A Glassy Chondrule in NWA 2833

Northwest Africa 2833 was a single 2,112 gram stone purchased in Erfoud, Morocco in 2005. It was classified as an L3.8 ordinary chondrite, S2, W3 by Wittke and Bunch and published in 2006.

In the hand a slice is busy with crowded chondrules, some of them large. This slice is about two inches across near the top.

The thin section used here.

Thin section in transmitted light.

Thin section in cross-polarized light, XPL

A barred olivine chondrule near the center of the section in XPL, see above.

Another, just left of top center.

This chondrule appears to be built around a relict grain (blue).

The star of our show is this partially glassy, one millimeter diameter chondrule.

The chondrule cross section is outlined by fine, colorful olivine bars. Here, if we didn’t know better, we might think that the dark interior portions are minerals in optical extinction or an opaque material.

When animated, this GIF steps through views of our field with polarizing filters in four different positions. Other minerals go in and out of extinction but the chondrule’s dark interior area remains black.

In plane polarized light, PPL, we see light through the chondrule so it is not opaque, though the surrounding matrix is opaque. Therefore the mesostasis that remained dark in XPL is glassy.

Chondrule with bars, glassy mesostasis and quench crystallites in PPL.

Chondrule with bars, glassy mesostasis and quench crystallites in XPL.

Here the crystallites look like frost on a window but we should remember that they formed in a three dimension volume through which this is a thin slice.

Their crystalline nature shines in PPL.

John Kashuba

John is a natural history enthusiast living in Oregon.