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I have a piece of rock that I believe to be potassium feldspar but I'm not certain.

The rock leaves a white streak, can scratch glass and be scratched by a penknife and has a pearly luster on unbroken faces.

There are inclusions of quartz and what I assume to be mica through the rock. There is a browner material also spread through the specimen and on one side there is a black material, pyroxene maybe?

The piece was found in Co Tyrone, Northern Ireland.

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Yes, it is a K-feldspar, most likely microcline or less likely orthoclase.

Your rock is a pegmatite: a very coarse grained granite. These are usually attractive rocks with pretty and unusual minerals in them. For example your black mineral: I've see columbites that look like that. It's probably not a pyroxene.

The brown stuff is probably some sort of patina or brown clays sticking to your rock.

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