Thread: Help identify this "rock"

Hi,

Many years ago I found a small strange piece of "rock". I have never seen anything like it, although I can't say I've been looking that much for any others.

Anyway, I am curious what it is.

It is dense and solid like a stone, it's mostly black and somewhat smooth, possibly a bit brownish black. It has some strange brown features under that dark cover, the outermost layer is brown and very smooth. Under the thin smooth brown layer is is still smooth and brown but a much lighter brown.

Here is a picture of it: http://img269.imageshack.us/g/cimg1256k.jpg/

I think it was found somewhere in Sweden, but I can't remember where. Any ideas? I have some thoughts of what it may be, but as I know very little of the subject I better save them for a few posts as to not influence you with my thoughts.

You can put the images into your post quite easily, I think that would help you get more replies.





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As for your question... it's always difficult to tell from photos, but...

Is the rock magnetic? Could be magnetite rich. The grey shell invokes that thought in me.

The light brown, smooth bits are reminiscent of chert, suggesting silica rich. The rock mattrix could be mainly quartz with other mineral inclusions.

My guess at a stretch: Very fine sandstone pebble weathered in a magnetite rich environment.

Oh, yes good forgot about img tags. Thanks for posting the images.

I tested with a pretty strong magnet now and felt nothing at all, so I'd say it's not magnetic. This could however be that the grey shell is very, very thin, too thin for me to even guess how thin it is.

Still I'd say your guess sounds reasonable, if nothing else it's a starting point when I try to find out more.

Magnetite was a VERY long shot, so not surprised at all that it is wrong.

Now I think about it, I'm not even sure it's possible for a magnetite shell to from like that by natural processes.

Far more likely is that there is some mineral in you rock sample that simply weathers to a grey colour by a chemical reaction. Now: what minerals weather to a grey colour?

The picture may give a false impression of shininess. I should have had better lightning and skipped the camera flash. It is quite a lot darker and mat than it seems.

If the stone is quite hard, I would say it's a quartzite. The "shell" you mention could be a weathering product, but it looks like an oxidation rind -- the interior color of the stone and that brown streak suggest iron content.

My suggestion: rub it on a piece of glass. If it scores the glass, it's likely quartzite.

While you're at it, find a bit of hydrochloric acid and drip on it. Tell us if it fizzes.

See also if you can scratch the darker portion with the blade of a knife. If you can, you might be able to scrap a bit off into a powdery pile and then put a drop of vineger on it if you don't have HCL for the test in the first line. If it fizzes either from the HCL or the vinegar, then at least a portion of it is carbonate rock. If it doesn't fizz, then you can at least rule out the various carbonate rocks.

The reason this might be important is that the outer, darker portion looks like a built up patina more than a weathered basalt or sandstone. The texture isn't uniform the way you might expect. I'm wondering if the rock was first affected by carbonate deposition and followed by weathering in a river (or just years of handling).