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Black thin layer help id
  
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Robson Vieira




Joined: 05 Jan 2017
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Location: São Paulo

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PostPosted: Dec 23, 2019 10:33    Post subject: Black thin layer help id  

Hi everyone. I have found a little piece of basalt and it has a thin layer of a shiny black which I would like to know what it is. Follow the features of the LAYER only
Color: dark brown to black
Luster: submetallic
Streak: ochre to black
Hardness: 3 to 3,5 scratched by a fluorite chunk
Cleavage: none
The stuff is brittle a little bit.
Would it be just a chemical or organic deposition material?

The piece was found in a basaltic region.

Thanks



20191223_121948.jpg
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vic rzonca




Joined: 18 Nov 2008
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PostPosted: Dec 23, 2019 11:15    Post subject: Re: Black thin layer help id  

I would say it what's called a slickenside. It's a smoothly polished surface caused by frictional movement between rocks along the two sides of a fault. I collect in basalt's and have seen very large areas covered with this formation along fault zones.
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Robson Vieira




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PostPosted: Dec 23, 2019 12:15    Post subject: Re: Black thin layer help id  

Thanks vic. Its very cool fact. But how about the low hardness? I guess the material of the slickenside should be the same of the rock, wich is basalt in this case, that has high hardness. Maybe, the friction caused the decreasing hardness of the layer, i dont know?!
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vic rzonca




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PostPosted: Dec 23, 2019 12:36    Post subject: Re: Black thin layer help id  

I don't know all the metamorphic products that would be formed in the heat and pressure involved in that friction zone, but, some one of the learned folks here might have an idea. It could be a serpentine, hardness 3-6.
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David Mustart




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PostPosted: Dec 23, 2019 14:36    Post subject: Re: Black thin layer help id  

How about a pic of the opposite side of specimen?
If it is reddish brown, then possibly the shiny surface is desert varnish, a result of manganese deposition on exposed surface.

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David Mustart
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Robson Vieira




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PostPosted: Dec 23, 2019 19:13    Post subject: Re: Black thin layer help id  

A genius idea David Mustart, thanks. Exactly as you had stated. The sample is brown at the opposite side. Would you say it solved the question? How can I detect the Mn presence to elucidate the ID?

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