Friday, October 10, 2008

Weekend Fun No. 7

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After a long hiatus another implementation of the Weekend Fun puzzles! I've been searching through my pet rocks and I found this one. It is just right I think for asking: What is it and how does it form? I left away scale and other clues. The shown sample has been found in Germany by a friend of mine some years ago. Which isn't really any help because these can be found worldwide as far as I know. Have fun guessing! I'll solve this weekend fun puzzle sunday or monday depending on the number of comments unless someone get's it right first. Good luck!



6 comments :

Lockwood said...

Looks like a septarian nodule. Generally thought to be the result of dessication of a clay/mud lump or concretion, then infilling with (most often) calcite, sometimes quartz or other secondary mineral. Nice dendrites too! Look like pyrolusite.

Lukysh said...

Looks like a Jigsaw Breccia, in the right side looks a small and angular fragments and then may be indicate is product or some hydraulic or phreatic event. The cement looks siliceous, but I'm not sure. Also the dendrites, lockwood was right looks like pyrolusite.

A Life Long Scholar said...

breccia is the word which comes to my mind too. And in response to local stresses, since the angular fragments still line up with the parts on the other side of the breaks--no transport, just local brittle failure and re-cementation by whatever fluids were wending their way through the new cracks (and, quite probably, widening the cracks in the process).

In the absence of tests for hardness or being able to check for fizzing in response to acid, I'd go with the statically safest guess of silica rich fluids for the crack-filling cement. The cherty look of the angular fragments could be because it is chert, or could be a lime or mud stone that just looks cherty because of the cement not only being in the cracks.

Anonymous said...

I'm guessing hydrothermal breccia.

Silver Fox said...

I might have guessed hydrothermal breccia, also - because I'm not familiar with the septarian nodule. Also, it somehow didn't look quite right!

Lost Geologist said...

It may indeed look a little misleading but then again if it would be the perfect textbook example there would be no need to guess for you or stretch your imagination, no? :-)