THE AMERICAN MINERALOGIST, VOL. 51, JANUARY-FEBRUARY, 1966
KAMACITE AND TAENITE SUPERSTRUCTURES AND A METASTABLE TETRAGONAL PHASE IN IRON METEORITES
A. R. ReuslEN AND E. N. C-q.uexoN,Geology Department, fl niver sity oJ W,is c o n sin, M adis o n, W'is c o n sin.
Assrnacr
Kamacite and taenite in iron meteorites have cubic superstructures in which the unit cell edgesare respectively three times and two times those of the disorderedphases. Certain iron meterorites also contain a tetragonal phase that seemingly has the same chemical composition as kamacite. This is consideredto be a transitional state in the transformation of -y-phasealloy into a-phasealloy (kamacite).
INrnooucrroN X-ray powder photographshave been obtained from the iron-nickel phasesin a number of iron meteorites.The meteoritesexamined include hexahedrites,octahedrites and nickel-rich ataxites (Table 1) in the form of polishedslices that are currentlv the subject of microscopicinvestiga- tions. For tr-ray analvsiseach specimenhas been sampledin two wa.vs:
1. by gently scratching the surface i,vith a biological needle; 2. by drilling powder from the surfacewith a tungsten carbide bur. The first method yields a minute samplethat can be mounted on the tip of a gelatin fiber, following the method of Sorem (1960).Samples of this type have been tr-rayed in a 114.6mm diameter camera using FeKa radiation and exposuresof about 72 hours. The secondmethod yields a considerabll' larger sample that can be mounted on a glass fiber using collodion as adhesive.Samples of this type have been r-rayed in a 114.6 mm diameter camera using FeKa radiation and exposures of about 8 hours. In both casesthe r-ray film has been mounted accordingto the Ievins-Straumanis(asvmmetric) method. The powderpatterns obtained from scratchsamples differ considerably from thoseobtained from drilled samplesof the samemeteorite. Whereas the drilled samplesyielded the simple cubic patterns of kamacite (a Fe-Ni ailoy) and/or taenite (7 Fe-Ni alloy), the scratch samplesyielded com- plex patterns containing many lines. Analysis of the complexpatterns has indicated
(1) the existenceof a metastable tetragonal phasehaving the samecomposition as kamacite and (2) the presence of kamacite and taenite superstructures in which the unit cell edges are respectiveiy three times and trvo times larger than those of the driiled samples. Thesesuperstructures and the tetragonalphase are lost when powder is drilled from the meteorites. A. R. RAMSDEN AND E. N. CAMERON
PnBvrous Wonr Relatively little attention has been devoted to r-rav analysisof iron meteorites.Young (1926) r-rayed polishedareas of kamacite and taenite in the Carlton meteoriteand establishedthat kamacite is body-centered cubic (like a-iron) and that taenite is face-centeredcubic. He also estab- lished that the Widmanstdtten orientation of kamacite in taenite is due to the developmentof the (110) planesof kamacite parallel to the (111) planesof the taenite.Derge and Kommel (1937)extended these studies of
Tasra 1. Inor- Mnrponrtn's Exeutxnp rN trrts Srt'ov
Bulk Nickel bvmDol- Reference Lontent 70
Union Hexahedrite 5 .63 Henderson (1941) Hex River Mountain Hexahedrite (H) 5.68 Cohen (1905) HorseCreek Hexahedrite (H) 5.86 Goldberg, Uchiyama and Brown (1951) 5.98 Henderson and Perry Linwood Coarsest octahedrite (Ogg) (re4e) o. o{l Goldberg, Uchiyama and Brown (1951) Bischtttbe Coarseoctahedrite (Og) 6.48 Cohen(1897) Mesa Verde Park Mediumoctahedrite (Om) ? .77 Howell (1890) Carlton Fine octahedrite (Of) J12 \12.68 Goldberg,Uchiyama and Brown (1951) Linville Nickel-rich ataxite (D) 16 32 Cohen(1898) Twin City Nickel-rich ataxite (D) 29.91 Mason(1962) orientation in iron meteoritesby usingback reflectionLaue photographs. Owen and Burns (1939) were unable to prepare s.vntheticiron-nickel alloys that were rich in iron and in a state of complete thermal equilib- rium. For this reasonthey r-rayed a number of iron meteoritesin the hope that this would furnish information concerning the equilibrium conditions in iron-nickel alloys generally. The specimensused were filed from the meteoritesand the powders annealedat 330" C. in order to