMesa Verde

Park

Carlton

LinvilleTwin City

Coarseoctahedrite (Og)

Mediumoctahedr i te (Om)

Fine octahedrite (Of)

Nickel-rich ataxiteNickel-rich ataxite

Brown (1951)6.48 Cohen (1897)

?

J12 .77 Howell (1890)

\12.68 Goldberg, Uchiyama andBrown (1951)

16 32 Cohen (1898)29.91 Mason (1962)

(D)(D)

orientation in iron meteorites by using back reflection Laue photographs.Owen and Burns (1939) were unable to prepare s.vnthetic iron-nickel

alloys that were rich in iron and in a state of complete thermal equilib-rium. For this reason they r-rayed a number of iron meteorites in thehope that this would furnish information concerning the equilibriumconditions in iron-nickel alloys generally. The specimens used were filedfrom the meteorites and the powders annealed at 330" C. in order toobtain sharp c-ray reflections. The r-ray films showed only the simplepatterns of kamacite and/or taenite. For comparison, they also r-rayedunannealed powders. These yielded the same spectra except that the l ineswere badly defined due to distortion of the metal.

More recently, Stulov (1960) has r-rayed a number of iron meteoritesand has reported the existence of superlattice reflections in all of them.Unfortunatelv. Stulov used unfiltered iron radiation and also added

KAMACITE AND TAENITE SUPERSTRUCTURES 39

sodium chloride to the powders to serve as an internal standard. As aresult, the multiplicity of lines obtained (both c and B reflections beingpresent) make the data difficult to interpret without ambiguity.

Stulov assigned indices to the observed superlattice reflections but doesnot discuss these results beyond mentioning that they could be due toordering. However, on the basis of the indices given, the supercell wouldappear to have a unit cell twice that of kamacite. The existence of super-lattice reflections in these samples is rather surprising in view of the factthat the specimens were obtained by drilling or filing powder from themeteorites. However, examination of Stulov's results shows that onlyfour such reflections are identified and that generally only one of them(200), is present together with at most one other l ine. Hence it wouldseem that the powders contain only a fragmentarv record of the originalordering.

TnB TBrnacoNAr- PHASE

Ea,irlence for a tetrogonal phase. The evidence for a tetragonal phase ap-pears only in the r-ray powder patterns of scratch samples; it is absentfrom the corresponding drilled samples. The structure was first recog-nized after comparing the d-values obtained from the Union and HorseCreek meteorites. The powder pattern of the Union meteorite contains 26lines, four of which have d-values that correspond to the (110), (200),(211) and (220) reflections of kamacite. These reflections, however, areweak. The powder pattern of the Horse Creek meteorite contains 25 lines,none of which belong to kamacite.

A comparison of the two patterns shows that 18 of the unknown lines inthe Union meteorite match 19 of the l ines in the Horse Creek meteorite,one of the latter being an d.rot2 doublet. Furthermore, the lines in commoncan all be indexed on the basis of a tetragonal unit cell, but not on thebasis of a cubic or hexagonal unit cell. Examination of the powder pat-terns from all of the iron meteorites studied shows that this tetragonalpattern appears in six of the nine scratch samples but is absent from allthe drilled samples. The three scratch samples that do not show thispattern are from the two nickel-rich ataxites (Linville and Twin City)and from the Hex River Mountain hexahedrite.

The d-values of the tetragonal phase are summarized in Table 2. Celldimensions have been determined for the Union. Horse Creek. Bischti ibeand Carlton meteorites and are approximately as follows:

,(A) c/a"(A)UnionHorse CreekBischtiibe

Carlton

7 . r 2/ . l o

/ . r o

7 . r 0

5 . 7 05 . 8 1J . l +

J . / . )

0 8 00 . 8 20 . 8 00 . 8 1

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A. R, RAMSDI]N AND N N. CAMERON

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KAMACITE AND TAIJNITE SUPL,RSTRUCTURES 4I

