A Note on Metallographic I Meteorites George F. Vander Metal Physics Research, Carpenter Technology Corporation, Reading, PA 19612

Meteorites are particularly fascinating subjects for metallographic examination, not merely because of their extraterrestial origin, but also because of the rich variety of microstructural constituents encountered. While their structure can be assessed use of the familiar nital and picral etchants used extensively for steels, application of selective etchants, including those that produce selective coloration, greatly increases the amount of infor­ mation obtained by light microscopy. The article describes efforts to preferentially darken or color microstructural constituents in iron meteorites: kamacite (ferrite), Neumann bands (mechanical twins), taenite (austenite), cohenite (Fe-Ni carbide similar to ce­ mentite), martensite and schreibersite/rhabdite (Fe-Ni phosphides). The influence of con­ centration gradients, deformation, and reheating on the structure can be clearly observed by the use of tint etchants.

INTRODUCTION iron portion of meteorites, as in the iron meteorites, is opaque to and Meteorites have fascinated engineers and requires reflected-light studies. scientists for many years. Research on me­ The classification of iron meteorites is a teorites began with mineralogists and ge­ complex Classification is based on ologists and the major phases and constit­ macrostructural and microstructural char­ uents in iron meteorites were identified acteristics or chemical ,-,,..,.,.,,.. -e-,,..,.,,i.-ir'n and named prior to Henry Clifton Sorby's Ga, Ge, Ir contents). The two "UQYIPn,,, first observation in 1863 of the microstruc­ tunately. are consistent. In terms, ture of polished and etched manmade met­ there are three categories of iron meteo- als. Iron meteorites are but one of several rites-hexahedrites, and general categories of meteorites. ataxites. Those that do not these Fe-Ni alloys with nickel contents of three groups are cate- 4.3-34%, although few have levels >20%; gory-anomalous. and most are in the range of 5-10% Ni. The hexahedrites Small amounts of cobalt 0.4- crystals of kamacite 1.0%), sulfur (up to about phos- boundaries are phorus (up to about 0.3%), and carbon subgrain boundaries are common. Their to about 0.2%) are usually present, while structure may be altered trace levels of many other elements are de­ cosmic which tectable. There are also numerous types of the kamacite stony and stony-iron meteorites. The stony grains. Most contain Ni in the matter can be studied using reflected range of about 5.2-5.8%. or transmitted light on thin sections. The Octahedrites received name based 223

©Elsevier Science Publishing Co., Inc., 1992 MATERIALS CHARACTERIZATION 29:223-241 (1992) 655 Avenue of the Americas, New York, NY 10010 1044-5803/92/$5.00 224 G. F. Vander Voort on their macrostructural charac­ crostructure may also be altered cosmic teristic, visible to the unaided eye after shock and reheating events, as well as etching, caused the of kamacite terrestial corrosion. on the octahedral of the n1"'ln1"_.t-",~'n_ Ataxites contain higher Ni contents than ite (i.e., The octahedrites typically about 15-18%, but contain of Ni than the hex- do not exhibit gross macrostructural ahedrites, in the range of n-""r'TH~·h patterns. Instead, their macro­ although a have Ni contents. structure is basically featureless. Relatively The kamacite in octahedrites con- few ataxites have been found. The kama­ tains more Ni in hexahedrites, 5.5- cite are equiaxed and small in size, 7.5% versus 5.0-5.5%. There are six basic typically <30/-Lm in diameter. types of octahedrites; five are based on the width of the kamacite grains. As the Ni content increases, the kamacite width de­ MICROSTRUCTURAL FEATURES creases. Thus, octahedrites are classed in five steps from coarsest (lowest Ni) to fin­ In 1861, Reichenbach named the two major est (highest Ni) and then plessitic, where phases in iron meteorites, kamacite and the kamacite width is <0.2 rnm, and they taenite, and their mixture, plessite. Ka­ exist in discontinuous short fibers or spikes macite is the body-centered cubic ex phase with an octahedral orientation. The mi- known to metallurgists as ferrite, while

(a) (b)