obtain sharp c-ray reflections. The r-ray films showed only the simple patterns of kamacite and/or taenite. For comparison,they also r-rayed unannealedpowders. These yielded the samespectra except that the lines were badly defineddue to distortion of the metal. More recently, Stulov (1960)has r-rayed a number of iron meteorites and has reported the existenceof 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 a result, the multiplicity of lines obtained (both c and B reflections being present) make the data difficult to interpret without ambiguity. Stulov assignedindices to the observedsuperlattice reflectionsbut does not discussthese results beyond mentioning that they could be due to ordering. However, on the basis of the indices given, the supercellwould appear to have a unit cell twice that of kamacite. The existenceof super- lattice reflectionsin these samplesis rather surprising in view of the fact that the specimenswere obtained by drilling or filing powder from the meteorites. However, examination of Stulov's results shows that only four such reflections are identified and that generally only one of them (200), is present together with at most one other line. Hence it would seemthat the powders contain only a fragmentarv record of the original ordering. TnB TBrnacoNAr- PHASE Ea,irlencefor a tetrogonalphase. The evidence for a tetragonal phase ap- pears only in the r-ray powder patterns of scratch samples;it is absent from the corresponding drilled samples. The structure was first recog- nized after comparing the d-values obtained from the Union and Horse Creekmeteorites. The powderpattern of the Union meteoritecontains 26 lines, four of which have d-values that correspondto the (110), (200), (211) and (220) reflectionsof kamacite. These reflections,however, are weak. The powder pattern of the Horse Creek meteorite contains 25 lines, none of which belong to kamacite. A comparisonof the two patterns showsthat 18 of the unknown lines in the Union meteoritematch 19 of the linesin the Horse Creekmeteorite, one of the latter being an d.rot2doublet. Furthermore, the lines in common can all be indexed on the basis of a tetragonal unit cell, but not on the basis of a cubic or hexagonal unit cell. Examination of the powder pat- terns from all of the iron meteorites studied shows that this tetragonal pattern appears in six of the nine scratch samplesbut is absent from all the drilled samples. The three scratch samples that do not show this pattern 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. Cell dimensionshave beendetermined for the Union. Horse Creek.Bischtiibe and Carlton meteorites and are approximately as follows:
"(A) ,(A) c/a Union 7.r2 5.70 080 Horse Creek /.lo 5.81 0.82 Bischtiibe /.ro J.l+ 0.80 Carlton 7.r0 J. /.) 0.81 A. R, RAMSDI]N AND N N. CAMERON
-ol -rl eFev ::, t= = E o€H€H+lH+irrue H!, :H;" .-':9 d \o o.46 ^no o-+aN .c) € a @ -N€:\oo\ l+c)N€ lr l+ o-' - ' F c) 9^ oN;N4:4 9-€N\OO'€O c)-+N€cr' v:' t O.-€reOOo\ lO a O'oO4qq I t, O-F | 4 C =-O\- -oo < Br -:o9 + €6+o$o o9o+ € c9 a vai A s-=ilh;E$lgRR€lu = I il E s +rj oioi oi -i 6iJ-j -j *'J z OY, o 4nC)N F N aamN\oOO\\O g i: 3? Fr =psh;D;$ EsR€ N4 H o llt4tlllllnTll Fl F 'o o c a BA 6OO .E\/ NOI IN P al o i j Fl *NJ € 4 83813€€-€9-gi4Ho N E -eo! F 9 r.! F] a.l.q d s CI-+acnNc)+r@,-to, i!€r_.cp 9q Fr D --r++c|6qoooC4NNC+ & o- ,j- Jso€e+o"eNN€o-o- o N +iOONNNNNNiiiHH* -64o\OO\NO4rOr:9i+tN @ 6idD\o6+-FNon€r,-!ra)r. - :6+N4€e<'--NN\o\osqd Nn+rooNNNNNNi:Hiie i OiO:O4C)OOOO-a-OOQQ 9 - - -FrNi.6i NNO€ T -lio.-**c mro'DQ\Nto$oAS d :*=8883$ggRe33[gs3 a * o- HFN<{ioiN€NN-drr€iN \O \O-Ji@c)eN€N\OOQLQO.NN q\ :€n€oA4nD€NNOQ€9- O ts --*,o*j-i .i .i .i .i 6i J-jJJJ* J o-iFoNoNo-oQ-o+++ s Oc)O-Oc)N-C)NFOF!H:N N i-:*NONHeNo$drNdNo 4 KAMACITE AND TAIJNITE SUPL,RSTRUCTURES 4I Composition oJ the tetragonal phase. Microscopic examination of the meteoritesin reflectedlight has failed to reveal a discretephase 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. There is no obvious reason why the tetragonal pattern should appear in the o-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 under the microscope. Modal analysis indicates that they contain similar amounts 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 meteoriteshave essentiallythe the samechemical composi- tion, r'i,2.: Fe Ni Co PSCr Union 93.09 5.63 nd nd Hex River Mountain 93 59 5 .68 0.66 0.2s 0.08 0.02 Nevertheless,the r-ray pattern of the Union meteorite is tetragonal with only faint lines attributable to kamacite, whereas the r-ray pattern of the Hex River Mountain is cubic and wholly attributable to kamacite. It would seem,therefore, that the tetragonalphase in the Union meteor- ite has the same chemical composition as the kamacite in the Hex River Mountain. 