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Author Topic: What is this rock?  (Read 2993 times)

Offline AJK30

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What is this rock?
« on: 23/02/2010 00:12:49 »
Hi there! I have a quick question as to the identity of this rock. No one will ever mistake me for a geologist, but it interests me, as do most things, so I may try to learn a bit more about it. Anyway, I often find interesting rocks, usually while fishing, and occasionally grab one and bring it home with me. The following stone was found on a small island of a creek in Eastern PA. I'm very interested in it's identity, how it was formed, ect.; so if there is anything you can tell me about it I would appreciate it! And don't be afraid to go into excruciating detail  ;D

newbielink:http://s59.photobucket.com/albums/g282/whiteshark_2006/?action=view&current=DSC01691.jpg [nonactive]

newbielink:http://s59.photobucket.com/albums/g282/whiteshark_2006/?action=view&current=DSC01690.jpg [nonactive]

newbielink:http://s59.photobucket.com/albums/g282/whiteshark_2006/?action=view&current=DSC01694.jpg [nonactive]

newbielink:http://s59.photobucket.com/albums/g282/whiteshark_2006/?action=view&current=DSC01693.jpg [nonactive]

newbielink:http://s59.photobucket.com/albums/g282/whiteshark_2006/?action=view&current=DSC01692.jpg [nonactive]

Thanks!

Aaron


 

Offline stereologist

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What is this rock?
« Reply #1 on: 23/02/2010 02:50:57 »
My guess is that you are looking at a shale with calcite streaks in it. The bulk of the stone could be like a shale that has cracked. The cracks have been filled in with calcite deposited by waters percolating through the fractured rock. The orange staining is possibly due to iron compounds.
 

Offline Bass

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What is this rock?
« Reply #2 on: 23/02/2010 06:34:33 »
Welcome to the forum AJK30.  We love interesting rocks- especially with great photos. 

A few questions:

How heavy is the rock?  More or less dense than your average garden stone?
Can you scratch the surface with a knife? 
Can you try a drop of acid to see if the rock fizzes?
If you rub the rock on your most expensive porcelain bone-white china dining plate, does it leave an orange or red streak (just kidding about using your best china, but if you have an old piece of white porcelain- no glaze- give it a try).
Any other information would be helpful...
 
The orangish-brown is definitely iron oxide (like rust).  There are some fine-grained, shiny, metallic looking spots in several of the photos- possibly specular hematite (iron crystals)?  Looks like conchoidal fracture in several spots (like broken glass), which makes me suspect flint or chert.  Appears to have faint stratigraphic layers in photos 2 and 5.  Photo 4 looks like load casts- imagine pushing down on wad of silly putty sitting on top of bowl of jello.  Possible chert nodule?   
 

Offline AJK30

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What is this rock?
« Reply #3 on: 23/02/2010 07:16:44 »
Welcome to the forum AJK30.  We love interesting rocks- especially with great photos. 

A few questions:

How heavy is the rock?  More or less dense than your average garden stone?
Can you scratch the surface with a knife? 
Can you try a drop of acid to see if the rock fizzes?
If you rub the rock on your most expensive porcelain bone-white china dining plate, does it leave an orange or red streak (just kidding about using your best china, but if you have an old piece of white porcelain- no glaze- give it a try).
Any other information would be helpful...
 
The orangish-brown is definitely iron oxide (like rust).  There are some fine-grained, shiny, metallic looking spots in several of the photos- possibly specular hematite (iron crystals)?  Looks like conchoidal fracture in several spots (like broken glass), which makes me suspect flint or chert.  Appears to have faint stratigraphic layers in photos 2 and 5.  Photo 4 looks like load casts- imagine pushing down on wad of silly putty sitting on top of bowl of jello.  Possible chert nodule?  

Thanks for the welcome and answers so far!

Some of the info I'll have to get back to you on tomorrow because it's too late and I need to sleep :). I did the knife test with a box cutter I had handy and it didn't seem to scratch the rock. In fact, it bent the blade a bit. It feels pretty heavy/dense. I guess probably a bit heavier than the "average stone", if there is such a thing. The outside is very smooth which I kind of assumed was an effect of the water, though it was on the shore. In pic one you can see some small holes and inside of them it looks like the rock is more coarse than on the outside.

I will try to answer the other questions tomorrow. I think I have some porcelain to try that test, but the only acid I think I have handy would be vinegar. Is that strong enough?

EDIT: It doesn't leave any streaking on the porcelain.

Thanks!
« Last Edit: 23/02/2010 18:57:27 by AJK30 »
 

Offline Bass

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What is this rock?
« Reply #4 on: 23/02/2010 19:01:47 »
Thanks for the welcome and answers so far!

Some of the info I'll have to get back to you on tomorrow because it's too late and I need to sleep :). I did the knife test with a box cutter I had handy and it didn't seem to scratch the rock. In fact, it bent the blade a bit. It feels pretty heavy/dense. I guess probably a bit heavier than the "average stone", if there is such a thing. The outside is very smooth which I kind of assumed was an effect of the water, though it was on the shore. In pic one you can see some small holes and inside of them it looks like the rock is more coarse than on the outside.

I will try to answer the other questions tomorrow. I think I have some porcelain to try that test, but the only acid I think I have handy would be vinegar. Is that strong enough?

Thanks!

If the rock is that hard, you don't need to worry about the acid test.  Vinegar will work on some carbonate rocks, most geologists use dilute hydrochloric acid.  With the hardness and conchoidal fracture, I'm sticking with chert (extremely fine grained silica).  The porcelain test may or may not leave a brown streak- but the amount of limonite on the rock indicates the presence of iron.
 

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What is this rock?
« Reply #4 on: 23/02/2010 19:01:47 »

 

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