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Author Topic: What has caused the cavities within these round stones found in Panama?  (Read 2693 times)

Offline mia531

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FOUND THESE round stones which are a milky lilac colour on a beach in Panama. Many of those I found had these geometric cube type hole where something had obviously once been. What are they made of and what would the missing cubes have been?
« Last Edit: 25/10/2012 09:38:44 by chris »


 

Offline Bill S

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A few clues might help here such as size, hardness, streak and reaction with acid.

If these objects were very common, it would be worth looking for a possible local origin for them.  If they are very hard they would have to have been in the sea for a long time to become so rounded; but if the material is soft they might not have been sloshing around for very long, and could be from a local outcrop.
 

Offline mia531

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Hi bill,

Thanks for responding.The nodules we found were all approx 5-6cm in diameter and they seem quite hard and glassy. They are really very lilac in colour and streak of similar slightly grey colour. The 'gaps' inside some of them I'm told are rhomboid shaped , not cubes. Could they be chalcedony do you think?
 

Offline Bass

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Chalcedony or jasper is likely, and can come in a variety of colors depending on impurities.  I agree that the interior voids are rhomboid, which would make them some sort of carbonate - most likely calcite - that has since dissolved leaving the void.
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Offline mia531

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Thank you so much! Is this kind of thing common to find on a beach?
 

Offline Bass

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not really common anywhere.  cool rocks!
 

Offline mia531

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wow!!! Yipee!!!! Thank you for enlightening me!
 

Offline Bill S

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Bass, picking up on some of your points, further questions arise.

Chalcedony or jasper is likely”.  Something very hard would be needed to reveal the streak.   Mia has checked the streak, so unless she used something with a hardness greater than 7, the “matrix” is not silica.

What sort of environment would be responsible for calcite crystals embedded in cryptocrystalline silica?

…most likely calcite - that has since dissolved leaving the void”.  What could permeate the silica so as to dissolve the calcite?  How/when could that happen?

The fact that each pebble seems to have a more-or-less central inclusion suggests that the matrix formed around the crystals.  Even if the matrix is calcareous, the mechanism could present a problem.

Unless the matrix is considerably softer than the inclusions, one would expect fracture to occur through the inclusions rather than always at a distance from them.
 

Offline Bass

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white streak- which may be the porcelain instead of the rock

Hydrothermal (epithermal).  Not uncommon for calcite to form first, then be replaced or enclosed with silica.  Open-space filling in epithermal fracture zones is common.
 

Offline mia531

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Thank you guys for responding.I'm a complete novice so it is very interesting...
 

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