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 iron known as e-iron. The r-ray data for e-iron (Takashi and Bassett, 1964) show no correlation with r-ray lines observedin the present study. More- over, the present data cannot be indexed in terms of a hexagonalunit cell whereasall the lines can be indexed on the basis of a tetragonal unit cell. It is concluded,therefore, that the structure reported here is not related to a high pressuree-type structure, but is a tetragonalphase. Kamacite and the tetragonal phase cannot be distinguished under the microscopeand they seem to have essentiallythe same composition. Furthermore, the tetragonal phase is metastable and converts to kama- cite when subjectedto the heat and stressesof drilling (Table 5). The absence of the tetragonal pattern in the *-ray films of the nickel-rich ataxites (Linville and Twin City) and, more especially,of the Hex River Mountain hexahedrite, indicates that this structure is not produced by scratching the meteorites but is a true property of the meteorite surface. It is suggestedtherefore that the tetragonal phase is an intermediate stage in the transformation of .y-phaseinto the a-phase (kamacite), and 42 A. R. RAMSDEN AND IJ. N. CAMERON that it is able to coexist with kamacite of the same compositiononly becauseof the extremely sluggish approach to equilibrium exhibited in the Fe-Ni system.Such cubic-to-cubic transf ormations by wa,vof a tetrag- onal transition structure are known to metallurgists, (see for example Smoluchowskiet 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 tetragonalphase the c repeatis essentiallytwice the unit cell edgeof the correspondingkamacite to which it transforms in the drilled sample. For example. in the Carlton meteorite.r"r :5.73 A and the celledge of the correspondingdrilled kama- cite is 2.8698A. Sitrcekamacite and taenitesuperstructures are identified in the presentwork, this relationshipsuggests that the tetragonalphase also may be a superstructure in which the dimensionsof the supercellare doublethose of the disorderedstate. Assuming this to be true, then it can be seenthat, in the absenceof the superstructure,the disorderedtetrag- onal phasewould have one dimension,(the c repeat), the samelength as the unit cell edge of the, presumably disordered,kamacite to which it transforms in the drilled powder. This is strong support for the proposi- tion that it is a transitional stage in the 7-o transformation. Differences in cooling history might account for the preservation of this phase in somemeteorites and its absencein others. TnB TarNrrE SuPERSTRUCTURE Raid,encefor a taenite swperstrwcture.The evidence for a taenite super- structure appearsonly in the r-ray powder patterns of scratchsamples; it is absent from the correspondingdrilled samples.The evidenceis best documented in the patterns from the two nickel-rich ataxites Linville and Twin City. The scratch powder pattern of the Twin Citv meteorite contains 13 lines,nine of which index on the basisof a cubic unit cell with edge(,4) approximately 7.168A 1l|alt" 3). The four remaining reflectionscan be assignedto a body-centeredcubic kamacite cell, one of the lines being a supercellreflection (Table 4). The drilled sampleyields a pattern contain- ing only nine lines (Table 5), five of which belongto a face-centeredcubic taenite in which the cell edge (a) is approximately 3.582A, and four of which belong to a body-centered cubic kamacite the cell edge of which could could not be determined.It is clear that the unit cell edgeof the taenite in the drilled sample (presumablydisordered) is haif that of the taenitein the scratchedsample. The scratch powder pattern of the Linville meteorite is particularly complicated and contains 46 measurable lines. Yet, all of these can be indexed on the basisof a taenite supercellwith,4:2a and a kamacite KAMACITE AND TAENITE SUPERSTRUCTURES 43 Teglo 3. d-Ver-uns ol rrm TenNrrB Twin City Linvilie (D) (D) hkl I dcutc ) [*r d""r" I*r 100? 