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Where it was Found: I found the rock laying on the sidewalk near some landscaping at my apartment complex in Raleigh, NC.

Size: The largest intact portion of the rock is about 2.2 cm in diameter.

Description: It's a somewhat heterogeneous black/grey/white rock with clear/white crystalline structures and blacker mica-like cleavage layers. The rock is very brittle (you can see "crumbs" in the photos generated from simply setting the rock down and rotating it to take photos). However, given the brittleness, I'd say it's actually more dense/heavy than I would expect based on its size.

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The next photo shows "crumbs" demonstrating the two major rock-types. This photo suggests that what I had initially described as mica-like flakes, might in fact be thicker structures, but it's hard to tell.

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Here are some additional photos. (note that any yellow color in the pictures is from a nearby light, and the rock itself has no yellow hues):

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I apologize that I don't have better pictures as my hand lens is damaged and I do not have a good camera or scope to use currently.

This and this are the closest I can find to resembling my rock, but I don't think either is right. But based on these photos, my best guess is some kind of mica, pyrite, or scheelite mixed with quarts?? I'm an ecologist, so I have a limited knowledge of geology at this scale.

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  • $\begingroup$ I've asked a geologist friend of mine to ID it, but he hasn't come up with an answer yet. I just found out about USGS's Ask a Geologist, so maybe I will send a message there as well. $\endgroup$ – theforestecologist Feb 18 '17 at 17:49
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    $\begingroup$ What an extremely well-written question (and what a nice sample)! $\endgroup$ – Jan Doggen Feb 18 '17 at 18:39
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    $\begingroup$ Are you sure those cubic clear crystals aren't halite? Will one dissolve in water? $\endgroup$ – Spencer Feb 18 '17 at 20:57
  • $\begingroup$ @Spencer i just don't know rocks well enough to know either way about the clear crystals. I'll try dissolving some in h20 when i get back tonight $\endgroup$ – theforestecologist Feb 18 '17 at 22:52
  • $\begingroup$ Probably not halite. Halite and micas form in completely different environments. Can you scratch the clear crystals with a knife? $\endgroup$ – Gimelist Feb 18 '17 at 23:26
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My best guess is your rock is a granitic pegmatite.
Your rock is composed:

  • Quartz,
  • Biotite mica
  • probably hornblende (brown colored) not scheelite.
    (it is impossible to say without examining the rock in more detail).

Although scheelite can be found in the granites of North Carolina, it is relatively rare. Scheelite is usually very fluorescent (glowing bright blue under short wave UV light).

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