Composition oJ the tetragonal phase. Microscopic examination of themeteorites in reflected l ight has failed to reveal a discrete phase respon-sible for the tetragonal pattern in the r-ray films. In particular, the hexa-hedrites appear to have a uniform matrix that looks like kamacite. Thereis no obvious reason why the tetragonal pattern should appear in theo-ray film of the Union meteorite and not in that of the Hex River Moun-tain meteorite. Both are hexahedrites that look very much alike underthe microscope. Modal analysis indicates that they contain similaramounts of rhabdite, (3.0 vol/6 in the Hex River Mountain meteorite,and 3.8 vol/6 in the Union meteorite), and according to Henderson(1941), both meteorites have essentially the the same chemical composi-tion, r'i,2.:

Fe Ni CoUnion 93.09 5.63 ndHex River Mountain 93 59 5 .68 0.66

Nevertheless, the r-ray pattern of the Union meteorite is tetragonalwith only faint lines attributable to kamacite, whereas the r-ray patternof the Hex River Mountain is cubic and wholly attributable to kamacite.It would seem, therefore, that the tetragonal phase in the Union meteor-ite has the same chemical composition as the kamacite in the Hex RiverMountain.

Origin of the tetragonal phase. Recent work by Takashi and Bassett(1964), Clendenen and Drickamer (1964) and Bundy (1965) has demon-strated the existence of a high pressure hexagonal polymorph of ironknown as e-iron. The r-ray data for e-iron (Takashi and Bassett, 1964)show no correlation with r-ray lines observed in the present study. More-over, the present data cannot be indexed in terms of a hexagonal unit cellwhereas all the lines can be indexed on the basis of a tetragonal unit cell.It is concluded, therefore, that the structure reported here is not relatedto a high pressure e-type structure, but is a tetragonal phase.

Kamacite and the tetragonal phase cannot be distinguished under themicroscope and they seem to have essentially the same composition.Furthermore, the tetragonal phase is metastable and converts to kama-cite when subjected to the heat and stresses of dri l l ing (Table 5). Theabsence of the tetragonal pattern in the *-ray films of the nickel-richataxites (Linville and Twin City) and, more especially, of the Hex RiverMountain hexahedrite, indicates that this structure is not produced byscratching the meteorites but is a true property of the meteorite surface.It is suggested therefore that the tetragonal phase is an intermediatestage in the transformation of .y-phase into the a-phase (kamacite), and

P S C rnd

0 . 2 s 0 . 0 8 0 . 0 2

42 A. R. RAMSDEN AND IJ. N. CAMERON

that it is able to coexist with kamacite of the same composition onlybecause of the extremely sluggish approach to equilibrium exhibited inthe Fe-Ni system. Such cubic-to-cubic transf ormations by wa,v of a tetrag-onal transition structure are known to metallurgists, (see for exampleSmoluchowski et al., 1951), but they have not been reported for iron-nickel alloys.

It is of interest to note that in the unit cell of the tetragonal phase the crepeat is essentially twice the unit cell edge of the corresponding kamaciteto which it transforms in the drilled sample. For example. in the Carltonmeteor i te . r " r :5 .73 A and the cel l edge of the corresponding dr i l led kama-cite is 2.8698 A. Sitrce kamacite and taenite superstructures are identif iedin the present work, this relationship suggests that the tetragonal phasealso may be a superstructure in which the dimensions of the supercell aredouble those of the disordered state. Assuming this to be true, then it canbe seen that, in the absence of the superstructure, the disordered tetrag-onal phase would have one dimension, (the c repeat), the same length asthe unit cell edge of the, presumably disordered, kamacite to which ittransforms in the drilled powder. This is strong support for the proposi-tion that it is a transitional stage in the 7-o transformation. Differencesin cooling history might account for the preservation of this phase insome meteorites and its absence in others.

TnB TarNrrE SuPERSTRUCTURE

Raid,ence for a taenite swperstrwcture. The evidence for a taenite super-structure appears only in the r-ray powder patterns of scratch samples; itis absent from the corresponding dril led samples. The evidence is bestdocumented in the patterns from the two nickel-rich ataxites Linville andTwin City.

The scratch powder pattern of the Twin Citv meteorite contains 13lines, nine of which index on the basis of a cubic unit cell with edge (,4)approximately 7.168 A 1l|alt" 3). The four remaining reflections can beassigned to a body-centered cubic kamacite cell, one of the l ines being asupercell reflection (Table 4). The dril led sample yields a pattern contain-ing only nine l ines (Table 5), f ive of which belong to a face-centered cubictaenite in which the cell edge (a) is approximately 3.582 A, and four ofwhich belong to a body-centered cubic kamacite the cell edge of whichcould could not be determined. It is clear that the unit cell edge of thetaenite in the dril led sample (presumably disordered) is haif that of thetaenite in the scratched sample.

