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Granitic gneiss, Gneissic granite, Granite

Original Post
Bryce Adamson · · Unknown Hometown · Joined Apr 2015 · Points: 656

What are the real distinctions between the three? Are there clear and obvious distinctions at the intersections, or is it more of a "grey" matter.

And what is banded granite? Is this something different or another name synonymous with one of the above?

I can tell the WEML at Farley from North Conway, but where the differences aren't so clear, they are mostly lost on me.

Justin Barrett · · Russellville, AR · Joined Mar 2016 · Points: 85

Well, granite is an igneous rock with a phaneritic texture, meaning visible crystals. Gneiss on the other hand are metamorphic rocks that are banded; formed in layers caused by heat and pressure. That's the main, big difference between the two. Granitic gneiss is gneiss with a similar composition of granite (mainly quartz, feldspar, and biotite). It can have bands, but it is not truly a granitic rock, as it was not formed in the same manner. Gneissic granite is the same thing as granitic gneiss, just you swapped the words around.

The term banded refers to sedimentary or metamorphic rocks, where there's layers of different rock compositions. Due to how granite forms, there won't be layers, or bands.

I've never heard of a banded granite. Not technical terms anyways. My guess is that someone is referring to a gneiss. Some rock types are also mistakenly identified as granite too, like quartz syenite.

jaypg · · New England · Joined Aug 2009 · Points: 10

TIL Reddit post for sure.

Guideline #1: Don't be a jerk.

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