Tuesday, May 22, 2018

Orbicular Granite

I've been meaning to get to this post for sometime, seeing as how the trip was made in the summer of 2017, and the post related to this one, set earlier in the same day and not all that far back up the road, posted in September of last year. What we'll see here is some unexpected orbicular granite, found by MOH.
In this post, you'll see several pictures of orbicular granite, which in this case is composed of orbicules with dark centers and lighter outer rims.
I didn't collect a single specimen, partly because the outcrop didn't lend itself to being hammered on, partly because it seemed like the single outcrop should be left alone.

The inner dark center of most orbicules consists of black, interlocking and sometimes radial crystals of probable amphibole. It's fairly common that the next and only other layer is the outer rim of nearly white feldspar (not sure what kind), but in some cases (above and below) the second ring is composed of a white and black ring of intergrown feldspar and mafic mineral (pyroxene or amphibole are most likely), with white feldspar forming a third ring outside the second ring. The outer light-colored ring sometimes shows hints of actually consisting of as many as four or more separate feldspar-dominated rings or layers.
I'm not really up on the formation of orbicules, so if anyone with more knowledge than I wants to jump into the comments and explain, that would be great.
The discoverer of this small exposure provides a bit of scale for the orbicules, including a few protruding like eggs from the rock mass. It looks like many of the small spaces between the larger, often touching orbicules contain smaller orbs.
This particular orbicule appears to have at least three identifiable outer feldspar shells, set apart by somewhat vague mafic rings.
Bonus grebe.

I'm not giving details of the location of this tiny outcrop in order to preserve it. I didn't find anything about this locality by Googling, so this might be a new discovery.

4 comments:

rich said...

Very nice, never seen anything like it. Look forward to someone providing more insight.

on-the-rocks said...

Is there ANY possibility of it being a glacial erratic? Or perhaps a xenolith freed from its former granitic enclosure?

(It is a cool rock.)

Vinay Krishnan said...

Pretty spectacular rock..read about orbicular texture back in college...would love to come across same in field

Silver Fox said...

I think there is only a slight chance of it being an erratic -- I think the area is unglaciated. It is in a granitic area; I suspect it is just an uncommon formation.