The scratch powder pattern of the Linvil le meteorite is particularlycomplicated and contains 46 measurable lines. Yet, all of these can beindexed on the basis of a taenite supercell with,4:2a and a kamacite

KAMACITE AND TAENITE SUPERSTRUCTURES

Teglo 3. d-Ver-uns ol rrm TenNrrB

43

Twin City(D)

Linvilie(D)

hkl

I*rIdcutc

) [*r d""r"

100?1 1 01 1 1200?21021r2202213103 1 1222320321400330420L11

3325 1 1MO522433.).t I

533

622

7 .168

3 . 5 8 4

2 927

2.065

1 . 7 9 2

1 . 2 6 5

1 .078

1 054

t .o32

7 050'

3 . 5 1 6 1

2.841

2 . O 7 0

| . 7 9 0

| 266

1 .080

1 .051

2

1 5

l . )

l )

100

t . )

I J

o J l

44

15

I

15

5 .0554 . r 2 7

3. 1982.9182 . 5 2 72.3822 . 2 5 5

.2 1502 .0581 .9831 . 9 1 51 . 7 8 61 68.51 .5981 . 5 6 0t . 5 2 41 . 3 7 5| . 2 6 4| . 2 4 51 . 2 2 6r 2081 .090

r .077

1 .054

1 . 0 3 1

4 8 3 1 14.239 60

3.340 1002 879 802 . 5 2 6 8 02.398 |2 . 2 7 9 1 02 . 1 8 7 1 02 070 101 . 9 8 1 11 . 9 1 0 |r . 7 8 4 1 0| . 6 7 3 1| . 6 1 4 1 0t . 5 7 2 1t .542 101 . 3 7 4 1r . 2 6 6 1t . 2 5 6 || . 2 2 9 11 . 2 0 8 1r .092 1

[1 .081* , 1{ -1 1 . 0 8 1 a r 11 .047 1

1r.oas"' 1l .1.034@, 1

I These two lines may belong to the tetragonal phase2 A very diffuse line that could not be measured.

Note: Only those lines which index in terms of a taenite supercell are listed in this

table Other iines present on the films index in terms of a kamacite supercell (Table 4).

A number of large discrepancies between d"r," and dcals are the result of errors madelvhen

measuring the position of the lines (see note. Table 2).

supercell with r4:3o (Tables 3,4). Nine very faint l ines on this fi lmcannot be measured. The unit cell edge of the taenite supercell is approxi-mately 7 .t46 h. The drilled sample yields a pattern containing only nine

lines (Table 5), f ive belonging-to a face-centered cubic taenite with cell

edge (o) approximately 3.589 A, and four belonging to a body-centered

A R. RAMSDEN AND E. N. CAMI:.RON

T,uln 4 d-Velurs ol rrm Keuectrr:

Trvin City(D)

Linvil le(D)

Carlton(o0

Mesa Verde Park(Om)

I : e l dcarc dut u I'"1

2002202 2 132032140032233O/411 2 .0263 3 1332430s30/333 1 .490600/442 1.433542640721 1 . 169642

822/660

4 301 .+ 4S53 043 3 0322 868 2 .9532 386 2 .4552 299 2 2272.151 2 1242 086 2 0972 028 2 0311.97s 1 967| 925 t .824| 7 2 1 1 . 6 9 91 656 1 4811 . 4 3 4 1 . 4 3 51 2 1 2 t . 2 1 81 1 9 3 t . r 9 71 1 7 0 1 . 1 7 01 . 1 5 0 1 . 1 5 3

1 015411 . 0 1 4

1 015ar

4 .308 4 .399

2 . 1 3 4 2 1 2 42 090 2 1012 031 2 031

1 927 I 828

1 659 1 4901 436 1 .435

1 r 7 3 1 . 1 7 1

2 029 2 031

1 . 6 5 7 1 . 4 9 01 . 4 3 5 1 4 3 5

1 . t 7 1 l . r l l

100

3040

70

30

11 0

100

5

1050

io

r00

t10

50

11 01051I1

1006051

1 030

1I

70I

10

1 01 015 I 015 40

Note Only those lines which index in terms of a kamacite supercell are listed in this table.

cubic kamacite with unit cell edge approximatell ' 2.869 A. As with theTwin Cit-v the unit cell edge of the taenite in the dril led sample is halfthat of the taenite in the scratch sample.