FIG. 1. Examples of plessite morphologies: (a) comb plessite in Odessa (4% picral); (b) pearlitic plessite in Odessa (unetched, DIC); (c) comb and spheroidized plessite in Odessa (unetched, DIC); (d) comb, pearlitic and spheroidized plessite in Odessa (2% nital): (e) pearlitic plessite in Arispe (4% picral): (f) net (right) and cellular plessite in Gibeon (2% nital): (g) black plessite in Gibeon (4% picral): (h) acicular plessite in Canyon Diablo (4% picral); (i) altered plessite in Box (c) Hole (4% picral). Metallography of Fe Meteorites 225 taenite is the face-centered cubic phase alloys. Plessite 1) occurs in numerous known to metallurgists as austenite. Ples­ configurations two that resemble site is a mixture of kamacite and taenite ferrite-cementite mixtures in steels, that (ferrite and austenite) that occurs in me­ is, pearlitic and spheroidized ples­ teorites but not in manmade, iron-based site. Note that meteorites are named after

(d) (e)

(g)

(h)

FIG. 1. (Continued) 226 G. F. Vander Voort the place where they fell. Table 1 lists de­ grains as films or as triangular patches tails of the meteorites whose microstruc­ ("wedges") at kamacite junctions. The Ni tures are shown in this article. content within taenite is nonuniform [2, Neumann bands, that is, mechanical despite the extraordinarily slow twins, are observed in kamacite, be it in rate of meteorites (typically from 1 to 250°C hexahedrites, octahedrites, or ataxites (as per 106 years between 700 and 400°C long as the kamacite is more than about Determination of the prior-taenite 20/-Lm wide [1]). Because hexahedrites are size has rarely been done, mainly single crystals, Neumann bands can be one must have an exceptionally large quite long; up to several centimeters is not imen. Buchwald [1], in his study the unusual. The vast majority are formed by 20,140kg Agpalilik Cape York extraterrestrial collisions, not by their im­ found a prior-taenite grain size pact with the earth. The band width is usu­ x 1.5 x 1.5m (yes, meters!). ally narrow, typically 1-10/-Lm (Fig. The structure of the taenite generally quite complex and very Plessite is observed in octahedrites ing (Fig. Starting at the kamacite-facn­ where it becomes more common as the Ni ite interface and moving inward, ob­ content increases. The morphology of the serve that etching with nitaI reveals a series plessite mixture also changes with Ni con­ of zones. First, we see a very narrow zone, tent, and a variety of forms has been ob­ 1-2/-Lm wide, called clear taenite 1 served and classified (creating jargon to which contains 51.4-45.6 ± 1.3% Ni and rival anything developed by metallogra­ is ordered Fe-Ni [5]. Next is a phers!). Also in octahedrites, taenite can be brownish appearing zone, up to about observed between adjacent kamacite 25/-Lm wide, called the "cloudy zone"

Table 1 Meteorites Examined in This Work

Composition (Wt. %)[11

Meteorite" Fall location Type Ni Co p s c

Arispe Sonora, Mexico Coarse 6.70 0.47 0.3 Octahedrite Box Hole Northern Territory, Medium 7.67 0.49 0.11 0.05 Australia Octahedrite Canyon Diablo Arizona, USA Coarse 7.10 0.46 0.26 ~1.0 ~1.0 Octahedrite Coahuila Mexico Hexahedrite 5.59 0.45 0.28 Gibeon Southwest Africa Fine 7.93 0.41 0.04 Octahedrite Henbury Northern Territory, Medium 7.51 0.45 0.09 Australia Octahedrite Mundrabilla Western Australia Medium 7.8 ~0.7 0.26 ~8.0 ~1.0 Octahedrite North Chile Tocopilla, Chile Hexahedrite 5.59 0.48 0.30 Odessa Texas, USA Coarse 7.35 0.48 ~0.25 0.5 ~O.2 Octahedrite Toluca Mexico Coarse 8.14 0.49 0.16 0.7 Octahedrite Washington Colorado, USA Ataxite 9.9 0.6 0.39

a Meteorites are named after the place where the fall occurred. Metallography of Fe Meteorites