7.168 7 050' 15 110 5.055 4831 1 111 4.r27 4.239 60 200? 3.584 3.5161 l.) 210 3. 198 3.340 100 21r 2 927 2.841 l) 2.918 2 879 80 220 2.527 2.526 80 221 2.382 2.398 | 310 2.255 2.279 10 311 .2 150 2.187 10 222 2.065 2.O70 100 2 .058 2 070 10 320 1.983 1.981 1 321 1.915 1.910 | 400 1.792 |.790 t.) 1.786 r.784 10 330 1 68.5 |.673 1 420 1.598 |.614 10 L11 1.560 t.572 1 332 t.524 t.542 10 511 1.375 1.374 1 MO 1.265 | 266 IJ |.264 r.266 1 522 |.245 t.256 | 433 1.226 |.229 1 .).t I r 208 1.208 1 533 1.090 r.092 1 [1.081*, 1 622 1.078 1.080 15 r.077 {- 11.081ar 1 oJl 1 054 1.051 I 1.054 1.047 1 1 44 t.o32 2 15 1.031 1r.oas"' l.1.034@, 1 I These two lines may belong to the tetragonal phase 2 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 discrepanciesbetween d"r,"and dcalsare the result of errors madelvhen measuring the position of the lines (seenote. Table 2). supercellwith r4:3o (Tables 3,4). Nine very faint lines on this film cannot be measured.The unit cell edgeof the taenite supercellis approxi- mately 7.t46 h. The drilled sample yields a pattern containing only nine lines (Table 5), five belonging-toa face-centeredcubic taenite with cell edge (o) approximately 3.589A, and four belongingto a body-centered A R. RAMSDEN AND E. N. CAMI:.RON T,uln 4 d-Velurs ol rrm Keuectrr: Trvin City Linville Carlton Mesa Verde Park (D) (D) (o0 (Om) I: el dcarc dut u I'"1 200 4 301 .+ 4S5 1 4.308 4 .399 30 220 3 043 3 032 10 221 2 868 2.953 10 320 2 386 2.455 5 321 2 299 2 227 1 400 2.151 2 124 I 2.134 2 124 1 322 2 086 2 097 1 2 090 2 101 10 33O/411 2.026 r00 2 028 2 031 100 2 031 2 031 100 2 029 2 031 100 331 1.97s 1 967 60 332 | 925 t.824 5 1 927 I 828 5 430 | 721 1.699 1 s30/333 1.490 t 1 656 1 481 10 1 659 1 490 10 1.657 1.490 30 600/442 1.433 10 1.434 1.435 30 1 436 1.435 50 1.435 1 435 40 542 1 212 t.218 1 640 1 193 t.r97 I 721 1. 169 50 1 170 1.170 70 1 r73 1.171 io 1.t71 l.rll 70 642 1.150 1.153 I 1 0154110 822/660 1.014 1 015 I 015 40 1 015ar 10 Note Only those lines which index in terms of a kamacite supercell are listed in this table. cubic kamacite with unit cell edgeapproximatell' 2.869A. As with the Twin Cit-v the unit cell edgeof the taenite in the drilled sampleis half that of the taenite in the scratchsample. Examination of the other powder patterns has shown the presenceof the taenite supercellin the medium octahedriteMesa Verde Park. How- ever, the f-ray pattern of this meteoriteis predominantly due to kama- cite and the tetragonal phaseand, as a result, the taenite lines are only faintly visible. Only three of the measurablelines belong to the taenite supercelibut other very faint lines on the film can be recognizedas be- longing to this pattern even though thev cannot be measured.An e-ray film of the drilled sampleof this meteoritecontains oniy the simplekama- cite and taenitepatterns with the kamacitepredominant (Table 5). Composit,ionof the taenite.A curve relating the compositionof 7 Fe-Ni allovs to the unit cell edge has been establishedby Jette and Foote (1936),Bradlev et al. (1937)and Owen and Yates (1937).We have ana- 11,zeda seriesof synthetic 7 aliovs and, except at 80/s nickel, our data fall on this curve (Fig. 1). Accordingto this curve,the taenitedrilled from the Trvin Citv meteor- ite averagesaboft28/6 nickel.This value is in reasonableagreement with the reported bulk nickel content of the meteorite (29.91/6-Mason KAMACITD AND TA ENTTE SUPERSTRUCTURES 45 T*l-n 4-(continned) Linwood Bischttibe Union Hex River Mountain (oeg) (og) (H) (H) dotn l 4.304 4.340 30 2 8i3 2 844 10 2 876 2 883 1 2 388 2 477 1 2.o8i- 2 088 I 2 032 2 03r 100 2 029 2 03r i00 2 022 2 031 100 2 033 2.03r 100 1 9i7 1.914 5 1.958 I 993 10 1 724 f.i11 1 1.725 t.724 1 1.657 