Examination of the other powder patterns has shown the presence ofthe taenite supercell in the medium octahedrite Mesa Verde Park. How-ever, the f-ray pattern of this meteorite is predominantly due to kama-cite and the tetragonal phase and, as a result, the taenite l ines are onlyfaintly visible. Only three of the measurable l ines belong to the taenitesuperceli but other very faint l ines on the fi lm can be recognized as be-longing to this pattern even though thev cannot be measured. An e-rayfi lm of the dril led sample of this meteorite contains oniy the simple kama-cite and taenite patterns with the kamacite predominant (Table 5).

Composit, ion of the taenite. A curve relating the composition of 7 Fe-Niallovs to the unit cell edge has been established by Jette and Foote(1936), Bradlev et al. (1937) and Owen and Yates (1937). We have ana-11,zed a series of synthetic 7 aliovs and, except at 80/s nickel, our data fallon this curve (Fig. 1).

According to this curve, the taenite dri l led from the Trvin Citv meteor-ite averages aboft28/6 nickel. This value is in reasonable agreement withthe reported bulk nickel content of the meteorite (29.91/6-Mason

KAMACITD AND TA ENTTE SU PERSTRUCTURES

T*l-n 4-(continned)

45

Linwood

(oeg)Bischttibe

(og)Union

(H)Hex River Mountain

(H)

ldotn

2 8i3

2 0321 9 i 7

1 724

1 437

1 173

2 844

2 03r1 . 9 1 4

f . i 1 1

1 435

1 . 1 7 1

2 031I 993

1 .486| 43r| 286

1 . 1 1 0

2 883

2 .03 r

t . 7 2 4

t . 422

1 171

1

100

1

70

80

2 876

2 033

1 . 7 2 5

1 .438

1 . 169

10010

6011

20

2 0221 . 958

l . o J l

1 4301 239

1 168

30

1

Ii00

1 060

80r . r 71 1 171

1 0

1005

1

j-O

100

4 . 3 0 4 4 . 3 4 0

2 388 2 477

2.o8 i - 2 0882 029 2 03r

1 . 6 5 7 1 . 4 9 0i . 4 3 5 1 4 3 3

1 . 0 1 6 I 0 1 6 8 0 I 0 1 5 1 0 1 4 6 5 1 016 1 014 70

1962), and \'vith a modal analysis which indicates that the meteorite

contains 93.2 vol/6 taenite. Similarly, the r-ray data for the Linville

meteorite indicate that the taenite in the drilled sample contains about

32/6 nickel.It must of course be recognised that taenite in meteorites is

of extremel-v- variable composition, (Feller-Kniepmeier and Uhlig 1961;

Goldstein and Ogilvie, 1965), and that the nickel content can exceed 50/6

in those regions close to a taeuitefkamacite interface. Ifence, composi-

tions determined by tr-ray means are) at best, only average values for the

taenites contained in the samples.

Possible framework for the taenite superlattice. A possible framework for

the taenite superlattice is illustrated in Fig. 2. This structure is formed by

combination of eight body-centered cubic cells. Nickel atoms occupy four

of the body-centered sites in such a way as to avoid being nearest neigh-

bours and to avoid being in cubes that are side by side. With the four

n ickel s i tes lu l ly occupied as shown, the st ructure corresponds to the

composition FerNi. Hume-Rothery and Raynor (1962) have pointed out

that:,,Since the compositions AsB and AB usually mark definite states in the completion of zones

of solute atoms, we may always expect superlattice formation at those compositions at lot'

temneratures if the size factors are suitable."