K

,N

tQOfL m (a) -

(b)

(b)

FIG. 3. Examples of taenite wedges and martensite (c) meteorites: (a) taenite wedge in Odessa showing thin CT-l zone at the a--y interface, the dark CZ, 2. Examples of Neumann bands (mechanical terior retained taenite (CT-2) and martensite (very twins) in meteorites: (a) Neumann bands (N), light because no tempering has occurred) (2% nital); subgrain boundaries (sb), piessite (P), taenite (T) and (b) taenite wedge in Canyon Diablo showing the thin kamacite (K) in Gibeon (2% nital); (b) Neumann bands CT-l, the dark CZ, retained taenite (CT-2) and dark Odessa (2% nital); (c) Neumann bands intersecting etching martensite in the interior (2% nital); and (c) prismatic and platerhabdites in North Chile (2% nital, acicular piessite in Arispe containing martensite (4% DIC). picral). 228 G. F. Vander Voort where spinodal uecomposmon rites. The most common is called coh­ very fine, enite which is orthorhombic and ordered Fe-Ni identical to cementite a surrounding cohenite contains Ni Ni) of martensite and cobalt accord- serve a zone of about to Scott (quoted by Buchwald [1]), and the same width called the dear taenite 2 has the formula (Fe,Ni,CohC. It appears (CT-2) zone 25.8-28.1% white when being quite which may be ordered [5]. The struc- hard 1100 it will stand in relief. tural details in these three zones can only an alloy carbide with a cubic be observed by transmission electron mi- structure and the formula croscopy. In the interior where the (Fe,Ni,Coh3C6 , has been observed in only nickel content is martensite plate- a few meteorites. It is smaller in size than lets are observed within a retained taenite slightly less hard, and also white matrix. These platelets may be quite coarse when When one considers the and can be observed at low magnifications slow cooling rates that meteo­ by light microscopy. the taenite rites are subject to, rates as dose to equi­ wedge must be at least wide to pro- librium conditions as one can ever hope to duce martensite platelets. get, it seems surprising that carbides Carbides can be observed in iron meteo- would form rather than graphite. Graphite

(a) (b)

(c) (d)

FIG. 4. Examples of cohenite in meteorites-(a-c): cohenite (C) and schreibersite (5) in Canyon Diablo (2% nital): (d) cohenite and schreibersite in Arispe (4% picral). Metallography of Fe Meteorites 229 is observed in meteorites, but cohenite is Ni content is rather variable. When the more common. phosphorus content is above about 0.06%, Phosphorus is present in meteorites in very small schreibersite precipitates are ob­ levels much greater than in steels. Hence, served, frequently along subboundaries it is not surprising that phosphides are a and a-a or a-"{ phase boundaries. As the commonly observed meteoritic constitu­ phosphorus level increases, the particles ent. Although the phosphides are all te­ get larger. Because of its brittleness, schrei­ tragonal with the formula (Fe.Nij-P, two bersite usually exhibits extensive cracking. types are defined, schreibersite and rhab­ These massive particles contain 10-15% dite. Both were identified and named Ni, and they precipitate from the taenite. mineralogists in the mid-19th Century. Particles that precipitate later, generally at Schreibersite (Fig. 5) is yellow to brown­ a-a or a-"{ phase boundaries, contain 20­ ish when polished and more brittle than 50% Ni, are smaller in size, more ductile, cohenite, although slightly less hard. The and free of internal cracks. Rhabdites (Fig. 6) exhibit prismatic or platelike shapes. are commonly ob­ served in hexahedrites and many octahed-

(a) (a)

(b) (b) FIG. 6. Plate (P) and prismatic-shaped rhabdites in FIG. 5. Examples of schreibersite in meteorites: (a) north Chile: (a) note the difference in appearance of brittle (note cracks) schreibersite in Odessa the Neumann bands (N) compared to the plate rhab­ (unetched): (b) small schreibersite with dites (2% nital): (b) note that the twins have fractured Ni content between two taenite grains in To­ the intersected prismatic rhabdites (arrow) (2% nital, luca (4% DIC). 230 G. F. Vander Voort

FIG. 9. Sphalerite in Canyon Diablo (unetched).