1.490 10 l. oJl 1.486 60 1 437 1 435 j-O i.435 1 433 60 1 430 | 43r 1 1.438 t.422 70 1 239 | 286 1 1 173 1.171 100 r.r71 1 171 80 1 168 1.110 20 1.169 1 171 80 1.016 I 016 80 I 015 1 014 65 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 coursebe recognisedthat taenite in meteorites is of extremel-v-variable composition, (Feller-Kniepmeier and Uhlig 1961; Goldsteinand Ogilvie, 1965),and that the nickel content can exceed50/6 in those regions close to a taeuitefkamacite interface. Ifence, composi- tions determined by tr-ray means are)at best, only averagevalues for the taenitescontained in the samples. Possibleframework for the taenite superlattice.A possible framework for the taenite superlattice is illustrated in Fig. 2. This structure is formed by combinationof eight body-centeredcubic cells.Nickel atoms occupyfour of the body-centeredsites in such a way as to avoid being nearestneigh- bours and to avoid being in cubesthat are side by side. With the four nickel sites lully occupiedas shown, the structure correspondsto the compositionFerNi. 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." 46 A. R. RAMSDEN AND E. N. CAMERON lots€ I ts-6N O^ E: .F- tr stE${r.eF988.q".,18 E N NN l-i g | -6:i ;v;6;;E "{"4 i n++Aoo:db+ a'o Fl - laHo:-oN@SF SO6 N loorts+N:ooo €b E 6; N6ii-i---i-- .ir - 138: tBeS- R E :6 "<"< NO€+-O:+ObOD C) rlNNol9€r@o-sro NlaOrl$N:O!O@b ! i No --e8 lstatte @ d o<, ! >E E Pi *d:9 i$on Q - d + €; E R:53!lSl:llE"3i E ! a NNNd *llattotQtto rrvt/€r@rrr r.l * r.l -ll8tt3t3l g I€ F c56 BrrFrrR,Sr o{ Nlloll+lHl E - .-i -rrsttetst H 4 cF. x ?3 $$bo NIINIIOINI HZ NlloJl*Jil t a 6i6;*'* i I lo lio lo I *.8 tlot €t€l E! s + 9: o + o I tN I FD I N I I i dO I N I I O lts+ li I lO € I ! ,..; ,..i ...i -.i J33 3SrJ.i3 iio oNiiNN FC NNNONN KAMACITE AND TAT,NITE SUPERSTRUCTURES 47 Bradlel, and Jay (1932) originally proposed the above structure for orderediron-aluminum alloys. They showedthat aluminum enteredthe body-centeredsites (occupiedby nickel in Fig. 2) until the composition FegAl was reached. On further addition of aluminum, the aluminum atoms distributed themselveson all eight body-centeredsites until, at the compositionFeAI, thesewere all occupiedby aluminum, giving a caesium chloride structure, It is suggestedthat the nickel atoms in taenite may 5 3 Frc. 1. Relationship between unit cell edge and composition in 7 Fe-Ni alloys. behave in an analogousmanner and exhibit superlattice formation over a range of compositionin the region FeaNi. Unfortunately, the calculated structure amplitudes for the proposed structure show that this model would not give rise to the large number of superlattice reflections observedin the taenite and hence the model is not a complete explanation of the patterns. Nevertheless, the general agreement between the taenite compositions and the composition of the model would suggestthat the conceptis basicallycorrect. Possibly small displacements of the atoms within the structure might give rise to a model able to satisfy the complex conditions required to obtain the ob- served superlattice reflections.We are unable to proposea detailed struc- A. R- R,AMSDEN AND E N. CAMERON ture for such a model but would note that magnetic exchangeforces within the lattice might causethe necessarydisplacement of the atoms. Oldering has not been detectedin FesNi in the laboratory, although Hoselitz (1944) suggestedthat it might occur. However, on the basis of electricaland magneticmeasurements, order has beenreported at the composition FeNie (Wakelin and Yates 1953) and at the composition FeNi (Paulev6,,el al, 1962).In view of the great age of meteoritic iron O rroN I wrcrrl Frc. 2. Possible framervork for the taenite suoerlattice. it would seemreasonable to concludethat the superlatticeformation is primarilr an orderingphenomenon. TTTO K,T.IT.ICITE SUPERSTRUCTURE Evid,enceJor a kamacitesuperslructwre. The evidencefor a kamacite super- structure appearsonly in the r-ra-vpowder patterns of scratch samplesl it is absentfrom the correspondingdrilled samples.The evidenceis well documentedin all of the patterns (Table 4) except that of the Horse Creekmeteorite, and it is