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* . 8

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A. R. RAMSDEN AND E. N. CAMERON

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KAMACITE AND TAT,NITE SUPERSTRUCTURES 47

Bradlel, and Jay (1932) originally proposed the above structure forordered iron-aluminum alloys. They showed that aluminum entered thebody-centered sites (occupied by nickel in Fig. 2) unti l the compositionFegAl was reached. On further addition of aluminum, the aluminumatoms distributed themselves on all eight body-centered sites unti l, at thecomposition FeAI, these were all occupied by aluminum, giving a caesiumchloride structure, It is suggested that the nickel atoms in taenite may

Frc. 1. Relationship between unit cell edge and composition in 7 Fe-Ni alloys.

behave in an analogous manner and exhibit superlattice formation overa range of composition in the region FeaNi.

Unfortunately, the calculated structure amplitudes for the proposedstructure show that this model would not give rise to the large numberof superlattice reflections observed in the taenite and hence the model isnot a complete explanation of the patterns. Nevertheless, the generalagreement between the taenite compositions and the composition of themodel would suggest that the concept is basically correct. Possibly smalldisplacements of the atoms within the structure might give rise to amodel able to satisfy the complex conditions required to obtain the ob-served superlattice reflections. We are unable to propose a detailed struc-

A. R- R,AMSDEN AND E N. CAMERON

ture for such a model but would note that magnetic exchange forceswithin the lattice might cause the necessary displacement of the atoms.

Oldering has not been detected in FesNi in the laboratory, althoughHoselitz (1944) suggested that it might occur. However, on the basisof electrical and magnetic measurements, order has been reported at thecomposition FeNie (Wakelin and Yates 1953) and at the compositionFeNi (Paulev6,, el al, 1962).In view of the great age of meteorit ic iron

O rroN

I wrcrrl

Frc. 2. Possible framervork for the taenite suoerlattice.

i t would seem reasonable to conclude that the superlattice formation ispr imar i l r an order ing phenomenon.

TTTO K,T.IT.ICITE SUPERSTRUCTURE

Evid,ence Jor a kamacite superslructwre. The evidence for a kamacite super-structure appears only in the r-ra-v powder patterns of scratch sampleslit is absent from the corresponding dril led samples. The evidence is welldocumented in all of the patterns (Table 4) except that of the HorseCreek meteorite, and it is particularly evident in the Linvil ie and Carltonmeteorltes.

As mentioned earlier, the r-ray' pattern of the Linvil le meteorite con-tains 46 measurable l ines. Of these, 27 belong to the taenite superstruc-

KA M A CI TI' A N D TA F:,N I TI':. SU P I'RSTRU CTU RDS

ture (Table 3). The remaining 19 l ines can be indexed on the basis of acubic unit cell (Table 4) in which the cell edge is approximatel,'- S.603 A.Nine very faint l ines on the fi lm cannot be measured. The drii led samplevields a pattern containing only nine l ines (Tiable 5), f ive of which belongto a face-centered cubic taenite in which the cell (a) is approximately^ - ^ ^ ?3.589 A, and four of which belong to a bodl--centered cubic kamacite inwhich the unit cell edge (o) is approximately 2.869 A. It is clear- that theunit cell edge of the kamacite in the dril led sample (disordered) is onethird that of the kamacite in the scratched sample. This relationship alsohoids for the other meteorites that have been examined:

Scratch Sample(A)

Drilled Sample(A)

UnionLinwoodBischtiibeMesa Verde Park

Carlton

8 . 5 88 6208 6088 6098 . 6 1 7

2 8682 . 8 7 02 8692.8692 . 8 7 0

Composit'ion oJ the kamacite. The composition of the kamacite in thesemeteorites is not known accurately at this time but it may be estimatedfrom the unit cell edge of the dril led kamacite. We have measured theunit cell edge of zone refined iron and ol 2/6Ni and 4/6Ni syntheticalloys at 25" C., and have used the data to set up a composition/celledge curve. Unfortunately, an allegedll.- 6/6Ni alloy was found to con-tain both a and 7 phases and could not be used in this work. However,according to the electron probe measurements of Feller-Kniepmeier andUhlig (1961), kamacite in the Grant meteorite contains 6.6/6 nickel, andkamacite dri l led from this meteorite in the present work has a unit celledge of 2.869b h. This information has been combined with the data ofthe svnthetic alloys to obtain a composition/cell edge curve that is ap-plicable to the present work (Fig. 3).