FIG. 7. Daubreelite inclusion with a chromite rim in METAllOGRAPHIC TECHNIQUES Box Hole (unetched). specimen preparation of rites, particularly those with rather coarse iron meteorites is basically identical to that kamacite grains. The particles for steels. The major problems are generally exhibit numerous transverse elimination of polishing scratches, control cracks, while the prismatic particles are of preparation-induced deformation in the usually crack free. Rhabdites will be kamacite, and control of polishing relief as­ cracked by mechanical twins in the ka­ sociated with the hard cohenite or schrei­ macite that intersect the rhabdites, as il­ bersite. Rhabdite platelets and schreiber­ lustrated in Fig. site are rather brittle and exhibit extensive A number of other mineral phases can internal cracking. Regardless of the care be observed in such as FeS taken in specimen preparation, these (troilite), CrN (carlsbergite), graphite, dia­ cracks will always be present. mond, FeCr2S4 (daubreelite, Fig. 7), Nital is, far, the most common etch- FeCr2 0 4(chromite, Fig. silicates (Fig. 8), ant used to reveal the microstructure of ZnS (sphalerite, Fig. and so forth. These meteorites. Concentrations from 1 to 5% are less commonly observed, but they can have been utilized. Perry [6], however, be readily identified. preferred a 5% picral solution (4% is more commonly used) over a 5% nital solution because its action was slower and more controllable. However, a 5% nital solution is rather strong, and most nital users prefer 1 or 2% solutions because they are slower acting. Perry also reported use of equal parts of nitric and acetic acids for etching Ni-rich ataxites. Many of the etchants used for maraging steels would also be satisfac­ tory for the ataxites. Perry [6] claimed that the best etchant for identifying schreibersite (and all P-rich areas) is a neutral sodium picrate solution used boiling. The solution consists of 2g picric acid dissolved in an aqueous solution

FIG. 8. Silicates next to black plessite in Gibeon (4% of 25g sodium hydroxide dissolved in picral). lOOmL of water. This is the familiar alka- Metallography of Fe Meteorites 231 line sodium picrate solution. states gation, and are of considerable value to the that the solution can be neutralized but metallographer and does not say what to use. Kreye used tint etchants in their study states that neutral sodium picrate is made of the Coahuila and Gibeon meteorites. by adding a saturated aqueous acid Color contrast is observed with bright field solution to a concentrated caustic soda so­ in some cases, contrast can be lution. A yellow precipitate of sodium pic­ further enhanced use of polarized light rate forms, which is collected (plane polarized or by slightly off- and dried. (Dry picrate is an '..J'"f../J..'-'u.... crossed polarized Coloration is pro- it must be handled with A 1% duced by the of an interference film aqueous solution is prepared and neutral­ over either matrix phase, the second ized by adding aqueous picric acid. phases, or all of the structure, that is, an- [6] says that this solution can be made cathodic, or reagents. The without going through the films consist of an sulfide, complex process, but his details are mC'OII1D!lete molybdate, elemental or chro- does say that if the solution is it is mate. The purpose of this article is to dem­ "ineffective;" if it is it will color onstrate the value of certain tint etchants both carbide and phosphide, while if it is as compared to the use of the very com- neutral, it only colors the used nital or reagents. Results To darken cohenite, Perry used boil- with a few other selective etchants will also ing alkaline sodium picrate (4g, rather than be presented. the usual 2g, picric acid is added to a 100mL aqueous solution containing 25g NaOH).The solution must be freshly EXAMPLES made. It can be used electrolytically at room temperature at 6V dc. Berglund Small specimens of a of meteorites states that alkaline sodium will (see Table 1) were mounted and polished darken phosphides as well as cementite. using techniques identical to those for Perry [6] reported limited use of Mura­ irons and steels Specimens were kami's reagent. Numerous versions of ground 600-grit SiC paper, using Murakami's reagent have been used water as the They were then pol- but employed the standard mixture: ished using a medium nap synthetic suede 109 potassium ferricyanide and cloth and 3- and diamond abrasive sium hydroxide in 100mL of water. He followed by an aqueous slurry of 0.05/l-m states that cohenite is colored after 5-10s de agglomerated alumina. This was fol- immersion in a boiling solution, while the lowed by polishing with 0.035/l-m phosphides are unaffected. Perry colloidal silica The general structures that the surfaces etched this way were assessed etching with either 4% exhibited scratch patterns, regardless of or 2% nital. Picral is excellent for re­ the care he took in polishing. vealing the structure of plessite and for the Dorfler and Hiesbock utilized vapor- study of rhabdites. 10 compares deposited ZnSe, the interfer- plessite as revealed with 4% pi- ence layer method var- cral or 2% nital. Picral has revealed the ious constituents in meteorites. Little dif­ taenite globules and with great clar- ference is observed between kamacite ity. Nital out the taenite but also a- and taenite (pink), but the second phases a grain and boundaries and the and minerals exhibit more distinct color Neumann bands. Picral does not bring out differences. the Neumann but can be ob- Tint etchants produce color variations as served differential interference con- a function of ori- trast either as polished and segre- or after DIC is useful for ex- 232 G. F. Vander Voort