particularly evident in the Linvilie and Carlton meteorltes. As mentionedearlier, the r-ray' pattern of the Linville meteorite con- tains 46 measurablelines. Of these,27 belong to the taenite superstruc- KA M A CI TI' A N D TA F:,NI TI':. SUP I'RSTRU CTU RDS ture (Table 3). The remaining 19 lines can be indexed on the basisof a cubic unit cell (Table 4) in which the cell edgeis approximatel,'-S.603 A. Nine very faint lines on the film cannot be measured.The driiled sample vields a pattern containingonly nine lines (Tiable5), five of which belong to a face-centeredcubic taenite in which the cell (a) is approximately ^ -^^ ? 3.589A, and four of which belong to a bodl--centeredcubic kamacite in which the unit cell edge(o) is approximately2.869 A. It is clear-that the unit cell edge of the kamacite in the drilled sample (disordered)is one third that of the kamacitein the scratchedsample. This relationshipalso hoids for the other meteoritesthat have been examined: Scratch Sample Drilled Sample (A) (A) Union 8.58 2 868 Linwood 8 620 2.870 Bischtiibe 8 608 2 869 Mesa Verde Park 8 609 2.869 Carlton 8.617 2.870 Composit'ionoJ the kamacite. The composition of the kamacite in these meteoritesis not known accuratelyat this time but it may be estimated from the unit cell edge of the drilled kamacite. We have measuredthe unit cell edge of zone refined iron and ol 2/6Ni and 4/6Ni synthetic alloys at 25" C., and have used the data to set up a composition/cell edge curve. Unfortunately, an allegedll.-6/6Ni alloy was found to con- tain both a and 7 phasesand could not be used in this work. However, accordingto the electronprobe measurementsof Feller-Kniepmeierand Uhlig (1961),kamacite in the Grant meteoritecontains 6.6/6 nickel, and kamacite drilled from this meteorite in the presentwork has a unit cell edgeof 2.869bh. This information has been combined with the data of the svnthetic alloys to obtain a composition/celledge curve that is ap- plicable to the presentwork (Fig. 3). Jette and Foote (1936)have also measuredthe unit cell edgesof svn- thetic a Fe-Ni alloys at 25o C. Their original data have been converted from kX to A by multiplying by 1.00202and the results are shown in Fig. 3. The differencesbetween the two curves probably represent ex perimental errors on our part. The samplesused in the present work have not been annealedand hence the r-ra1'films contain broad lines due to distortion of the metal. These lines do not separate into aptz doublets at high 20, and errors probably arisewhen measuringtheir po- sitions. However, such errors also appear in the data for the drilled A. R, RAMSDL.N AND E. N, CAMERON kamacites and therefore do not affect the results when usinq our own curve. All the drilled sampleshave been *-rayed at 25" C. The results indicate kamacite compositions ranging from 3.5/6Ni to about 7.87o1:{i.In par- ticular, the s-ray data indicate that kamacite in the Linville meteorite containsabout 6.6/6Ni and kamacite in the Carlton meteorite contains 'l I 3 Atot$rcPEtcENt 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-ray method indicates only the average nickel content of a sample. Possibleframeworh for the kamacitesuperlatLice. By analogy with the pre- viously describedtaenite supercell,a possibleframework for the kamacite supercell is shown in Fig. 4. The supercell is formed by combination of 27 body-centeredcubes with nickel atoms occupying four of the body- center sites. The nickel atoms are distributed in such a wav as to avoid KAMACITE AND TA]JNITE SUPERSTRUCTURL,S 51 orite. The kamacite supercell reflections are particularly well developed in both thesemeteorites. Furthermore, no kamacitesare known to con- tain more than 7.8/6 nickel. As with the taenite supercell,it is necessaryto postulate a more com- plex model in order to account for the many observedsupercell reflec- tions, and it is suggestedthat magneticexchange forces within the lattice o[q a f,rct:r Frc. 4. Possible framework for the kamacite superlattice. might cause small displacements of the atoms in such a way that the necessarvconditions for diffraction are satisfied. Srcxrr.rcaNcEor. THE KalracrrB aNn TanxrrB