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Chunk of pegmatite?
https://gemologyonline.com/Forum/phpBB2/viewtopic.php?f=28&t=22919
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Author:  Lefty [ Sat Jul 23, 2016 2:27 pm ]
Post subject:  Chunk of pegmatite?

For my sins, asking for ID from a photograph :)

Image

Image

Found it in a creek that flows beside an old abandoned gold mining settlement. There is still gold in the ground but it became uneconomical to extract using 19th century methods. A number of the miners were also poisoned by arsenopyrite I believe.

The creek flows down out of a granite mountain range directly behind the old mining town site. Most of the stones in the creek appeared to be typical mottled granite but this piece caught my eye with it's very much larger crystals - the big white rectangular crystals are a feldspar I guess? Can't see in the photo but there are also fairly large shiny black crystals embedded in it.

Might be worth a wander up the creek sometime.

Author:  abeck [ Thu Jul 28, 2016 8:56 pm ]
Post subject:  Re: Chunk of pegmatite?

looks like (feldspar?) crystals in a groundmass of andesite(?). Definitely not pegmatite, but you might find a dike or sill of this material around the area.

Author:  Lefty [ Thu Jul 28, 2016 10:24 pm ]
Post subject:  Re: Chunk of pegmatite?

Thanks abeck. I still get confused as to exactly what constitutes a pegmatite when I look at a rock - intrusive igneous rock made up of crystals above a certain size, primarily quartz, mica and feldspar but occasionally containing some more interesting crystals was one such definition I heard.

You can't really see it in the photo but it contains a lot of shiny black crystals, schorl tourmaline I guess.

Author:  Stephen Challener [ Thu Jul 28, 2016 10:49 pm ]
Post subject:  Re: Chunk of pegmatite?

I've held back on commenting because like abeck said it looks a lot like a very coarse porphyry in the photo, but your description (shiny black crystals) would be a bit less typical of that, and it would have to be one hell of a coarse porphyry. It's certainly a bit of an oddball one way or another.
Tourmaline would definitely not be typical of a porphyry, so if you can get a close look at it and check for those triangular cross-sections that would be great. More likely I think you'll see either square (pyroxene) or diamond shaped (amphibole).
Edit: to clarify I mean volcanic porphyry here.

Author:  Icehut [ Wed Dec 28, 2016 8:18 pm ]
Post subject:  Re: Chunk of pegmatite?

Looks like feldspar porphyry for sure. Some of it is great for cutting. Look up flowerstone or Chinese writing stone. I collect it from the Fraser River in BC and the north shore of Lake Superior on Ontario, Canada.

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