New Raymer LL4 Opaques

New Raymer is a metamorphic type 4 so we wouldn’t expect to see dark matrix and sharply defined chondrules. Still, there are opaque areas that beg to be investigated.

Our thin section sample scanned with transmitted light. It is 28mm wide at the base. The character of this sample varies somewhat across its length. This thin section is one of over 100 that were deaccessioned by Arizona State University in 2014.

We’ll look closer at the area outlined in the lower right which contains one large and many small opaque spots.

The outlined area through a microscope in plane polarized transmitted light. The large dark bleb is visible, but details are difficult to discern.

In oblique incident light the large dark spot appears to be metal as do other areas surrounding opaques and mineral grains.

Still closer, we see that the brightly reflective material is both silver colored and bronze colored ̶ probably metal and troilite, respectively. This thin section has a cover slip which makes standard reflected light microscopy impossible. That is why we used oblique lighting here.

This tracery adorns irregularly shaped opaques at the right center of the outlined area.

In the transmitted light scans the left side of our sample appears lighter than the right side.

The outlined area through a microscope in plane polarized transmitted light. The area enclosed by the dark rim is obviously a porphyritic chondrule.

In oblique incident light we see that the chondrule is armored and that the wandering opaque line is surly composed of terrestrial weathering products including rust from meteoritic metal.

The armoring appears to be a fine mix of metal and sulfide. Note the crack traversing the lower right corner of the frame and attendant rust staining.

The same view as above in cross-polarized light. The thin section was not “double-polished” so mineral grains appear rough, that is, not glassy.

John Kashuba

John is a natural history enthusiast living in Oregon.