Optical Microscopy of Meteoritic Metal

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CHELYABINSK (LL5, S4, W0)

The Chelyabinsk fireball of February 16, 2013 was witnessed across much of Russia, and documented by numerous video cameras.  The shock wave damaged buildings, and many residents were injured by flying glass shards.  In the days and months after the fall, local residents recovered numerous fusion-crusted stones with individual masses ranging from < 1 gram to ~1.8 kg.  The total known mass is estimated to be between 100 and 500 kg.  Chelyabinsk is classified as LL5 (S4, W0).  The majority of stones display a light-colored lithology, whereas some stones appear darker due to high percentages of shock melt.  The following descriptive work was performed on a fully-crusted, "white lithology" specimen of 1.6 grams.

Photo Courtesy of Svend Buhl/Meteorite Recon.

 

Figure 1: Fusion crust in plane-polarized light, showing thin veins of metal + sulfide near the stone's surface.  The material to the lower left is epoxy.

Figure 2: Fusion crust as viewed in crossed-polars. In this illumination mode, the fusion crust is well-delineated.

Figure 3: The fusion crust is relatively porous (black voids).

 

Figure 4: Metal + sulfide veining is common in the fusion crust.

 

Figure 5: The fusion crust contains metal-sulfide melt textures of variable metal:sulfide ratios.

Figure 6: Closeup of the previous image showing metal-sulfide dendrite structure formed by melting/fast solidification in atmosphere.

Figure 7: Taenite shows a twinning structure.  This type of structure is not observed in the metal of H chondrites, but seems relatively common in LL chondrites.  The twinning structure occurs in shock stages S1 - S4, and apparently is not correlated with shock level.  It may represent a unique transformation in relatively Ni-rich metal as found in LL chondrites.

Figure 8: Taenite twinning.

 

 

 

Figure 9: Taenite twinning.

 

Figure 10: Complex intergrowth of taenite, kamacite (deeply etched), and troilite.

 

Figure 11: Large troilite mass contiguous with chromite (gray) at the lower right.  The fractures in FeS correspond to grain boundaries.

 

 

Figure 12: The same troilite mass as preceding image, as viewed in crossed-polars to show the polycrystallinity.

 

Figure 13: Chromite (gray) associated with troilite (same region as in Figure 11).

 

Figure 14: Close-up of chromite.

 

Figure 15: Chromite viewed in crossed-polars.  This chromite is anisotropic (much weaker anisotropy than troilite) and displays a cross-hatched twinning (or perhaps exsolution?) structure.  Cleavage occurs parallel to one of the twinning directions.  The anisotropy and cleavage are both unexpected for chromite.

Figure 16: Another occurrence of chromite.

 

 

Figure 17: Close-up of the previous image.

 

Figure 18: The chromite as viewed in crossed-polars.  As before, cleavage directions are parallel to the twinning.