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Thin Sections

  CV3

TKW 2 tons. Observed fall 8 February 1969, at the village of Pueblito de Allende, Chihuahua, Mexico.



          ^Smithsonian

Steve writes:

The February 1969 Allende meteorite is a famous and scientifically important carbonaceous chondrite that most of Paul’s readers are quite familiar with. It’s also been the subject of many prior MPOD posts, including two previous ones from me (8/22/17 and 8/26/17). Allende has provided significant contributions to meteoritics – as well as having the honor of being the first extra-terrestrial material to pass through NASA’s Lunar Receiving Lab as a test prior to the moon rocks returned by the Apollo missions – but I won’t elaborate here as there is a lot of online information that Paul’s readers can easily find if interested.

My prior posts featured individual stones, end cuts, and slices. In today’s post, I thought it might be fun to show three thin sections taken from the same Allende plug (grouped in Photo 1 and shown in greater detail in Photo 2’s animated GIF comparison with each slide’s xpol image on top and its corresponding white light image on the bottom). This particular plug had a pronounced CAI that can be seen through the series of slices. Their full gigapixel resolution xpol and white light mosaics, respectively, can be seen on Gigapan:

Slide 1, xpol

Slide 1, white light

Slide 2, xpol

Slide 2, white light

Slide 3, xpol

Slide 3, white light

For those new to Gigapan, click the diagonal arrows at the upper right of the image to go full screen, then use your mouse to pan around and your mouse wheel to zoom in and out.

It’s not clear whether these slides are adjacent thin sections, or whether others were created in between them – while there are some similarities between pairs of slides, even the larger chondrules don’t show any remnants in any others, so I suspect that either these are not adjacent, or there was a lot of material lost between slices.

Since individual xpol images by themselves are more artistic than scientifically useful, I thought I’d bridge the gap by providing some animated GIFs of a few more interesting areas (I assume this is somewhat like a post John Kashuba did over four years ago, though the link to his image is broken so I can’t say for sure). But because my stage is motorized for mosaicking, rotation is not possible; instead, while not the way petrology specimens are traditionally viewed, I simply rotated the source polarizer from extinction through seven subsequent steps of 22.5 degrees, with a final image minus the source polarizer to show what each area looks like in white light (that image persists longer so the viewer can tell when the cycle repeats). A more correct approach would have been to simultaneously rotate both the source and analyzer polarizers the same amounts, but the analyzer on my Ultraphot has a multi-turn knob that would have made this fairly tedious, and since these GIFs weren’t meant for any actual scientific analyses, I got lazy and only rotated the source polarizer.

The animated GIFs in Photos 3 through 6 show individual chondrules; Photo 7 shows a cluster of chondrules; Photo 8 shows a mixture of chondrules and CAI material; while Photo 9 focuses specifically on CAI material.