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Met Name

Met Type

Thin Sections

  Iron, IIIAB

TKW 15.5 tons. Fall not observed. Found 1902 in Oregon, USA.


   

Jnmczurich writes:

Willamette is classified as an iron, IIIA(B) or iron-ungrouped, Om (1.05 ± 0.10 mm), recrystallized and annealed twice in cosmos.

Looking to its chemical composition, Willamette has following approximate composition: 7.62% Ni, 0.45% Co, 0.14% P. The Sulfur content of Willamette is unreported.

Following is some information to accompany the images of my little unetched /color etched metallography examination.

Images 1-3: I asked my long-time friend Marc Jost, Space Jewels Switzerland, to split my former 35.6 g Willamette partial slice in two thinner partial slices with his wire-saw. A 0.30 mm wire was used to split the partial slice. Some of you might think, "why the hell is he cutting a large 35.6g partial slice of this famous meteorite small?". The idea behind was, to make a nice metallographic preparation of the Willamette iron, but I can't do metallography on such a large sample. Fortunately I have a second and a bit larger Willamette piece in my collection – the story of this larger Willamette piece will be another MPOD, coming soon – and therefore it was acceptable for me to cut the 35.6g partial slice smaller and to trade the smaller parts to collector friends of mine.

Image 3: With my own cutting equipment I sectioned the thinner and smaller 12.7g partial slice into four pieces. The red frame shows the 2.3g sample, used for metallographic preparation.

Image 4: The hot embedded 2.3g sample. Dimensions: 13.5 x 12 x 1.9 mm. Hot embedding is carried out with thermally curable Duroplast plastic granulate under pressure in a small pressing tool. The embedded sample can thus be prepared with sharp edges. It is better suited for microscopic images than non-embedded specimens, which are then mostly polished somewhat roundish near the edges.

Image 5: A fast cellphone picture through a simple old one-tube laboratory microscope to check for scratches on the polished face of the embedded partial slice, unetched condition. Poor quality of the picture, but the old microscope used is not a photo microscope.

Image 6: Same partial slice in color etched condition. The recrystallized structure is clearly to see, but also relicts of the primary Om structure (former kamacite bars, delimited by thin taenite layers and more or less degenerated plessite fields) are recognizable.

Color etchings:

For color etchings the sample preparation must be perfect, absolutely no scatches are allowed in the finished polished face. It takes a lot of time and effort to get an acceptable polishing result. The final polishing steps, after the use of wet sandpaper, is done with different type (very fine to extra fine) diamond paste and finally with (Alumina) Oxid Polishing Solution (OPS). The etching itself is a so-called wet etching without using a cotton ball or any kind of direct contact with soft brush or similar tools to the surface to be etched. Color etchings are difficult in preparation but I like these etchings very much and I love the challenging work on it. However, I always have to ask a nearby laboratory for a private working time slot and depending on the laboratory workload I have to wait days, sometimes weeks for using their professional laboratory equipment including their chemicals by my own. A good, microscope-compatible color etching can be produced, for example, with the etchant "Klemm-I". This has given me the best results so far for various iron meteorites. Beware of color etching: if you touch the etched surface then you destroy it. Handling is quite difficult.

Image 7: Degenerated plessite field, alternating wide kamacite (K) and thin taenite (T). Schreibersite (S) delimits the plessite field to the right side. Some further schreibersite inclusions. Unetched condition. Magnification: 50x.

Image 8: The same motive but color etched. Blue, brown and redbrown are kamacite, the small mosaic-like areas are close-meshed plessite fields/areas, the block-like white inclusions are schreibersite inclusions, the thin white stripes inside of the large degenerated plessite field is taenit. The „scratches" are annealed Neumann bands, stopping at the border to the next kamacite grain. The large degenerated kamacite field in the center of the picture looks misplaced into the kamacite grain structure. It is a relict of the former Om structure. Magnification: 50x. One of my favourite motives.

Image 9: Two connecting but different looking small plessite fields. Annealed Neumann bands to see on the left side in picture. Unetched condition. Magnification: 50x.

