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See comments below.   Iron, IIIAB-an

TKW 63 kg. Observed fall 3 April 1916.



   

jnmczurich writes:

A huge fireball appeared over the middle part of Germany on April 03, 1916 in the afternoon.

After collecting many eye and ear witness reports, Alfred Wegener (famous German physicist, also known for the approved "continental drift theory"), initiated a meteorite search. The Treysa iron meteorite was found by a forester only 800 m from Alfred Wegener’s calculated impact point. The iron mass was found in a 1.6 m deep hole in the Rommerhausener forest, some eleven month after the fall happened. .

For detailed information see Buchwald, Handbook of Iron Meteorites, pages 1232-1235. Pretty interesting to read! .

Image 1: These writings are also very interesting to read:

• Alfred Wegener: "Das detonierende Meteor vom 3. April 1916, 3 1/2 Uhr nachmittags in Kurhessen" (1917)
  
  
• Franz Richarz: "Auffindung, Beschreibung und vorläufige physikalische Untersuchung des Meteoriten von Treysa" (1918) .
  

Both writings are available as a combined reprint: Elwert, Marburg 2001, ISBN 3-7708-1160-7, in German language only.

Treysa is classified as an iron, IIIAB-an, Om (0.90 ±0.10 mm) with shock-hatched Ɛ-structure. The iron is an anomalous IIIAB group member due to the chemical composition (Ni-Ga-Ge ratios, and also in its P-Ir ratio). The Treysa microstructure is exciting to look at compared to other IIIAB irons. There are a large number of plessite fields to see, partly developed as "classic" comb or finger plessite fields or also developed as martensitic plessite fields. Schreibersite occurs in different sizes and some troilite inclusions can also be found.

As a fresh iron meteorite fall, the thermally transformed / recrystallized outer zone (alpha-2 zone) of the meteorite is also clearly recognizable in some of the following microstructure pictures.

Image 2: Forester Hupmann posing with the Treysa meteorite, shortly after the discovery of the iron mass was made by him in spring time 1917.

Image 3: Old newspaper clipping. Forester Hupmann received the previously announced reward of 300 Reichsmarks (old German money) for finding the Treysa meteorite. That was a lot of money for the time. Lucky guy!

Images 4 and 5: Treysa main mass, waiting for visitors in Mineralogical Museum of the Philipps University zu Marburg/Germany. The images of the main mass were taken during the 100 years celebration of the Treysa meteorite fall in April 2016. From late 2015 through early 2016 I took the opportunity to do some metallographic work on a 13.2 g partial slice of Treysa.

Image 6: 13.2g thick Treysa partial slice wire-cut in two halves. The partial slice on the right was used for metallographic work. Let's now look close at a few microstructure images and discuss them together. I decided to focus on images taken from the color etched sample only.

Image 7 – Embedded Treysa partial slice, polished and color etched. The sample needs to be perfectly polished before etching (Klemm-I wet etching/precipitation etching was used). Next, alcohol cleaning and drying with a hair dryer. Never (!) touch the polished face with cotton or fingers during or after the etching. The red marked fields of interest will be shown in detail in different magnifications and briefly described.

Image 8: magnification 50x. Overview of recrystallized kamacite bars, comb plessite field and multiple layered fusion crust.

Image 9: magnification 200x. Detail of comb plessite field. Alternating Ni-rich taenite (white) and kamacite recrystallized in grains (blue/brown).

Image 10: magnification 200x. Detail of multiple layered fusion crust - probably created due to the mass tumbling during atmospheric flight.

Image 11: magnification 50x. Overview of kamacite bars recrystallized in grains (blue/brown), two recrystallized plessite fields, taenite (white), fusion crust on top.

Image 12: magnification 200x. Multiple layered fusion crust, kamacite recrystallized in grains.

