January 31, 2017 at 12:48 pm #18469
after having good luck in another ‘black’ shale area I decided to search for more. I noticed there were alot if gray and black portions in this shale but when I started looking closer it wasn’t the same. What is causing this shale to appear black on its surface and what are the white lines that seem to branch and spiderweb? On one rock I could scratch the black surface and it turned white like the lines. I though it might just be that the shale was wet but it did not dry or change color.
Attachments:You must be logged in to view attached files.January 31, 2017 at 12:53 pm #18474
Here are more pictures of the rocks and of the outcrop I took them from. The outcrop was not very uniform either.
Attachments:You must be logged in to view attached files.February 1, 2017 at 11:38 am #18483Eleanor GardnerModerator
Again, @jayson-kowinsky can likely provide more detail, but I’m thinking you’re looking at calcite (or quartz) veins within the shale. You can get dendrites (non-fossil crystal growths) of calcite and other minerals on parallel surfaces like shale layers or some fine-grained limestones. As for why the shale is black, it is likely due to high organic content when the layers were deposited.February 1, 2017 at 7:21 pm #18497Jayson KowinskyParticipant
You have come across some very oil rich shale. Unfortunately, I don’t see anything that looks fossil to me.
What I see is that oil rich shale has allot of mineral precipitation going on, causing the different patterns and colors, some of which you can rub off (thin layers of precipitation).
The white areas that seem to branch again look like mineral precipitation lines that follow weak spots and microscopic cracks in the shale. The oilyness (if that’s a word) probably helps spread it. This is similar to how dendritic mineral growth occurs. The dendritic growth looks like little trees and stuff forming on the rock. Yours is not quite the same, as it has more of the branching that you see. If you google “dendritic mineral growth” you can see how this type of precipitation occurs.
JaysonFebruary 1, 2017 at 7:24 pm #18498Jayson KowinskyParticipant
Woops! I just wrote that reply and just saw that Eleanor (@egardner) said basically the same thing! I think we’ve reached a consensus!February 2, 2017 at 7:23 am #18503
Thanks for the help!
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