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Identity HelpFossil? Mineral? Or Something Else?

6th Dec 2019 20:49 UTCMatthew Droppleman

00320870015756651395482.jpg
I have no idea what this is I found it at Lake Texoma.... I have a couple of other specimens if others are needed. The mineral/fossil in question is the brownish stuff. The specimen itself is fist-sized. The specimen shows a slight glassy look when turned in the light. Checked on another fossil-identification website and got nothing. So, is it a mineral? 

6th Dec 2019 21:31 UTCKevin Conroy Manager

It looks like a cross-section of a gastropod.

6th Dec 2019 21:44 UTCMatthew Droppleman

Cool

6th Dec 2019 22:57 UTCEd Clopton Expert

Is the brown material deposited on the surface of the matrix, or does it extend below the surface?  If the latter, Kevin's fossil cross-section is a good possibility.  If the former, it could be a calcite deposit that formed in a narrow crack.  The rock looks like limestone--is it?  And where is Lake Texoma?  The Mindat forum is read by people worldwide.

7th Dec 2019 01:44 UTCRuggy Holloway

Typical of material from this area.
Lake Texoma is on the Texas Oklahoma border.

Plenty of fossil material in North texas

7th Dec 2019 02:49 UTCDoug Daniels

Well, on the right side of the photo the brown material is clearly in the form of a "G', so must be a gastropod.  OK, maybe not, but most likely are cross-sections of fossils.  Does the matrix react with acid (indicating limestone)?

7th Dec 2019 19:25 UTCGeorg Graf

Hi, Matthew Droppelman,

your specimen looks for me like a fossil in limestone. The fossil consists of Calcite. To proof this, you can use drops of vinegear concentrate: Both Calcite and limestone = rock, constisting manly of very fine grained Calcite, with maybe some clay minerals, grains of Quartz and others, react with vinegear concentrate, generating gas bubbles. Calcite has Mohs hardness 3, can be easily scratched with a knife. - A further proposal: Enlighten it with UV light. Maybe you will see nice fluorescence colours.

Kind regards, Georg

7th Dec 2019 19:40 UTCBob Harman

07785070015757475716136.jpg
For reference, I have added a photo of silicified fossil gastropods (geodized fossils) from Indiana. They are mid Mississippian in age and are both solid or hollow, like typical geodes.     Interestingly, fossil shells, crinoids,  and brachiopods abound in surrounding Indiana limestone, but I only have 1 very small gastropod in limestone.   I have never figured out why this is......       CHEERS.....BOB

7th Dec 2019 21:07 UTCGregg Little

Bob;
The lack of gastropods may be due to the depositional environment, competition, preservation, etc.  When drilling carbonates the samples will often show a low range of varieties, or you might think of it as certain groups dominating a particular environment.  A lateral move into a different environment would likely show a different assemblage of fossils.  In my experience gastropods are often in lower energy depositional environments ("dirty" limestone) whereas many others prefer the more energetic environments like reefs and other types of bioherms.
 
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