FMF - Friends of Minerals Forum, discussion and message board

[Robson Vieira]

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

[vic rzonca]

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.

[Robson Vieira]

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?!

[vic rzonca]

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.

[David Mustart]

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.

[Robson Vieira]

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?

Thanks