Meteorites have fascinated mankind for centuries. Indeed, more than two dozen meteorites have been venerated by Indian tribes, aborigines, Arabs and other ancient peoples. The study of meteorites is part of the overall study of the origin of our solar system. There was a recent meteor explosion over the city of Chelyabinsk with up to 1000 injuries. Think what the damage would have been like if it hit a major city. Some asteroids are exceptionally large, and when they strike earth, they can make an immense crater. Some of these, as in Figure 1, are in arid climates and can be seen today. Such an impact near the Yucatan Peninsula has been claimed to have caused the extinction of dinosaurs.
There are three basic types of meteorites: stones, stoney-irons and iron. The classification of meteorites is a complex subject. For the iron meteorites, classification is based upon chemical composition, macrostructure and microstructure. Basically, iron meteorites “fall” (no pun intended!) into three categories – hexahedrites, octahedrites and ataxites. Some, however, do not fully fit the requirements of these groups and are classed as anomalous.
Displays of meteorites in museums generally consist of large, solid chunks of iron meteorites and of etched slices, as shown in Figures 2 to 6. These slices are ground smooth and then etched with a strong acid solution that brings out the growth structure. The octahedrites are commonly exhibited in this manner because they undergo a solid-state phase transformation where the kamacite (ferrite) nucleates and grows along the octahedral planes of the parent taenite (austenite) phase producing a beautiful etched pattern.
The growth of the kamacite phase in octahedrites occurs very slowly. Based upon the known rate of diffusion of nickel in iron, the cooling rate between about 700 and 400ºC, where the transformation occurs, has been found to vary from about 1 to 250ºC per million years! So, no graduate student, no matter how dedicated, can reproduce these macro- and microstructures in the laboratory!
It is important to examine the gross macrostructure (the structure revealed by etching a large ground section and viewing it with the unaided eye, or at low magnification) but much more information can be obtained by microscopical examination of polished and etched specimens. Because iron meteorites are opaque to light, they must be examined with a reflected light microscope, as used by metallographers in the study of man-made metals, such as irons and steels. Indeed, techniques for ferrous alloys work very well for Fe-Ni meteorites.
Iron meteorites are Fe-Ni alloys with nickel contents ranging from about 4.3 to 34 weight %. Most, however, have nickel contents from about 5 to 10%. Small amounts of cobalt are also present, generally from about 0.4 to 1.0%. Sulfur, phosphorus and carbon are also present but their amounts are quite variable. Trace levels of many other elements may be detected.
|Figure 3: This 375 kg mass from the Canyon Diablo fall is at Yale University. The surface is well preserved and is typical of many recovered pieces. Photo courtesy of Dr. Vagn Buchwald .|
The three basic types of Fe-Ni meteorites are defined by the bulk nickel content, and by the width of the kamacite phase in the octahedrites. Hexahedrites are single crystals of kamacite, that is, no grain boundaries are observed even in quite large specimens (the largest ones found on earth are about 20 feet in diameter). Hexahedrites usually have nickel contents in the range of 5.2 to 5.8 weight %. Other phases, chiefly iron-nickel phosphides, occur in hexahedrites; hence, not all of the nickel is in solution in the kamacite (ferrite) phase.
Octahedrites contain about 5 to 10 weight % Ni, and two main phases – kamacite and taenite (austenite). Upon solidification, taenite forms. With subsequent cooling, kamacite is nucleated on the octahedral planes of the taenite and grows in a preferred pattern. The kamacite that forms is lower in nickel content, generally about 5.5 to 7.5%, than the bulk Ni content. Thus, there is a movement of nickel atoms with the remaining taenite phase becoming enriched in nickel. As the bulk nickel content increases, the bandwidth of the kamacite grains decreases. Octahedrites are classed in five steps from coarsest (lowest Ni) to finest (highest Ni) and then plessitic (distinct bands no longer exist).
Octahedrites also contain two-phase mixtures of kamacite and taenite known as plessite. The morphology of these mixtures varies, and their amount increases, with increasing nickel content. A wide variety of rather colorful jargon has been created to describe these different plessitic patterns.