(a) (b)

(c) (d)

FIG. 10. Comparison of plessite revealed using 4% picral (a and c) or 2% nital (b and d) in Gibeon. Note that nital brings out the a-a grain boundaries within the plessite and the Neumann bands, which picral does not reveal. amination of picral etched specimens as bit of effort to polish the kamacite free of additional information may be obtained; preparation-induced deformation and Fig. 11 shows such an example. In some scratches. Vibratory polishing will even­ meteorites, if nital is used, there are so tually remove these problems, so long as many Neumann bands visible that it is dif­ the prior steps were properly executed. ficult to assess the rhabdite content. Con­ This generally does introduce substantial sequently, seeing less of the overall struc­ relief at cohenite, rhabdite, and schreiber­ ture can be beneficial in certain instances. site particles, and generally produces Both picral and nital bring out the CT-l, enough relief to view plessite with DIC in CZ, and CT-2 zones within taenite wedges, the as-polished condition, as shown in Fig. and in some taenite strips. When marten­ and c). site is present within the wedge, nital usu­ The writer has tried a number of tint ally reveals its structure better unless the etchants and selective etchants on meteo­ meteorite experienced some degree of rite specimens. This study has not been cosmic reheating after the martensite completed, and only a limited number of formed, that is, some tempering has oc­ meteorites have been examined, so the re­ curred. After the general structure is as­ sults must be viewed as preliminary. sessed, the etch is removed, and vibratory Nevertheless, the results have been quite polishing is repeated. Sometimes it takes a and more work of this type should

Metallography of Fe Meteorites

8. G. F. Vander Voort, Metallography: Principles and lective etching, in AppliedMetallography, Nos­ Practice, McGraw-Hill Book Co., New York (1984). trand Reinhold Co., Inc., New York (1986), pp. 9. G. Dorfler and H. G. Hiesbock, Investigations on 1-19. a quantitative mineralogical characterization of 14. E. Hornbogen and H. The ml,rrr,,,,h"l1r+l1"O meteorites by modal analysis, Meteorite Research, of two iron meteorites (Coahuila and Springer-Verlag, New York (1969), pp. 669-682. Zeitschrift fur Metallkunde 61:914-923 (1970). 10. H. E. Buhler and H. P. Hougardy, Atlas of Inter­ 15. G. Hallerman and M. L. Picklesimer, ference LayerMetallography, Deutsche Gesellschaft ence of the of metal specimens fur Metallkunde, Oberursel, Germany (1980). precision 11. E. Beraha and B. Shpigler, Color Metallography, iron Probe Microanulueis American Society for Metals, Metals Park, OH Marton (eds.), Acauemic (1977). pp. 197-225. 12. G. F. Vander Voort, Tint etching, in MetalProgress 127:31-33, 36-38, 41 (March 1985). 13. G. F. Vander Voort, Phase identification by se- Received June 1992; accepted June 1992.