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Please help identify. Looks like a geode not sure
  
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CompTrekkie




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PostPosted: Jan 29, 2018 10:53    Post subject: Please help identify. Looks like a geode not sure  

I am trying to identify the type of rock and what mineral I am looking at here. Thank you all.


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Ballast Point, Tampa Bay, Tampa, Hillsborough County, Florida, USA
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Bob Harman




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PostPosted: Jan 29, 2018 11:01    Post subject: Re: Please help identify. Looks like a geode not sure  

The background in your photo is quite distracting. Never-the-less, I think you have a piece of silicified limestone where most of the actual limestone has weathered away, leaving the resistant quartz areas lining some irregular cavities. Hard to see, but I think the cavities might be lined by small druzy quartz crystals. The whole appearance is like a cousin of the well known agatized Tampa Bay Coral. BOB
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CompTrekkie




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PostPosted: Jan 29, 2018 11:11    Post subject: Re: Please help identify. Looks like a geode not sure  

I am at work right now. I have already pressured washed it and it looks much better, I will get better pics when I get home tonight to help out. Thank you for the info.

The out side is covered in impressions of sea shells and coral.
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kushmeja




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PostPosted: Jan 29, 2018 11:13    Post subject: Re: Please help identify. Looks like a geode not sure  

Bob Harman wrote:
The background in your photo is quite distracting. Never-the-less, I think you have a piece of silicified limestone where most of the actual limestone has weathered away, leaving the resistant quartz areas lining some irregular cavities. Hard to see, but I think the cavities might be lined by small druzy quartz crystals. The whole appearance is like a cousin of the well known agatized Tampa Bay Coral. BOB


Agree with Bob. This looks very similar to the agatized corals from Tampa Bay and other areas of FL, but without the appearance of coral fossilization.
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Kevin Schofield




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PostPosted: Jan 29, 2018 11:56    Post subject: Re: Please help identify. Looks like a geode not sure  

Hi CT,
what you have here is a silica nodule, better known in my (original) part of the world as a flint nodule. As Bob has already said, it formed when the original carbonate rock was replaced by cryptocrystalline silica. The white "rind" around the edges, rather than being crystals growing into a geode, are remnants of the original limestone that are only partly silicified. Flints form in chemical microenvironments during relatively shallow burial of the original carbonate sediment (work in the chalk of the UK suggests the order of a few metres) where the Eh/pH conditions become suitable for the dissolution of the biologically-stabilized opaline silica in the skeletons of sponges and diatoms. This silica is itself not happy to be in solution at low temperature and pressure and generally redeposits locally as a silica gel that largely replaces the original carbonate (one presumes that it is rendered more soluble by the same conditions that favour the silica coming out of solution). With further burial, the silica gel dehydrates to become a flint.
Kevin
ps to the Forum regulars and purists, I apologize for posting before submitting an introduction. Will do soon, I promise!
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