NWA 2824 Diogenite

Ted Bunch and other researchers have described many shock induced features in this odd diogenite. We found some to photograph in a few thin sections, all viewed in cross-polarized light.

Brecciation is a common result of shock. The clasts in this polymict breccia have a variety of textures. Mechanical twinning, also called shock twinning, is apparent in numerous mineral grains. This diogenite is rich in plagioclase. The plagioclase, gray to white in cross-polarized light, melted under shock and recrystallized to striking spherulitic textures. Also, adjacent to these melts we found mineral grains with heat affected edges.

Clast textures vary widely. Some contain plagioclase. Some clasts are plagioclase.

 

 

 

The color banding in these mineral grains is indicative of shock twinning.

 

 

 

Recrystallized shock melted plagioclase is patterned with intersecting zones of radiating fibers. Portions of fibers show gray and black when they are near and in optical extinction when the thin section is viewed in cross-polarized light.

 

 

 

 

 

 

 

The most distinctly altered grain margins we found were on mineral grains well exposed to the plagioclase melt pockets.

 

 

Finally, two grains with closely spaced polysynthetic shock twinning.

 

 

 

 

 

About the Author

John is a natural history enthusiast living in Oregon.
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