SupnnsrnucruREs wakelin and Yates (1953) have demonstrated the existenceof ordering in the region FeNia by measuring the electrical resistivity and magnetic properties of synthetic iron-nickel ailoys. They found that ordering was 52 A. R. RAMSDENAND ]J N. CAMERON extremely sluggishand took place over a lange of compositionfrom 50 to 80 atomic per cent nickei, with the maximum change in structure sensitiveproperties occurring at the compositionFeNi:. More recentlr', Paulev6 et at. (1962) have demonstratedthe existenceof a cuAu type superlatticein a synthetic alloy of compositionFeNi (50:50). IIowever, ordering was so sluggishthat it could only be establishedunder neutron irradiation in a magnetic held. Since taenite and kamacite contain far lessnickel than thesesynthetic alloysit followsthat ordering,if it occurs, should be an extremely sluggishprocess. However, in vieu' of the great age of meteoritic iron, (about 4X10e 1'ears),it u'ould seemreasonable to concludethat superlatticeformation is indeed an ordering phenom- enon. Sincethe criticai temperaturefor orderingof iron-nickelalioys are low, (498" C. for FeNi3 -Wakelin and Yates 1953,320o C. for FeNi-Paulev6 et ol. 1962),it is probable that the critical temperaturesfor ordering of taenite and kamacite are equall-vas low, if not lou'er. It follows, there- fore, that if ordering has taken place, then the meteoritesmust have cooledto thesetemperatures at sometime in the past sr.rfftcientlvIemote to allow formation of the superstructuresto take place. If the critical temperatures\ rere known and details of the superstructurescould be determined,then, in principle, it should be possibleto calculatethe time required for formation of the superstructuresand henceobtain a better insight into the cooling historiesof these bodies. TnB ANou'q.LousHoRSE CnBex X{urBonrrB The Horse creek meteorite is a hexahedrite (Table 1). However, it haslong beenknown as an anomalousiron becauseof its unusualpseudo- octahedral structure. The pesudo-octahedralstructur-e arises through orientation of schreibersiteand perrl-ite lamellae along rvhat appear to be octahed.ralplanes in the metal, thus producing a prominent Widman- stetten pattern. However,Perrl- (1944)has pointed out that theseplanes are not really octahedraland that sectionsof the meteoritein rn'hichthe-v intersect at right anglesshow a rectangularpattern not a squareone. The r-ray pattern of the scratch sampleis unusual among all the me- teoritesexamined in that it doesnot contain kamacite lines.However, it does contain a u-ell developedtetragonal pattern (Tabte 3). The cell dimensionsof this phaseare approximatel,va.:7.16 A and c:5'81 A' As with the other meteorites,the tetragonal phaseis unstable and con- verts to a cubic "kamacite" in the drilled powder. Horvever, the unit cell edgeof 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 Besidesthe materials here discussed,we have :u-ra1,edan additional 16 iron meteoritesas drilled powders. All show the simple patterns of kamacite and/or taenite. Nloreover, out of the total of 25 meteorites onl1.the "kamacite" in the Horsecreek meteoritehas given this unusually Iow value for the unit cell edge. Using an electron probe microanalyzer,Fredricksson and Henderson (1965) have shown that the "kamacite,' contains 2.5/6 siliconin solid solution together with about 3.9/o nickel and that the meteorite also contains about 30/6of perryite, nickel siiicide, a new mineral having the approximatecomposition Ni 81%, Si 1270,Fe 3/6 and p 5/e.It seems, therefore,that the Hoise creek meteorite is indeed an anomarousiron. Apparenth- the presenceof silicon markedly reducesthe cell edge of kamacite,and silicon may also be responsiblefor the greaterstabilitl- of the tetragonalphase in this meteor:ite.This stability is indicated by the Iack of kamacite in the scratch sample and the appa'entrv tetragonal orientation of the schreibersitelamellae. CoNcrusroNs The metallic phasesin iron meteoriteshave beeninvesitgated b1.r-ray anall'sis.It has beenfound that (1) many iron meteorites contain a metastable tetragonal phase that seemingly has the same 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 