Jette and Foote (1936) have also measured the unit cell edges of svn-thetic a Fe-Ni alloys at 25o C. Their original data have been convertedfrom kX to A by multiplying by 1.00202 and the results are shown inFig. 3. The differences between the two curves probably represent experimental errors on our part. The samples used in the present workhave not been annealed and hence the r-ra1'fi lms contain broad linesdue to distortion of the metal. These lines do not separate into aptzdoublets at high 20, and errors probably arise when measuring their po-sit ions. However, such errors also appear in the data for the dril led

A. R, RAMSDL.N AND E. N, CAMERON

kamacites and therefore do not affect the results when usinq our owncurve.

All the drilled samples have been *-rayed at 25" C. The results indicatekamacite compositions ranging from 3.5/6Ni to about 7.87o1:{i. In par-ticular, the s-ray data indicate that kamacite in the Linville meteoritecontains about 6.6/6Ni and kamacite in the Carlton meteorite contains

'l

Atot$rc PEtcENt NtcxEt

Frc. 3. Relationship between unit cell edge and composition in a Fe-Ni alloys.

about 7.8/6Ni. As with taenite it must be recognised that the r-raymethod indicates only the average nickel content of a sample.

Possible frameworh for the kamacite superlatLice. By analogy with the pre-viously described taenite supercell, a possible framework for the kamacitesupercell is shown in Fig. 4. The supercell is formed by combination of27 body-centered cubes with nickel atoms occupying four of the body-center sites. The nickel atoms are distributed in such a wav as to avoid

I

3

KAMACITE AND TA]JNITE SUPERSTRUCTURL,S 51

orite. The kamacite supercell reflections are particularly well developedin both these meteorites. Furthermore, no kamacites are known to con-tain more than 7.8/6 nickel.

As with the taenite supercell, i t is necessary to postulate a more com-plex model in order to account for the many observed supercell reflec-tions, and it is suggested that magnetic exchange forces within the lattice

Frc. 4. Possible framework for the kamacite superlattice.

might cause small displacements of the atoms in such a way that thenecessarv conditions for diffraction are satisfied.

Srcxrr.rcaNcE or. THE KalracrrB aNnTanxrrB SupnnsrnucruREs

wakelin and Yates (1953) have demonstrated the existence of orderingin the region FeNia by measuring the electrical resistivity and magneticproperties of synthetic iron-nickel ailoys. They found that ordering was

o [ q

a f,rct:r

52 A. R. RAMSDEN AND ]J N. CAMERON

extremely sluggish and took place over a lange of composition from 50

to 80 atomic per cent nickei, with the maximum change in structure

sensitive properties occurring at the composition FeNi:. More recentlr ',

Paulev6 et at. (1962) have demonstrated the existence of a cuAu type

superlattice in a synthetic alloy of composition FeNi (50:50). IIowever,

ordering was so sluggish that it could only be established under neutron

irradiation in a magnetic held. Since taenite and kamacite contain far

less nickel than these synthetic alloys it follows that ordering, if i t occurs,

should be an extremely sluggish process. However, in vieu' of the great

age of meteorit ic iron, (about 4X10e 1'ears), it u'ould seem reasonable

to conclude that superlattice formation is indeed an ordering phenom-

enon.Since the crit icai temperature for ordering of iron-nickel alioys are low,

(498" C. for FeNi3 -Wakelin and Yates 1953, 320o C. for FeNi-Paulev6

et ol. 1962), it is probable that the crit ical temperatures for ordering of

taenite and kamacite are equall-v as low, if not lou'er. It follows, there-

fore, that if ordering has taken place, then the meteorites must have

cooled to these temperatures at some time in the past sr.rfftcientlv Iemote

to allow formation of the superstructures to take place. If the crit ical

temperatures \ rere known and details of the superstructures could be

determined, then, in principle, it should be possible to calculate the time

required for formation of the superstructures and hence obtain a better

insight into the cooling histories of these bodies.

TnB ANou'q.Lous HoRSE CnBex X{urBonrrB

The Horse creek meteorite is a hexahedrite (Table 1). However, it

has long been known as an anomalous iron because of its unusual pseudo-

octahedral structure. The pesudo-octahedral structur-e arises through

orientation of schreibersite and perrl-ite lamellae along rvhat appear to

be octahed.ral planes in the metal, thus producing a prominent Widman-

stetten pattern. However, Perrl- (1944) has pointed out that these planes

are not really octahedral and that sections of the meteorite in rn'hich the-v

intersect at right angles show a rectangular pattern not a square one.

