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Kamacite and Taenite Superstructures and a Metastable Tetragonal Phase in Iron Meteorites

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Kamacite and Taenite Superstructures and a Metastable Tetragonal Phase in Iron Meteorites 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 Creek meteorite, 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<f iSON.o 24= * +d-; -i Gi 6i 6i ,-; J J -'; -i iv,d U X.::' 9-€N\OO'€O c)-+N€cr' v:' t O.-€reOOo\ lO<ir=i,Dr l€ {, os+N4co'b+ 6os.o€- ld o : r' r;+-; -j ooi oi oi -i .iJJJ J -i i".s q 9;b u;: I O ODO OOi4O-i I = =--l '6 N oa--.o99Q=\o SAY N +-@ o4o\cc<roccN i FN:NQ On+O-ON\O o€ ft r:itOO NNNNN:i 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<iOONNN NNdi 3 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 *<j*, o-j .i .i -i -i .i -i J-iJ*JJ* ''j 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.
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