Submit a Picture

Where is My Picture?!

The Queue

Met Name

Met Type

Thin Sections

9 grams. 51 x 49 x 1 mm.   R3-5

TKW 1100 grams. Fall not observed. Found 2004.
 

Steve writes:

Rumuruti (R-chondrite) meteorites are a rare group, with even fewer known examples than Martian meteorites. Compared to ordinary chondrites, they are more oxidized and have small but significant differences in chemical composition and oxygen isotopes that consequently suggest they aren't related. More than half of the R-chondrites are regolith breccias - as inferred from signs of implanted solar wind - and analysis of noble gases in the NWA R-chondrites suggests at least 16 different falls and a major collision event on their parent body sometime between 15 and 25 million years ago.

The lighter gray R-chondrites (like NWA 4288 that I previously posted on MPOD) have been interpreted by some as being fresh, while the reddish-brown color of ones like NWA 6491 shown here was sometimes attributed to terrestrial sulfide alteration. But more recent findings indicate R-chondrites formed in both dry and aqueous conditions, giving rise to their disparate colorations. In particular, the presence of water in NWA 6491, coupled with an appearance of fresh fusion crust and the absence of weathering veins within its interior, suggests pre-terrestrial hydrous alteration either on the R-chondrite parent body or in the nebular environment.

After considering several of Bob Falls' slices of NWA 6491, I chose the 9.0 gram, 51 x 49 x 1 mm slice shown in Photo 1 because of the dark breccia on the right that speculation at the time suggested might be possible impact material. Later, I acquired a large thin section collection from Edwin Thompson that included a slide from this fall. I found some of its features intriguing and thought they might be worth sharing, though I must apologize for the poor quality of the accompanying thin section photos (while I have a Zeiss Ultraphot III microscope that provides crisp views through its eyepieces, for some reason my Amscope digital microscope camera gives distorted images; consequently these photos were taken with a Galaxy S7 cell phone using eyepiece projection).

I took Photo 2 to show the unusual bubble trains seen in the blue area (note how some change direction); but after cropping, I noticed it looked a little like Africa - fitting since it's an NWA! Photo 3 is a collage showing some of the various features seen across the thin section. NWA 6491 is a breccia, and Photo 4 shows some fine-grained material that appears to have formed or flowed around the various features (Photo 5 is a blowup of a couple of these, the bottom half being an enlargement of the upper right bifurcated vein in Photo 4 and a pool of this material above it). What I found intriguing was the shearing present in some of them (one example is seen in the lower right corner of Photo 3). Finally, Photo 6 shows an area taken with four different rotations of polarized light; I picked it because of the orthogonal crystalline structure in the center that was unique within this thin section.

  Click to view larger photos     #1     #2     #3     #4     #5     #6