Image 13: high magnification 500x. Kamacite in the thermally changed alpha-2 zone of the meteorite. During flight the kamacite was briefly heated to >750° C and, due to the low temperature of the inside of the iron meteorite, quickly cooled down again (quenched) almost immediately, transforming it to a martensitic structure of "flake-like" grains. This change in structure can be seen much better with color etching than with etching in nitol (ethanol and nitric acid).

Image 14: magnification 50x. Great overview of a larger plessite field, partly fine taenite islands within small kamazite fields and alternating taenite lamellae and shock-hatched kamacite (kamacite fingers within the plessite field).

Image 15: magnification 200x. Taenite lamellae (white) and shock-hatched kamacite (brown, feather structure). Left and right finely structured plessite field with globular taenite/kamacite islands. One of my favourite pictures.

Image 16: magnification 500x. Detail of shock-hatched kamacite. Shock-hatched kamacite can be recognized by its feather-like structure. It is a relic of a multiple structural change in the atomic metal lattice: from body-centered cubic alpha-iron (bcc) to its high-pressure modification epsilon-iron (Ɛ iron) and a conversion back into alpha-iron. As a metal lattice structure, Ɛ iron shows the hexagonal closest packing (hcp). The structure only arises at very high pressures (>130 kbar) when high-energy pressure waves run through the iron. This is possible when knocking out masses of the meteorite parent body or in the event of a severe collision between two asteroids.

Image 17: magnification 50x. Overview of a large plessite field at the borderline of recrystallized alpha-2 zone and unchanged kamacite structure (to see into the lower part of the kamacite fingers).

Image 18: magnification 200x. Lower part of the plessite field with shock-hatched kamacite (feather-like, blue) between the taenite lamellae (white). Large shock-hatched kamacite bar in the lower part of the image.

Image 19: magnification 500x. Recrystallized "flake-like" kamacite grains between taenite lamellae (white).

Image 20: magnification 50x. Oooh, that is a crazy plessite field with many "fingers" of shock-hatched kamacite bars and thin taenite (white). The right hand part of the plessite field is finely structured plessite field with globular taenite/kamacite islands. What a beauty!

Image 21: magnification 200x. Spectacular detail of the large Plessite field with crossing kamacite "fingers".

Image 22: magnification 500x. Detail of the kamacite fingers. Taenite (white), shock-hatched kamacite (blue with brown "feathers").

Image 23: let’s have a look at Nickel element mapping of the same plessite field (see also image 20). Color code: blue = low Ni content (here about 8.5% in kamacite^*), green/yellow = high Ni content (here about 30% in taenite), yellow/red = very high Ni content (here about 40%). ^* Normally the kamacite should have about 6 to 6.5% Ni content, but in Treysa we found about 8.5% Ni content in kamacite.

Image 24: Phosphorous element mapping of the same plessite field (see also images 20 and 23). Color code: green = high P content (here about 10%). If we combine the high Ni content and the high P content, then we now understand that the elongated inclusion is a schreibersite inclusion (iron-nickel-phosphide). Chemical formulae (Fe, Ni)₃P. The semi-quantitative EDX measurement showed P approx. 10% by weight, Ni approx. 40% by weight, balance Fe.

Image 25: Treysa partial slice ultra-fine hand-polished with diamond past 1 micron and further polished with Alumina clay suspension. The Widmanstätten structure comes out very well with this high-quality polish, even without etching.

All microphotographs and element mapping measurements were taken with laboratory equipment from the accredited test laboratory Qualitech AG, Oberwinterthur/Switzerland. Sample preparation, color etching as well the microphotographs were performed by myself. We all at Qualitech including me had so much fun and fruitful discussions during this little Treysa project.

Side note 1: further information on the Treysa iron meteorite can be found in my facebook posting of today in the FaceBook group "Historic Meteorites" (private group, 182 members now). Have fun looking through it.

Side note 2: there are many more Treysa microstructure images, taken from color etched as well from nitol (ethanol and nitric acid) etchings (b/w images), and also element mapping and line scan images in my folders available that I would like to present on another occasion. I’ll let you know…

Comments are always welcome.