What made this volcanic rock look like this?

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Offline VistaHunter

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What made this volcanic rock look like this?
« on: 14/08/2012 01:15:19 »
Hello,

I have an interesting rock that I would like some help in identifying. I found this rock on my friends property up by Mt. St. Helens in Washington State. For a long time I actually thought this rock was a meteorite because it appears oriented, is weakly magnetic, and has very thin black coatings in areas that I thought may have been remnant fusion crust.

I know now that it is not a meteorite because I sent a sample in to New England Meteoritical Services for petrographic analysis. They informed me that it is a volcaniclastic rock composed of chalcedony.

The rock fascinates me very much and I would like to learn more about it.

How did it form?

Clearly it came from the bowels of Mt. St. Helens right?

Was it once a giant molten geode?

Did brecciation occur on impact?

Any insight would be much appreciated!!
« Last Edit: 14/08/2012 09:24:22 by chris »

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Offline VistaHunter

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Re: Volcaniclastic Brecciation
« Reply #1 on: 14/08/2012 01:16:31 »
Here is a slab photo.

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Offline VistaHunter

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Re: Volcaniclastic Brecciation
« Reply #2 on: 14/08/2012 01:17:41 »
What are the black inclusions within the matrix that I can scrap out with my fingernail? Carbonates?
« Last Edit: 14/08/2012 01:19:21 by VistaHunter »

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Offline VistaHunter

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Re: Volcaniclastic Brecciation
« Reply #3 on: 14/08/2012 01:18:59 »
What is the brown substance throughout the matrix that gives the agate a yellow golden hue? Sulfides?
« Last Edit: 17/08/2012 06:16:19 by VistaHunter »

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Offline VistaHunter

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Re: Volcaniclastic Brecciation
« Reply #4 on: 14/08/2012 01:21:13 »
One Crystal appears to have formed wavy bands or perhaps was melted and warped. Why?

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Offline VistaHunter

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Re: Volcaniclastic Brecciation
« Reply #5 on: 14/08/2012 01:21:59 »
Another crystal has formed or was recrystallized right in the middle of dense agate. Why?

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Offline VistaHunter

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Re: Volcaniclastic Brecciation
« Reply #6 on: 14/08/2012 01:23:15 »
Closeup of breccia.

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Offline VistaHunter

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Re: Volcaniclastic Brecciation
« Reply #7 on: 14/08/2012 01:23:38 »
More Slabs.

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Offline Bass

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Re: What made this volcanic rock look like this?
« Reply #8 on: 17/08/2012 04:24:38 »
What you found is not uncommon in near volcanoes.  Your rock is probably a lava flow breccia that had its voids filled with chalcedonic silica.  The banded "wavey" agate is typical, as the hot fluids circulated through the breccia and precipitated silica and/or replaced precipitated calcite.
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Offline VistaHunter

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Re: What made this volcanic rock look like this?
« Reply #9 on: 17/08/2012 06:15:46 »
Thanks Bass.

Can anyone tell me what the soft black inclusions within the matrix might be?

What might the brown substance be that leached into the agate and quartz?