On Wednesday I had my first experience cutting a rock to get a better interior view. On the outer rim of the George Mason University campus in a shed I spent the afternoon with Nik and Kristen, friends and fellow GMU Geology students. It takes about an hour to cut through a rock sample, so I had to carefully pick which particular one I wanted to cut the most. I chose a rock that had been collected out near the Blue Ridge Mountain that I initially thought was a piece of the Virginia basement complex. My assumption was that a clean view, free of weather rinds, would show off the gneissic foliation of pyroxene layers, maybe some folding, and possibly a garnet or two. I think years of looking at great hand samples in the lab have spoiled me, though, and my expectations exceeded the results. What I got was a better view of the foliated layers, and a lesson in tempering my expectations.
Nonetheless, I still find this to be an interesting rock and still stick with my original hypothesis that it is part of the Blue Ridge Basement Complex. According to the USGS website this formation I am referring to is a:
“Middle Proterozoic (Early or Pre-Grenville-Age) Gneiss. Layered pyroxene granulite. Medium- to dark greenish-gray, fine- to medium-grained, segregation-layered quartzofeldspathic granulite. Major minerals are quartz, plagioclase, potassium feldspar (includes assemblages with one alkali feldspar), orthopyroxene and clinopyroxene, and magnetite-ilmenite; garnet, hornblende, and reddish-brown biotite are widespread minor constituents.”
I know that is a mouthful so let’s break it down a little bit. The effects of the Grenville Orogeny on the Appalachians begin ~1.1 Ga meaning this rock is over 1 billion years old. The rock itself exhibits layers of greenish-gray, medium grained pyroxene minerals, but also includes minerals such as quartz, plagioclase, potassium feldspar (orthoclase) and a smattering of other “minor constituents”.
Looking at the picture above you can generally tell the different mineral types by color (since I have yet to figure out how to test hardness through a computer screen). The pink minerals are the orthoclase, the colorless minerals are the quartz; and the dark greenish-gray minerals are the pyroxene, though I do not know which type of pyroxene they are. Based on locality I would lean towards calling them enstatite, but I think some thin section work would give a more definitive answer.
What do you guys think? Any ideas or suggestions?