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Unitedstatesofamerica1

What kind of rock is this?

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 CAn you scratch it with a pin?  What color mark does it leave when scratched on the bottom of  a porcelain toilet tank lid?

Will it light on fire with a torch? COal?

WHere found( for geologic info)?

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13 hours ago, homefire said:

 It does look "platey"

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We have minor pockets of that here in the Florida Mtns just out of town.  Hit a small bit of it with a Propane torch and it will do the pop corn thing.  Eye Protection Required.

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Looks like massive Biotite to me.  The stated S.G. also match Biotite.  Will it scratch your fingernail or a 1970s penny?  If it peels off in flat sheets that are brittle and has a white streak, then mostly likely Biotite.

Vermiculite will have a "pearly" luster along the cleavage plans, the plats will have a "greasy" feel, yellow or brown streak and are pliable (flexes like plastic), not brittle like Biotite.

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Just to be sure, was the streak done on a "fresh" surface of the black colored mineral?  Will the black mineral scratch a piece of copper?  When you brake off a piece of the black mineral, do you get irregular, jagged edges or smooth edges with shapes that repeat?  Last, if you hit a piece of the black mineral with a hammer, does it shatter into small pieces or does it flatten out like a piece of copper wire hit with a hammer?

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@ d_day: Augite would be a good candidate but the hardness of this sample is to soft at somewhere between 3.5-4.5 for Augite.  I'm leaning toward some sort of copper mineral due to the softness and the green coloration on the sample.  Still the S.G. is low for a metal salt mineral.

@ poster: Can you give us a general area where you found the sample, I'm assuming its in eastern Pennsylvania.

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Here is a candidate mineral:  Alabandite a Manganese Sulfide, hardness of 3.5 to 4, black color, green streak, S.G. of 3.9-4.1, isometric crystal system which will give "pentagram" crystal faces.  Alabandite is a member of the Galena group.  What do the rest of you think?  A positive test for manganese & sulfide would clinch it.

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Yeah I missed where he mentioned it would scratch with a knife. That certainly rules out augite.

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