Begaa NWA 4910 LL3.1: Now This is a Chondrite!

For a long time Begaa had no official name and several stated classifications. Now Begaa is a synonym for NWA 4910 LL3.1. Whether this change is welcome, a comedown or something else is a matter of opinion. But we can agree that it is one fine meteorite. It’s a rock you can show any novice and say “THIS is a chondrite!”

Begaa Meteorite NWA 4910 LL3.1 Thin Section Photos

This cut shows a distinct internal orientation. Slice. Begaa NWA 4910 LL3.1

 

Begaa Meteorite NWA 4910 LL3.1 Thin Section Photos

Okazaki and Nakamura (2005) tell us that in addition to chondrules, matrix and CAI, chondrites contain, isolated mineral grains, metal sulfide lumps, igneous rock fragments from impact melt and metamorphosed chondrules and chondritic clasts. Begaa likely contains all of these. Thin section in transmitted white light. Begaa NWA 4910 LL3.1

 

Begaa Meteorite NWA 4910 LL3.1 Thin Section Photos

Radial pyroxene chondrule. The two dark bands radiating from the right side are laths of pyroxene in optical extinction. These dark bands would move to other locations, though still radiating from the same origin, if the polarizing filters were rotated. Thin section in cross polarized light. Begaa NWA 4910 LL3.1

 

Begaa Meteorite NWA 4910 LL3.1 Thin Section Photos

A radial pyroxene chondrule with numerous domains. In some, the laths are too fine to resolve with an optical microscope so they are called cryptocrystalline. Rotating the polarizing filters would cause “shadows” to move within these cryptocrystalline zones – revealing their similarity to the better defined RP sectors. Thin section in cross polarized light. Begaa NWA 4910 LL3.1

 

Begaa Meteorite NWA 4910 LL3.1 Thin Section Photos

A portrait of contrasting textures. On the left is a porphyritic olivine-pyroxene chondrule. The individual internal crystals show natural crystal faces. That is, the phenocrysts are euhedral. On the right is a granular olivine chondrule. Thin section in cross polarized light. Begaa NWA 4910 LL3.1

 

Begaa Meteorite NWA 4910 LL3.1 Thin Section Photos

A barred olivine chondrule with a thin rim (and some additional growth beyond it). There are several sets of bars, each in optical continuity with the adjacent rim. Thin section in cross polarized light. Begaa NWA 4910 LL3.1

 

Begaa Meteorite NWA 4910 LL3.1 Thin Section Photos

This is an enveloping compound chondrule since one chondrule completely encloses another. In this case, the enclosed chondrule is the bold blue and red barred olivine chondrule. Thin section in cross polarized light. Begaa NWA 4910 LL3.1

 

Begaa Meteorite NWA 4910 LL3.1 Thin Section Photos

This is the same enveloping compound chondrule as above. The polarizing filters have been rotated to where the enclosed BO chondrule is in optical extinction, highlighting its independent origin. Thin section in cross polarized light. Begaa NWA 4910 LL3.1

 

Begaa Meteorite NWA 4910 LL3.1 Thin Section Photos

A chondrule surrounded by metal. It is “armored”. Thin section in incident light. Begaa NWA 4910 LL3.1

 

Begaa Meteorite NWA 4910 LL3.1 Thin Section Photos

This roughly spherical, high-metal feature appears to have accreted in layers. Thin section in incident light. Begaa NWA 4910 LL3.1

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About the Author

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