Optical Microscopy of Meteoritic Metal

Home The Micrographs Field of View Techniques Contact Me

NWA 1584 (LL5)

Shock Stage: S2

Weathering: W1

Fayalite: 28.3

Ferrosilite: 25.2

Known Mass: 3250 g


 

Photo 1. Taenite surrounded by troilite. The taenite interior is martensite, constraining the central Ni concentration to < 30 wt.% Ni.  The cooling rate was sufficiently fast that taenite's central Ni concentration did not rise above 30 wt.% Ni. The whole-rock (bulk) metal composition is similarly constrained to < 30 wt.% Ni because the equilibrium taenite concentration increases during cooling. As shown in the plot below, this inferred bulk composition (<30 wt.% Ni) is consistent with the expected bulk metal composition for a chondrite containing olivine of 28 % Fa. Field of View = 1725 microns.

 

Plot of olivine Fa versus whole-rock (bulk) metal composition for 28 well-characterized chondrites. As expected from "Prior's Rule", metal Ni increases and olivine Fa increases through the H>L>LL sequence. The hatched regions mark olivine composition gaps between the chondrite groups. The metal Ni contents were calculated from wet chemical analyses reported by Jarosewich.

 


 

Photo 2. Close-up of the first image. Field of View = 863 microns.

 


 

Photo 3. Zoned taenite surrounded by kamacite. The kamacite displays numerous slip lines (somewhat curved). Field of View = 1725 microns.

 


 

Photo 4. Close-up of the previous image. The taenite core is martensite (very lightly etched). Field of View = 863 microns.

 


 

Photo 5. Kamacite with small amounts of taenite at the bottom. A euhedral spinel is patially surrounded my metal near the top. The surface of the metal has been modified by terrestrial oxidation. Field of View = 3450 microns.

 


 

Photo 6. Close-up of the previous image. Field of View = 863 microns.

 


 

Photo 7. Another close-up. Field of View = 345 microns.

 


 

Photo 8. Yet another closeup. Field of View = 173 microns.

 


 

Photo 9. Taenite in contact with kamacite. This taenite core is clear-etching. Field of View = 345 microns.

 


 

Photo 10. Close-up of Photo 9. Field of View = 173 microns.

 


 


Photo 11. Taenite in contact with toilite. Field of View = 1725 microns.

 


 

 

Photo 12. Close-up of the previous image. A few kamacite spindles occur surrounded by martensite. Field of View = 863 microns.


 

 

 

Photo 13. Close-up of the previous image, showing kamacite spindles. Field of View = 345 microns.


 

Photo 14. Yet another close-up. Field of View = 173 microns.

 


 

Photo 15. Yet another close-up, highlighting the textural relationship between troilite and metal. Locally, troilite protrudes into the metal cross-cutting the zoned taenite substructure. This enigmatic metal-troilite texture must have a low-temperature origin, as it appears to have formed after cloudy zone formed by spinodal decomposition. Typically, the troilite fingers share the same crystallographic orientation as nearby massive troilite (as indicated by common extinction behavior).  The troilite fingers are often associated with massive tetrataenite, kamacite, and metallic copper (orange).  The origin may involve the breakdown of cloudy zone by the process of discontinuous precipitation. Field of View = 345 microns.

 


 

Photo 16. Yet another close-up. Field of View = 345 microns.

 


 

Photo 17. Yet another close-up, sowing metallic copper associated with troilite and tetrataenite and small amounts of kamacite (deeply etched). Field of View = 173 microns.

 


 

Photo 18. Close-up of Photo 11, showing the top of the taenite. Field of View = 345 microns.

 


 

Photo 19. Taenite in which cloudy zone and martensite have decomposed to plessite. Field of View = 863 microns.

 


 

Photo 20. Close-up of the previous image. Field of View = 345 microns.

 


 

Photo 21. Yet another close-up showing a duplex structure. Field of View = 173 microns.

 


 

Photo 22. Taenite with martensite interior. Kamacite spindles occur surrounded by martensite. Field of View = 345 microns.

 


 

\

Photo 23. Taenite (with a clear-etching interior) abutting kamacite. Note that dark-etching cloudy zone is absent where taenite contacts kamacite, resulting in an asymmetrical taenite etching pattern. The taenite appears to be weakly polycrystalline (i.e., low angle grain boundaries) and evidence of shear can be seen toward the top. Field of View = 345 microns.

 


 

Photo 24. Taenite adjacent to troilite. The taentie interior has numerous troilite blebs.  The troilite blebs are probably fingers in three dimensions, and likely connect with the massive troilite below. Indeed, the troilite blebs and massive troilite share the same crystallographic orientation as demonstrated by the common extinction patterns. Field of View = 863 microns.

 


 

Photo 25. Close-up of taenite interior showing trolite blebs, tetrataenite, kamacite, and a small particle of metallic copper (orange). Field of View = 345 microns.

 


 

Photo 26. Yet another close-up. Field of View = 173 microns.

 


 


 

Photo 27. Field of View = 345 microns.


 

Photo 28. Field of View = 345 microns.

 


 

Photo 29. Field of View = 1725 microns.

 


 

Photo 30. Field of View = 863 microns.

 


 

Photo 31. Field of View = 345 microns.

 


 

Photo 32. Field of View = 345 microns.

 


 

Photo 33. Field of View = 173 microns.

 


 

Photo 34. Troilite rimming a relict chondrule. Field of View = 3450 microns.

 


 

Photo 35. Taenite (composed of several crystallites) adjacent to kamacite. Note the curvature of the kamacite slip lines. Field of View = 345 microns.

 


 

 

Map of NWA 1584 showing the locations of metal particles documented in this study. The numbers correspond to the photo titles above.