NWA 4657 CK4

I don’t think CK meteorites are much to look at except those rare slices with huge, gnarly CAI. You pretty much get dark chondrules in dark matrix. I have one fairly dense slice that someone polished which made features essentially disappear. Thankfully that wasn’t done to this slice.

Since this is a CK4 chondrules are easy to see scatted in the matrix. This view is an inch wide. NWA 4657.

We see a lot more in thin section under a ‘scope. This is an odd shot I made with reflected light plus some light coming through the section from below. It shows quite a mix of textures and materials in a portion of a large chondrule. This picture and the next show layering from multiple accretion and heating events.

Large grains are in the interior of the chondrule, around them (below) is a rim formed from melt. It is peppered with opaque blebs. Further out (yet lower) are a couple layers of fine mineral grains then a jacket of dust.

Evidence of turbulent times - fragments of broken chondrules.

In this portrait the BO chondrule at the right is missing its lower right side. The curved fragment in the picture above is a perfect match in size, shape and even orientation. But it’s hard to believe they are mates that got separated, stayed close during final parent body assembly and were sectioned perfectly in this slice. It is even harder to believe that they are not related.

This is the object in the left of the portrait above, a granular olivine chondrule. It contains blebs of opaque material and has a rim of coarse grains.

This large diffuse feature contains a couple round dark areas surrounded by bright grains.

It would be interesting to know what mechanisms were at work here.

The bars in this chondrule are the same color as the rim where they join it. That is, those bars and that portion of the rim are in optical continuity, the crystal structures are aligned. For example the bars that reach the rim from the noon position to one o’clock are light orange and so is the rim. To the right, the bars that touch the rim at two o’clock are green as is the rim. This continuity is common. The fun thing here is that this crystalline orientation extends beyond the igneous rim. We see that the mineral grains that were forming from the surrounding dust or matrix grew, “recrystallized”, with their crystal matrix in the same orientation as the nearby rim and the bars on the other side. This is seen best when the polarizing filters are rotated and, for example, the green bars, green rim and green speckles beyond go in and out of optical extinction together.

Speaking of rotating filters, this animated GIF steps through four pictures to emphasis the different crystal domains in this sharp chondrule through ninety degrees of rotation. The horizontal slide remained stationary and a horizontal polarizing filter above it and a horizontal polarizing filter below it were moved in unison. The filters began and remained with their direction of polarization in a crossed orientation to each other. Consider the bright, completely illuminated shot as zero degrees. The next picture was taken with the filters rotated (about a vertical axis) approximately 23 degrees. The next picture is after another 23 degrees. And the fourth after a total of about 68 degrees. At 90 degrees the view is the same as the first picture, which is displayed again.

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About the Author

John Kashuba
John is a natural history enthusiast living in Oregon.
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