Today’s slate deskcrop is a “roofing tile-quality” piece of maroon Metawee slate with an elliptical reduction spot. The reduction spots presumably originated when organic matter caused reduction of iron in a small spherical region of the shale protolith. When these slates were deformed and metamorphosed the reduction spots were also deformed, and as such, they can be used as strain markers. One of the NSF-REU funded undergrad summer research projects I was part of took measurements of the three principle axes of these reduction spot strain ellipsoids. I’d point to the abstract, but it seems GSA doesn’t have meeting abstracts from prior to 2001 online. D’oh!
Another thing that one can see nicely in this sample is the reflection of light off the cleavage surface. If you look closely in one of the higher resolution versions of this photo you might even to be able to detect a hint of a second (crenulation?) cleavage.
I have to admit that I’m not 100% sure which quarry this particular deskcrop comes from. I’ve elected to locate it at a quarry that exposes plenty of Metawee “tailings” – no doubt some of which display reduction spots with this texture.
Explore this and all of my 2010 Deskcrops and Outcrops in Google Earth!