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. Sometimes very high shock will go beyond fracturing and cleaving minerals to producing planer deformation features (PDFs). These are extremely fine, very closely spaced glass-like layers in mineral grains. In nature they are rare outside of quartz and feldspar but they have been described in this and other meteorites. We believe we have found several examples. 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.

 

 

This set of PDFs is in a small wedge shaped grain at the arrow. The field of view of the first photo is 3mm wide. The scale bar in the third photo shows that the PDF lamellae are indeed fine and closely spaced. PDFs align within minerals along distinct crystallographic planes.

 

 

 

Similarly, this set of PDFs is in an inconspicuous grain near the edge of the sample. The field of view of the first photo is 2.2mm wide. These PDFs photographed better, perhaps because the planes of the PDFs might be perpendicular to the face of the thin section so we are viewing the PDF cleanly edge-on.

 

 

About the Author

John is a natural history enthusiast living in Oregon.
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