the tetragonal phase and the kamacite and taenite superstructures are destroyed when mate- rial is drilled from the meteorites. rt has beenconcluded that the tetragonalphase is a transitional stagein the transformation of 7-phaseinto a-phase(kamacite). Becausest.nthetic iron-nickel alloys are known to order on the lab- oratorlr time scaleit has been concludedthat the kamacite and taenite superstructuresare due to an ordering process.However becauseof the small differencein scatteringpowers between Fe and Ni, simple ordered structureswould not producevisible tr-ray superstructurereflections. rn order to producethe large number of observedsuperstructure reflections a more complex model must be postulated in which the ordered atoms are svstematicallvdisplaced in position, possiblyas a result of magnetic exchangeforces. A detailed structure determination bv neutron diffrac- tion would be neededto verify such a model. The :u-ra1'data for the Horse Creek meteorite indicate that it has an unusual composition. Recent work by Fredricksson and Henderson (1965) has shorvn that silicon is present in the metallic phase of this meteolite. 54 A. R. RAMSDEN AND E. N. CAMERON Acr Rnlnnesces Bn.r.ntnv, A. J. en-o A. H. Jav (1932) Lattice spacings of iron-aluminum alloys. Jour lron Steel Inst. 125,339 ,A. H. Jav arqo A. Tevr-on (1937) The lattice spacingsof iron-nickel allovs Phil' IIag.23, 545-547. BuNov, F. P. (1965) Pressuretemperature phase diagram of iron to 200 Kbar, 900o C- Jour App. Phys 36, 61G.620. CmNonsrN, R. L. axn H G Dnrcre,urn (1964) The effect of pressureon the volume and lattice parameters of ruthenium and iron. J our. P hys Chem-S olid s, 25' 865-868 CcrrN, E. (1897)Ann. Naturhist.HoJmus.Wein-72,55. -- (1898)Ann. Naturhist. HoJmus.Wien 13,74. - - (1905)M eteor'itenkundeII eJt.3, 222. Doncu, D. exo A. l{. Koumr, (1937) The structures of meteoric iron' Amer. Jour. Sci. 34, 203. Fnr.Lrn-KNrnpunren, M. exl H. H. Ltr{Lrc (1961) Nickel analyses of metailic meteorites by the electron probe microanaiyser.()eochitn. Cosmoclti.m. Acta 21,257-265. FrunnrcxssoN, K. ,rN'oE. P. HsNonnsoN (1965) The Horse Creek, Baca County, Colo- rado, Iron Mete orite T r ons. Am. Geo p hy s. Un 46, l2l. Gor-osrerm,J. I. ,r.no R. E. Ocrr-vrn (1965) The growth of the rvidmanstiittan pattern in metallic meteorites. Geo c him Co sm o c him. A cta. 29, 893-920. Golnlonc, E, A. Ucnrriml,q. aNo H. BnowN (1951) The distribution of nickel, cobalt, gallium, palladium and gold in iron mete orites.Geochim. Cosmochim. Acta 2, 1-25. Hrxornsor, E P. (1941) Chilean hexahedrites and the composition of all hexahedrites. Am. Minerol 26, 545-550 ,rNn S H Ponnv (1949) Proc. U. S Nat. Mus.99,357. Hosnr.rrz, K. (1944) The iron-nickel phase diagram by magnetic analysis, and the efiectsof cold rvork. J our . Iron Steel Inst. 149, 193-204 Howar.r., E. E. (1390) Notice of two new iron meteorites from Hamilton County, Texas, and Puquios,Chili, S.A. Am. Jour.Sci.40,223. T{unr-I{orrrrnv, W. aNo G. V. R.cvnon (1962) The structure of metals and, alloys. Inst. Metal,s,Monograph. Jnrrr, E R rNo F Foora (1936) X-ray Study of lron-Nickel Alloys. Am' Inst. Mi.n. Met. Eng. Metols Tech.3. Tech.Publ'.67O- MnsoN, B (1962)Meteorites. John Wiley and Sons,New York. OrvnN, E. A. a,mnB. D. Bunxs (1939) r:ay study of some meteoric irons. Phil' Mag. ser.7, r28,497-519. KAMACITE AND TAENITE SUPERSTRACTURES 55 Paur.nvf, J., D. Deurn*rr, J. Leucr*,qNo L. N6nr, (1962) Une nouvelle transition ordre-desordredansFe-Ni(50-50). Colloqueslnter.centreNatl.Rech.sci.llg,24g-2s1. Pnnnv, S. H. (l9M) The metallography of meteroric iron. (J. S. Nat. Mus.&uil,. lg4. srrorucrrowsrr, R., J. E. MlvEn alro w. A. wrvr, (Eds.) (1951) phase Transformations in Solids. John Wiiey and Sons, New york. Sorrr'r, R. K. (1960) *-ray diffraction technique for small samples. Am. Mineral.45, 1104. Sru'ov, N. N. (1960) An r-ray investigation of the composition of some meteorites. M eteoritika 19, 63-84 (in Russian). Trrasrrr T. e*r w. A. B,lssnrr (1964) High-pressure polymorph of iron. science r4s, 383-385. we Monuscri,pt receive.d.,June 18, 1965; accepled.Jor publication, N o,ember29, 1965.