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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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Could be augite.

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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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On 12/5/2017 at 10:39 AM, 4meter said:

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.

Is there anyway to test if it is alabandite?

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Not without using strong acids and having training in wet chemical analysis of minerals.  Best bet is to take it to an Assayest and have them assay the sample or if you have a near by, community friendly college/university, you could take it to the chemistry department and they might test it for manganese for you.

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