NWA 5742 Howardite

It’s worth a few minutes to peruse a howardite just for the textures – viewing it in hand or with a loupe or ‘scope like we do here. Howardites are the compacted soil of the asteroid Vesta. They are a mixture of fragmented rock that has been excavated – from both shallow and deep – mixed and lithified by impacts. Breccias. Some are highly shocked overall. Those might take a good polish but for our purposes can look dark and unappealing up close. This one has a clean appearance.

The thin section was made from this two and a half gram part slice of NWA 5742. Pretty fresh material.

With light behind the thin section we see mineral fragments in generally light colors and some darker clasts. The clast sticking out of the curved edge at ten o’clock was cut from the bump on the edge of the slice seen in the picture above. The thin section is from the middle of the slice. The slice was three millimeters thick.

Up closer we see that most of the mineral fragments are angular.

This field is three millimeters wide. We see smaller and smaller fragments. Ultimately they are so fine that they don’t let light through and appear as dark matrix. Now, let’s look at some clasts.

This clast shows in the overall TS picture as a dark oval near the center of the bottom third. Here we see it in reflected and transmitted light. It is mostly needles and well shocked. Each frame is 1.5 mm wide.

This is the 10 o’clock bump. Obviously a different lithology from its surroundings. Reflected light on left and combined reflected and crossed polars on the right. Each frame is 2.2 mm wide.

This 3 mm wide field contains several clasts set in the “normal” groundmass. Reflected light. Below is the same field in combined reflected light and transmitted cross polarized light.

Combined reflected light and transmitted crossed polar light view of the same 3 mm wide field shown above.

These two bits of metal (one above the other) are brilliantly shiny in other light. Here we see they’ve stained a little of their surroundings. They are about 0.2 mm across. They are the only metal I saw on this slide.

And last, a pretty picture with crossed polars. This mineral fragment is about 0.4 mm wide.


About the Author

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