Friday, January 16, 2009

Friday Fragment: Graphic Granite

I am very fond of the term "graphic granite;" it seems to carry the resonance of "graphic violence," or "graphic sex." So is graphic granite extreme or uncensored? Yes and no, sort of. Granite is the endpoint of segregation of the lowest melting temperature components of igneous rocks, so it's kind of extreme to begin with (actually, aplites could be considered even more of an endpoint here, but anyway...). Could it get more extreme? Yes. Pegmatites are the result of very slow crystallization, and thus have very large crystals. Now granite is slow cooling to begin with, and has by definition, large (visible to the naked eye) crystals. Granite pegmatites are not terribly uncommon, and they can have enormous crystals.

The two main minerals here are quartz and orthoclase feldspar. Biotite occurs as an accesory in this sample (we defined accesory minerals as those making up less than 2% of the rock). A blade of biotite can be seen near the middle of the above photo. Quartz is the darker grayish glassy mineral, while orthoclase is the creamy gray mineral that makes up most of the rock. (I think I remember ID'ing sphene or some such from this outcrop at some point, but I didn't see any in my scan over this sample)The reddish stains are probably from weathering of the biotite. So graphic granite is a pegmatitic granite with a very distinctive texture. The quartz and orthoclase crystallize out in a series of (more or less) parallel lathes. Looking perpendicular to the axis of the lathes, there is a peculiar columnar texture: And a close-up (a crop from the above smallified image):
Looking down the axis of the lathes, on the other hand, the quartz looks like small cuniform marks on the rock- hence, like writing: graphic.
1610, "traced" (implied in graphical), from L. graphicus "picturesque," from Gk. graphikos "of or for writing, belonging to drawing, picturesque," from graphe "writing, drawing," from graphein "write," originally "to scratch" on clay tablets with a stylus. (From here)

As an aside, "graphite" is also from the same root. In the above picture you can see the graphic texture- note fingers for scale. The texture is even more clear in the picture below.
In the below crop from the full-size picture, you can see the cuniform-like patterns that the quartz crystals form. You can also see clearly that the orthoclase is the major component of this rock. I will try again when we get some sunshine; I couldn't really capture it with the flash. If you look back at the close-up picture of the quartz-orthoclase lathes, you can see that one of the quartz lathes branches from the lower right to the upper left. So in the upper left, even though the two areas of quartz are separated, crystallographically they are continuous! That is, it's all the same mineral grain. You can really see this with the orthoclase: as you tilt it back and forth, all of the cleavage surfaces of that mineral glint at the same time.

About 90% of this rock is made of two enormous, intertwined and intergrown mineral grains!

So, yes, this is a pretty extreme rock... a "graphic" granite indeed.

This was collected just south of Denio Junction, Nevada. An oblique view to the southeast in Google Earth shows the hillside; the road is route 140. This is about 4.7 miles south of the junction. (note the yellow pin marking the general area. Pin location in GE is 41.875615° -118.597880°. Actually the outcrop is fairly extensive, and this is unnecessarily precise.)And in map view, Denio Junction is at the 3-way intersection to the west of the irrigation circles.
Very nice pair of photos here, including one of a graphic granite polished as a cabochon. This site also reminded me of the term perthitic, which I was trying to recall last night, but the texture didn't show up well enough in the photos to mention. Wonkish piece on experiments regarding the crystallization of this rock here.

So there's my first "Friday Fragment," which I hope is a fairly regular feature highlighting bits and pieces of our wonderful planet's corpus.


Silver Fox said...

I like it (friday fragment)! I've probably driven by this outcrop at least a few times; never knew about it.

Silver Fox said...

Now I've driven by it for sure, last week, and didn't stop. :(