On the structure of an iron-nickel meteorite

Figure 1: This is a macro photograph of the Muonionalusta meteorite in the ground and etched condition showing the Wedmanstatten structure. The diagonal line in the lower right hand zone of the specimen is a prior-Taenite grain boundary.

An earlier version of this article appeard on rexp2research.com.

It is illegal in the United States to own a moon rock. However, this does not prevent one from possessing other interstellar material. Meteorites are a great example of material not from the earth. They have fascinated people for centuries and were once an important source of metal before smelting was available.

The Muonionalusta meteorite is estimated to have impacted the earth over one million years ago.[1] It was discovered in northern Sweden in 1906. The specimen shown in Figure 1 was obtained from the A.E. Seaman Mineral Museum at Michigan Technological University in Houghton, Michigan. The specimen was microscopically analyzed in the as-obtained “museum etched” condition. In this state, observations on the phase transformations which lead to its beautiful structure can be made. What follows is a microscopic journey into the origins of this structure.

Not much has been published on the structure of the Muonionalusta meteorite. Vander Voort [2] published a short paper on microstructure, highlighting color metallographic techniques and deformation from earth impact. Holtstam [3] wrote on the discovery of the mineral stishovite in some specimens from this meteorite.

Specimens from the Muonionalusta meteorite typically are composed of ca. 8.4 % nickel in iron. At this nickel concentration, this meteorite is classified as an octahedrite [1], owing to its symmetry. A partial phase diagram is given in Figure 2. Phase diagrams are important tools of scientists and engineers. They show the stable forms of the material at different temperatures and compositions. When crossing a line on the diagram, a crystal structure change occurs. These phase transformations are responsible for the properties of metals, including strength, hardness, density, etc.

Figure 2: Partial Iron-Nickel phase diagram.[4]

The high temperature Taenite phase is face centered cubic (FCC), just like Austenite or gamma-iron in the iron-carbon “steel” system. The low temperature Kamacite phase is body centered cubic (BCC), as is the case for Ferrite or alpha-iron again in the iron-carbon system.[5]

This meteorite likely came from an asteroid or planetoid that disintegrated.[1] During the long travel time in space, the Kamacite precipitated from the Taenite as the meteor very slowly cooled. This is the same process that is behind the crystallization of rock candy from a heated sugar-water solution. As the water cools, the solubility of the sugar decreases and ultimately it precipitates. The only difference here is that the precipitation is occurring from a solid solution as opposed to a liquid solution. Precipitation is a nucleation and growth process, so a nucleant, typically a defect, is required to get the process started. This is akin to the seed crystal when making rock candy.

From the phase diagram in Figure 2, Kamacite becomes thermodynamically stable below about 700 ºC for this meteorite’s composition. However, significant undercooling of 50 to 100 ºC is required for precipitation to begin, due to sluggish nucleation of Kamacite. Additionally, the kinetics of this solid state reaction are exceedingly slow. As a type IVA octahedrite meteorite [1], the cooling rate during the time of Kamacite precipitation is estimated to be 7–80 ºC per million years. No way exists to simulate this in the laboratory. Early researchers mistakenly thought they understood the precipitation based on laboratory quenching experiments, but they were not forming Kamacite, but rather another structure known as Martensite.[5]

Figure 3: Photomicrograph of Muonionalusta meteorite with Kamacite plates almost parallel to section plane. False color Nomarski DIC image to enhance relief.

The Kamacite precipitation is a diffusion controlled nucleation and growth process. However, the new crystals do not just form in random orientations. Kamacite preferentially precipitates on crystallographic planes of the Taenite which are favorably oriented, thus limiting the number of likely orientations of growth within a given grain or crystal of Taenite. The orientation relationship is {111}-FCC II {110}-BCC and [110]-FCC II [11I]-BCC.[5] The orientation relationship specifies what planes and vectors within those grains of the two phases will be parallel as the transformation progresses. The morphology of the Kamacite is parallel plates as shown in Figure 3.

Taenite is FCC and in FCC metals there are 4 unique {111} planes. Thus, one would expect plates of BCC Kamacite to have 4 unique orientations when precipitating from a Taenite crystal. In Figure 1, three of these planes show intersections in the cross-section plane, while the fourth is nearly parallel to the cross section plane and more difficult to detect, although it is evident in the upper right quadrant of Figure 1 and at higher magnification in Figures 3 and 4. (Figure 180 of Buchwald [5] shows this fourth plane very clearly for a different meteorite). This is the origin of the Widmanstatten or basket weave structure shown in Figure 1. The limitation on the possible orientations of Kamacite plates which precipitate from the Taenite causes the geometric order. The transformation of beta titanium to alpha titanium, although of different crystallography, gives rise to a similar Widmanstatten structure.

Figure 4: Widmanstatten structure of Kamicite in the Muonionalusta meteorite. “P” is the Plessite structure. “A” is a Kamacite lamella in the cutting plane.

In Figure 1, the prominent diagonal line on the right hand side is a prior-Taenite grain boundary. It is interesting that in this specimen, the Taenite grains had very similar orientations, as the respective Widmanstatten structures are very similar. It is also worth noting that by metallurgical standards, the prior Taenite grains were absolutely huge.

Figure 4 shows the Kamacite plates of the four possible orientations in the Widmanstatten structure. Location “A” is a lamella cut parallel to the cutting plane. Location “P” is the structure Plessite. Referring to Figure 2, Plessite is from the two phase region being made up of a mixture of Kamacite and Taenite. Figure 5 illustrates a common morphology for Plessite formation. The darker material near the Kamacite plates bounding the Plessite is likely rich in Taenite.[5] Figure 6 is a higher magnification image of Plessite, where the morphology makes it more apparent this is a two phase mixture.

Figure 5: Plessite surrounded by Kamicite plates. Muonionalusta iron meteorite, Nomarski DIC image.

Figure 6: Plessite in the Muonionalusta meteorite. Nomarski DIC image.

In summary, meteorite specimens in the “museum etched” condition, intended to prepare the beauty of the specimen for viewing with the unaided eye, can yield important information when viewed with optical microscopes.

The specimen investigated here was imaged at low magnification using an American Optical Spencer stereo microscope retrofitted to accommodate a Sony NEX 5n camera. Higher magnification Nomarski DIC imaging was perfumed on an Olympus BHS microscope with a Sony A7S camera. Nomarski DIC gives false color images which can reveal surface relief more clearly than bright field images. This is discussed further elsewhere.

References

1. Wikipedia

2. G. F. Vander Voort and F. E. Schmidt, “Microstructure of the muonionalusta octahedrite meteorite,” Microsc. Microanal., vol. 20, no. 3, pp. 848–849, 2014, doi: 10.1017/S1431927614005960.

3. D. Holtstam, C. Broman, J. Söderhielm, and A. Zetterqvist, “First discovery of stishovite in an iron meteorite,” Meteorit. Planet. Sci., vol. 38, no. 11, pp. 1579–1583, 2003, doi: 10.1111/j.1945–5100.2003.tb00002.x.

4. Tobias1984, CC BY-SA 3.0.

5. V. F. Buchwald, Handbook of iron meteorites, their history, distribution, composition, and structure. Berkeley: Published for the Center for Meteorite Studies, Arizona State University by the University of California Press, 1975.