Ataxites contain higher nickel contents that octahedrites, frequently in the 15 to 18% range. Unlike the octahedrites, they do not exhibit gross macrostructural patterns. Relatively few ataxites have been found. The kamacite in ataxites is equiaxed, that is, approximately equal in size in all directions. Also, their grain diameter is small, about 30 µm or less. Kamacite grains in octahedrites are quite long (up to several centimeters is not unusual) and narrow. Length-to-width ratios of 10 to 30 are commonly observed.
The kamacite phase in meteorites has a body-centered cubic crystal structure identical to that of α-iron in steels. Taenite has a face-centered cubic crystal structure identical to that of γ-iron in certain types of stainless steels. When kamacite is deformed, as in extraterrestrial impacts between asteroids, twinning occurs if the rate of strain is high (as in a collosion) and the temperature is low (as in outer space). These twins are called Neumann bands and they are ubiquitous in kamacite in hexahedrites and octahedrites, unless recrystallization has occurred due to reheating. Neumann bands are not commonly observed in ataxites, unless the grain size is rather large. Neumann bands are long (up to several cm is possible) and narrow (1-10 µm).
When kamacite forms and grows in the octahedrites, thin films of residual taenite can be observed between portions of some of the adjacent kamacite grains. Also, wedge-shaped patches of taenite can be observed at kamacite grain junctions. If these patches are relatively large, greater than about 50 µm in diameter, martensite may form in the central region where the nickel content is less than 25%. The nickel content of these taenite patches is highly variable. At the extreme surface, adjacent to the kamacite, nickel contents of about 50% occur in a very thin zone, 1-2 µm wide. Below this thin layer the nickel content gradually drops to about 28 to 30%. This zone is called the “cloudy” zone because of its appearance after etching with nital. Beneath this zone, the nickel content continues to decrease until martensite is observed when less than 25% is present. The structure of these regions is very complex. Transmission electron microscopy is required to study it properly.
Carbides can be observed in certain meteorites, but not all. The most common carbide in meteorites is called cohenite with the general formula (Fe,Ni,Co)3C. This basic type of carbide occurs in steels, and is called cementite (with the general formula M3C where M refers to a metal, mainly Fe, but also substitution of small amounts of Mn and Cr), but cementite in steels never contains Ni or Co. An alloy carbide, called haxonite, has been observed in a few meteorites. Both carbide types are very hard.
Phosphorus is very common in meteorites, but in much greater concentrations than in steels. When the phosphorus level exceeds about 0.06 weight %, which is quite common in meteorites, phosphides are formed. They have a tetragonal type crystal structure with the general formula (Fe,Ni)3P but different morphologies are observed. Schreibersite is the name given to globular shaped phosphides in meteorites. Their size is a function of the phosphorous content. Schreibersite has a variable nickel content, depending upon the temperature at which they nucleated, from about 10 to 50% Ni. The higher nickel content particles tend to be smaller in size and they are more ductile than the larger, lower Ni particles, which are often cracked.
Rhabdites are phosphides that have plate-like or prismatic shapes. These types of phosphides are most common in hexahedrites. In most instances, the prismatic-shaped particles are crack free but the plate-like shaped particles exhibit numerous transverse cracks.
Many other mineral phases have been found in meteorites but they are much less common than those discussed above. Also, these phases may be rather erratically distributed within a given meteorite, rather than being uniformly distributed. These microstructures will be illustrated in subsequent articles on meteorites in this column.
1. Vagn F. Buchwald, Handbook of Iron Meteorites: Their History, Distribution, Composition and Structure, three volumes, Center for Meteorite Studies, Arizona State University and the University of California Press, Berkeley, 1975.
George Vander Voort has a background in physical, process and mechanical metallurgy and has been performing metallographic studies for 45 years. He is a long-time member of ASTM Committee E-4 on metallography and has published extensively in metallography and failure analysis. He regularly teaches MEI courses for ASM International and is now doing webinars. He is a consultant for Struers Inc. and will be teaching courses soon for them. He can be reached at 1-847-623-7648, EMAIL: [email protected] and through his web site: www.georgevandervoort.com
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