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Andrew Alden

What a Phenocryst

By January 4, 2013

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phenocrystOver in the Geology Forum, people are always bringing in photos of rocks. When one visitor showed us a nondescript boulder on a Cornwall beach, I commented instead on the granite stones around it, each one studded with large phenocrysts of alkali feldspar. "What do you think of this one?" was our visitor's comeback, and there it was, one of the biggest ones I've ever seen. I just had to make room for it in the phenocryst gallery. While prepping the image, I determined that this "granite" is more of a quartz monzonite, and a very nice one it is. (Here's how to judge among the granitoids.)
K-spar phenocryst in quartz monzonite — Forum visitor photo


January 4, 2013 at 4:31 pm
(1) Bruce Davidson says:

Just wanted to applaud your geology instruction, particularly the link on “What’s my countertop made of”!
This just goes to show that nearly every question one might have is addressed on the Internet, if you know the right link. Ha
Big box stores call everything granite, that’s bugged me since grad school. Again, very nice job. :-)

January 6, 2013 at 9:25 pm
(2) Karen says:

There’s a spot along CA hwy 88 east of Kirkwood, where Husband and I once stopped at one of those wide spots in the road they use to terminate snowplowing. One of our cats had gotten carsick, so we stopped to clean up the cat and the carrier and change the towel. But we took a quick look around, and there were HUGE K-spar phenocrysts in the exposed granite. Gorgeous. Damned if I could find the spot again, and I didn’t have my camera with me at the time.

January 7, 2013 at 12:23 am
(3) Karen says:

Oops, I talked about phenocrysts in granite, but it was more likely granodiorite.

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