Iron nickel meteorite microstructure analysis
This is a macro photograph of the Muonionalusta meteorite in the ground and etched condition showing the Wedmanstatten structure.

Iron nickel meteorite microstructure analysis

Iron nickel meteorite microstructure analysis based on the Muonionalusta meteorite

An enlarged version of this posting intended for a more general reader is on Medium.

This posting presents an iron nickel meteorite microstructure analysis for a meteorite from the north of Sweden known as the Muonionalusta meteorite. Even unaided by a microscope, the microstructure is quite beautiful and intriguing. It shows how metallurgical and geological sciences intersect, even though there are some small language differences. Microscopy was performed using the facilities available to REXP2 Research LLC.

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

Not much has been published on the structure of the Muonionalusta meteorite. Vander Voort [1] published a short paper on microstructure, highlighting color metallographic techniques. Holtstam [2] wrote on the discovery of stishovite in some specimens from this meteorite. The Muonionalusta meteorite was discovered in northern Sweden in 1906.  It is estimated to have impacted the earth over a million years ago.[3] The specimen shown here was obtained from the  A.E. Seaman Mineral Museum at Michigan Technological University in Houghton, MI. The specimen was microscopically analyzed in the as-obtained “museum etched” condition. In this state, observations on its phase constitution can be made, however, analysis of deformation effects such as twinning would require further polishing and etching. 

Specimens from the Muonionalusta meteorite typically are composed of ca. 8.4 % nickel in iron. At this Ni concentration, it is classified as an octrhedrite.[3] A partial phase diagram is given in Figure 2. The high temperature Taenite phase is FCC, just like Austenite or gamma-iron in the iron carbon system. The low temperature Kamacite phase is BCC, as is the case for Ferrite or alpha-iron.[5] This meteorite likely came from an asteroid or planetoid that disintegrated.[3] During the long travel time in space, the Kamacite precipitation occurred as the meteor very slowly cooled. Kamacite becomes thermodynamically stable below about 700 C for this meteorite’s composition. However, significant undercooling of 50 to 100 deg. C is required for precipitation to begin due to sluggish nucleation of Kamacite.[5] Additionally, the kinetics of this solid state reaction are exceedingly slow and it can not be simulated in the laboratory.[5] As a type IVA meteorite [3], the cooling rate during the time of Kamacite precipitation is estimated to be 7-80 deg. C per million years.[5] 

Fe-Ni phase diagram. Iron nickel meteorite microstructure analysis.
Figure 2: Partial Iron-Nickel phase diagram.[4]

The Kamacite precipitation is a diffusion controlled nucleation and growth process. 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] 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 transformation of beta titanium to alpha titanium, although of different crystallography, gives rise to a similar Widmanstatten structure.

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.

Photomicrograph of Muonionalusta meteorite with Kamacite plates almost parallel to section plane. Nomarski DIC image. Iron nickel meteorite microstructure analysis.
Figure 3: Photomicrograph of Muonionalusta meteorite with Kamacite plates almost parallel to section plane. Nomarski DIC image.
Materials science and metallurgical consulting. Widmanstatten structure of Kamicite in the Muonionalusta meteorite. P is the Plessite structure. A is a Kamacite lamellla in the cutting plane.. Iron nickel meteorite microstructure analysis.
Figure 4: Widmanstatten structure of Kamicite in the Muonionalusta meteorite. P is the Plessite structure. A is a Kamacite lamella in the cutting plane.

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.

Plessite surrounded by Kamicite plates.. Muonionalusta iron meteorite, Nomarski DIC image. Iron nickel meteorite microstructure analysis.
Figure 5: Plessite surrounded by Kamicite plates. Muonionalusta iron meteorite, Nomarski DIC image.
Plessite in the Muonionalusta meteorite. Nomarski DIC image. Iron nickel meteorite microstructure analysis.
Figure 6: Plessite in the Muonionalusta meteorite. Nomarski DIC image.

In summary, meteorite specimens in the “museum etched” condition, intended to prepare the specimen for viewing with the unaided eye, can yield important information when viewed with optical microscopes.  Finer metallographic polishing and etching would bring out even more finer scale detail in iron nickel meteorite microstructure analysis.[1, 6]

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 on this site.

References

1. 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.

2. 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. 

3.  Wikipedia 

4. Tobias1984CC 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.

6. G. F. Vander Voort, “Metallography of Iron-Nickel Meteorites,” 2018. https://vacaero.com/information-resources/metallography-with-george-vander-voort/153771-metallography-of-iron-nickel-meteorites.html.

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