NWA 6704 – A New Ungrouped Achondrite

Greg Hupé tells about his new ungrouped achondrite in this issue of Meteorite-Times so this is a good time to show close up pictures of it. Dr. Tony Irving did the classification and we learned about this stone from his Meteoritical Bulletin writeup, an abstract and phone conversations. NWA 6704 and, evidently, at least one pairing are from a previously unknown parent body. Oxygen isotope values place it well away from the howardite – eucrite – diogenites and with the acapulcoite – lodranites. Mineral compositions distinguish it from this latter group.

It is a cumulate, the product of a magma in which solid mineral grains have settled in the melt. Here the grains are relatively large crystals of pyroxene (specifically orthopyroxene) which enclose small crystals of olivine, chromite and metal. These are surrounded by a plagioclase groundmass (specifically albite, the sodium end member of the plagioclase solid solution).

The beauty of this meteorite to the thin section hobbyist is in the bold mineral grains that show few signs of recent shock. Abundant trails of bubbles add interest. These appear to be the remnants of partially healed fractures. With a little luck the collector will find large bubbles and small, spherical and elongated, in simple trails and in undulating curtains. With the correct lighting they can appear quite intriguing.

The cut face of this sample is 36 mm across.

 

The large grains are orthopyroxene. They surround smaller grains of olivine and opaque minerals. The large dark area at the top is an orthopyroxene grain in optical extinction. Field of view is 8.1 mm wide.

 

This is a closer view of the same area. The white is albite, a plagioclase, which surrounds the large orthopyroxene grains. FOV is 3 mm wide.

 

Transmitted white light shows a few opaque minerals.

 

This view is in reflected light. The bright white mineral is awaruite, a nickel – iron alloy. Here the nickel content is very high at about 75%. The grey blebs are chromite. FOV is 2 mm wide.

 

The large tan orthopyroxene grain is crossed by numerous wavy ribbons of bubbles. FOV is 3 mm wide.

 

A closer view of trails of bubbles in transmitted light. FOV is 0.3 mm wide.

 

The same view in a different light.

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About the Author

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