The r-ray pattern of the scratch sample is unusual among all the me-

teorites examined in that it does not contain kamacite l ines. However, it

does contain a u-ell developed tetragonal pattern (Tabte 3). The cell

d imensions of th is phase are approximatel ,v a. :7.16 A and c:5 '81 A'

As with the other meteorites, the tetragonal phase is unstable and con-

verts to a cubic "kamacite" in the dril led powder. Horvever, the unit

cell edge of this "kamacite" is anomalousll 'smail (Table 5) and is in fact

far smaller than that of pure a-iron.

KAMACITE AND TAENITE SLIP]JRSTRT]CTURES .53

Besides the materials here discussed, we have :u-ra1,ed an additional16 iron meteorites as dri l led powders. All show the simple patterns ofkamacite and/or taenite. Nloreover, out of the total of 25 meteoritesonl1. the "kamacite" in the Horse creek meteorite has given this unusuallyIow value for the unit cell edge.

Using an electron probe microanalyzer, Fredricksson and Henderson(1965) have shown that the "kamacite,' contains 2.5/6 sil icon in solidsolution together with about 3.9/o nickel and that the meteorite alsocontains about 30/6 of perryite, nickel siiicide, a new mineral having theapproximate composition Ni 81%, Si 1270, Fe 3/6 and p 5/e.It seems,therefore, that the Hoise creek meteorite is indeed an anomarous iron.Apparenth- the presence of sil icon markedly reduces the cell edge ofkamacite, and sil icon may also be responsible for the greater stabil it l- ofthe tetragonal phase in this meteor:ite. This stabil ity is indicated by theIack of kamacite in the scratch sample and the appa'entrv tetragonalorientation of the schreibersite lamellae.

CoNcrusroNs

The metall ic phases in iron meteorites have been invesitgated b1.r-rayanall 'sis. It has been found that(1) many iron meteorites contain a metastable tetragonal phase that seemingly has thesame composition as kamacite and is indistinguishable from this mineral under the micro-scope, (2) kamacite and taenite in iron meteorites have superstructures and (3) both thetetragonal phase and the kamacite and taenite superstructures are destroyed when mate-rial is drilled from the meteorites.

rt has been concluded that the tetragonal phase is a transitional stage inthe transformation of 7-phase into a-phase (kamacite).

Because st.nthetic iron-nickel alloys are known to order on the lab-oratorlr t ime scale it has been concluded that the kamacite and taenitesuperstructures are due to an ordering process. However because of thesmall difference in scattering powers between Fe and Ni, simple orderedstructures would not produce visible tr-ray superstructure reflections. rnorder to produce the large number of observed superstructure reflectionsa more complex model must be postulated in which the ordered atomsare svstematicallv displaced in position, possibly as a result of magneticexchange forces. A detailed structure determination bv neutron diffrac-tion would be needed to verify such a model.

The :u-ra1' data for the Horse Creek meteorite indicate that it has anunusual composition. Recent work by Fredricksson and Henderson(1965) has shorvn that sil icon is present in the metall ic phase of thismeteolite.

54 A. R. RAMSDEN AND E. N. CAMERON

Acr<NowrBpcMENTS

This study is part of an investigation of the phases of meteorites under

National Aeronautics and Space Administration Research Grant NS

G-439, to the University of Wisconsin. The authors gratefull,v acknolvl-

edge this support. Our thanks are also due to Professor J. A. Mullendore(Department of Minerals and Metals Engineering, the University of

Wisconsin) for preparation of the svnthetic iron-nickel ailoys, and to

Professor S. W. Bailel. (Department of Geology, the LTniversitv of Wis-

consin) for crit icism of the manuscript. ' l 'he meteorites used in this in-

vestigation are on loan from the Smithsonian Institution, Lr. S. National

Museum, Washington, D. C. This loan is gratefuil.v acknowledged.

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KAMACITE AND TAENITE SUPERSTRACTURES 55

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Monuscri,pt receive.d., June 18 , 1965 ; accepled. Jor publication, N o,ember 29, 1965 .