Image 10: The same motive but color etched. Dark blue, light blue, brown and redbrown is kamacite. Annealed Neumann bands are present in various directions in all kamacite fields. The right, somewhat wider spheroidized kamacite field shows recrystallized kamacite grains on the inside. Taenite forms rounded precipitates. Very interesting to see. The left, somewhat narrower plessite field shows a significantly higher proportion of taenite. It looks like a wide taenite-rich lamella with only a small amount of kamacite inside or like a taenite accumulation. Magnification: 50x.

Image 11: Earlier parallel kamacite bars, delimited by thin taenite layers. Micro fine schreibersite precipitations are spread within the kamacite. Unetched condition. Magnification: 50x.

Image 12: The same motive but color etched and lower magnification. Dark blue, light blue and light brown are recrystallized kamacite grains within and across the former kamacite bars. Neumann bands are present. Magnification: 25x.

Image 13: Kamacite grain boundaries at a perfect angle of 120 °, ending in triple points. Some small schreibersite inclusions. Unetched condition. Magnification: 50x.

Image 14: Kamacite grain boundaries at a perfect angle of 120 °, ending in triple points. Some small schreibersite inclusions. Unetched condition. Magnification: 100x.

Image 15: Kamacite grain boundaries at a perfect angle of 120 °, ending in triple points, some Neumann bands. Color etched condition. Magnification: 100x.

Image 16: neighbor partial slice to the partial slice used for metallographic examination. Weak etched with alcoholic nitric acid. Kamacite grains and boundaries at a perfect angle of 120 °, ending in triple points, some annealed Neumann bands. This small 8x6 mm part of the Willamette iron meteorite is almost perfectly recrystallized, see description below.

Explanation to images 13-16 and also 17:

A short attempt of an explanation using the laws of thermodynamics and metallurgy: Grain interfacial tensions exist between individual grains of a metal structure. The equilibrium of these tensions gives the metallic structure its characteristic appearance. This applies in particular to recrystallized structures. Under the influence of time and temperature, every body tries to adopt the lowest possible energetic state. In the case of the recrystallized, granular structure of the iron meteorite Willamette, this means that when the triple lines with 120 ° angles are present, a relatively small grain boundary surface is opposed to a relatively large grain volume; and this is a state aimed at for energetic reasons. This shown part of the Willamette structure is almost completely recrystallized.

Image 17: Kamacite grains in various blue colors, schreibersite in single spots. Two close-meshed plessite fields with a high proportion of taenite (with a lot of white taenite inside). Also kamacite grains and boundaries at a perfect angle of 120 °and some triple points are visible. There are partially annealed out Neumann bands to see in the right lower corner. Color etched condition. Magnification: 50x.

Image 18: Detail of the larger Plessite field, see image 17. The high taenite content (white) in this plessite field is visually confirmed by this higher magnification. Kamacite is brown to redbrown and blue in color in between the white taenite. The fat elongated and almost vertical inclusion on the left side of the plessite field is a somewhat roundish polished schreibersite inclusion. Color etched condition. Magnification: 500x.

Image 19: Another nice but also degenerated, taenite-poor plessite field between recristallized kamacite grains. The corners and boundaries of the plessite field consist of taenite enrichments, similar structure to that shown in Figure 18. Neumann bands are present. Color etched condition. Magnification: 50x.

Image 20: Another misplaced and taenit-rich plessite field between kamacite grains. The former Om structure disappeared completely during the recristallization. Annealed Neumann bands are present. Color etched condition. Magnification: 100x.

Image 21: Inside of a kamacite grain annealed Neumann bands can be found in various directions. The elongated, white-looking precipitates are likely schreibersite precipitates (small prismatic schreibersite precipitates are also called rhabdites). Color etched condition. Magnification: 200x.

Schreibersite:

Schreibersite in Willamette is also associated with troilite inclusions, but troilite was not found within the small partial slice used for this metallographic color etching experiment.

Comments are welcome.