29 March 2010

New metaconglomerate sample

Posted by Callan Bentley

Courtesy of my scanner, and my generous student Saadet M., whose family runs a building stone business here in northern Virginia… behold a lovely metaconglomerate:


This is a really cool rock. I would rather have this than diabase for my countertop.

It doesn’t show up well in this scan, but the rock has a moderately well developed foliation in the matrix: I’d describe it as “phyllitic verging on schisty.”

The next two images are zoomed in on certain portions of the rock, showing what I think it really cool about this sample: some of the pebbles have started to fuse with one another, with the grain boundaries of one impinging on the grain boundaries of another. Kind of reminds me of a certain batch of fusilinids. Check it out – Here’s a small quartz pebble getting squished between a bigger quartz pebble and a pebble of something gray.

Here’s another close-up, showing the pebble at left impinging on the pebble at right. Furthermore, this photo shows the “fused” texture that has developed in this metamorphic rock, as a fracture propagates from lower left to upper right, totally ignoring the “discontinuity” of the grain boundary:


Pressure solution is dissolving the pebble at right, and the quartz that leaves is either leaving the rock entirely (perhaps filling a vein somewhere), or precipitating in low-pressure areas (“next” to pebbles, perhaps, as pressure shadows).

Very cool. Thanks again, Saadet!

Other geobloggers: have your students or colleagues or second cousins or co-coffee-shop-denizens brought you anything cool like this lately?