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Can Fossils Create "geodes"?


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I've got a Madagascar ammonite with crystals... are these technically "geodes"?

Here is an example (not mine):

cleoniceras-cleon.jpg

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Ooooo, that's a nice one!

If "geodized" means that internal voids have crystals growing in them, then this would certainly qualify.

I've seen geodized brachiopods, crinoids, and other such too.

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Absolutley. Does the fossil 'create' the geode...no, ground water and minerals create the geode. Fossils provide the empty space for the geode to form in.

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Mediospirifer

Same here. I have a small but pretty geodized horn coral, and several others in which the coral is completely filled with crystals. I also recently purchased a cool geodized clam fossil in which the calcite crystals are beautifully fluorescent.

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How about an amazing geodized trilobite:
 
IMG1.jpg
 
Above: Discovery of this geodized Flexicalymene meeki (Foerste) came at home while I was cleaning it. I noticed a difference in the color of calcite along the axial furrow. Taking another specimen of equal size, I weighed them, and found this one to be lighter. Taking my scriber, I picked a small hole in the light area, only to confirm my suspicion that it was indeed hollow, and filled with calcite crystals (CaCO3). Other specimens I dissected of the same species, were found to contain calcite and aragonite crystals. This hollow cavity extends from the occipital ring, and follows the axial lobe to the pygidium following the same course as the digestive tract.
 
Johnson, Thomas T. (1985)
Trilobites of the Thomas T. Johnson collection: How to find, prepare, and photograph trilobites.
Litho-Print Inc. Publishing, Dayton, Ohio, 178 pp.
 
 
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Here's one more geodized trilobite from the same author. This one is enrolled and quite spectacular! :fistbump:

 

IMG1.jpg

 

Johnson, Thomas T. (2003)
Discovering the mysterious trilobites.
Geolinea, Torreano di Cividale, Italy, 175 pp.
 
 
 
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  • 3 weeks later...
Macrophyseter

fossils cannot form geodes. Geodes form when gas bubbles are trapped in lava, and minerals penetrate the bubbles, and the whole thing hardens. Fossils cant form geodes, if it came from lava, everything would be just a rock. But minerals can penetrate anything that can be permineralized, and that causes crystals to form if the subject is hollow.

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fossils cannot form geodes. Geodes form when gas bubbles are trapped in lava, and minerals penetrate the bubbles, and the whole thing hardens. Fossils cant form geodes, if it came from lava, everything would be just a rock. But minerals can penetrate anything that can be permineralized, and that causes crystals to form if the subject is hollow.

Sorry, but couldn’t agree with that. It’s completely untrue to suggest that a geode can only form in volcanic circumstances. Geodes can have either sedimentary or volcanic origin and are most commonly found in basaltic lavas and limestones.

They’re very common in the Lower Warsaw Formation where Missouri, Iowa, and Illinois join, in host rocks which are predominantly argillaceous dolomite and dolomitic mudstone. They’re also scattered across the sedimentary rocks of the Mid-West in general.

What characterises a “geode” is that it’s a secondary structure formed by one of two processes:

- the partial filling of vesicles in volcanic to sub-volcanic rocks by minerals deposited from hydrothermal fluids

or

- the dissolution of sedimentary nodules or concretions that were deposited syngenetically (ie at the same time) within the rock formations in which they are found, followed by partial filling with the same or other minerals precipitated from diagenetic water, groundwater or hydrothermal fluids.

Additionally, a geode is characterised by having an outer mineral layer (as distinct from a vug, which does not), that is usually more resistant to weathering than the host rock. As such it can be isolated from its matrix as a discrete hollow structure and frequently weathers out of its own accord. Although usually hollow, it may be so completely filled that it’s almost indistinguishable from a nodule that has formed by accretion. A geode can form in any cavity and can be any shape but is usually spherical, ovate or oblate. Informally, the term is often confined to more or less rounded structures. The term “geode” may refer to the complete structure or just the mineral-lined cavity within it.

As such, there’s absolutely no reason why some fossil cavities cannot be described as “geodised”. Also, since the original organic structure determined the boundary of any cavities then (in a sense) the fossil has created the geode and may have contributed some of the minerals. In the case of the beautiful ammonite that kicked off this post then yes, it’s geodised, and technically each hollow chamber having a discrete mineral boundary and a crystal lining is itself a geode.

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Roger, thanks for taking the time to provide a complete explanation. ;)

Sorry, but couldn’t agree with that. It’s completely untrue to suggest that a geode can only form in volcanic circumstances. Geodes can have either sedimentary or volcanic origin and are most commonly found in basaltic lavas and limestones.

They’re very common in the Lower Warsaw Formation where Missouri, Iowa, and Illinois join, in host rocks which are predominantly argillaceous dolomite and dolomitic mudstone. They’re also scattered across the sedimentary rocks of the Mid-West in general.

What characterises a “geode” is that it’s a secondary structure formed by one of two processes:

- the partial filling of vesicles in volcanic to sub-volcanic rocks by minerals deposited from hydrothermal fluids

or

- the dissolution of sedimentary nodules or concretions that were deposited syngenetically (ie at the same time) within the rock formations in which they are found, followed by partial filling with the same or other minerals precipitated from diagenetic water, groundwater or hydrothermal fluids.

Additionally, a geode is characterised by having an outer mineral layer (as distinct from a vug, which does not), that is usually more resistant to weathering than the host rock. As such it can be isolated from its matrix as a discrete hollow structure and frequently weathers out of its own accord. Although usually hollow, it may be so completely filled that it’s almost indistinguishable from a nodule that has formed by accretion. A geode can form in any cavity and can be any shape but is usually spherical, ovate or oblate. Informally, the term is often confined to more or less rounded structures. The term “geode” may refer to the complete structure or just the mineral-lined cavity within it.

As such, there’s absolutely no reason why some fossil cavities cannot be described as “geodised”. Also, since the original organic structure determined the boundary of any cavities then (in a sense) the fossil has created the geode and may have contributed some of the minerals. In the case of the beautiful ammonite that kicked off this post then yes, it’s geodised, and technically each hollow chamber having a discrete mineral boundary and a crystal lining is itself a geode.

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I don't want to hijack this thread or Roger's recent explanation of "geodised fossils". But wouldn't agatized fossil go through about the same process? And what is the diffence between geodes and agates? Would it be that geodes are categorized by their cavity, and with that cavity having the ability to fully form crystal structures?

Edited by fossilized6s
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I don't want to hijack this thread or Roger's recent explanation of "geodised fossils". But wouldn't agatized fossil go through about the same process? And what is the diffence between geodes and agates? Would it be that geodes are categorized by their cavity, and with that cavity having the ability to fully form crystal structures?

A geode is a structure, that implies one of two possible modes of formation but says nothing about the mineral composition. Yes, it has to be hollow (and have a discrete mineral exterior shell) and have minerals crystallised within that. There is a wide range of possible minerals, but commonly: barite, calcite, celestite, chalcedony, dolomite, jasper, kaolinite, limonite, millerite, opal, pyrite, smithsonite, sphalerite, and almost any variant of macrocrystalline quartz.

Agate can be added to that list, but it most usually has a volcanic or metamorphic origin, and so is more often associated with volcanic types of geode rather than sedimentary types. Geologically, agate is an impure cryptocrystalline variety of silica composed chiefly of chalcedony, with the term being reserved for material which is banded. Informally, the term also extends to various (usually colourful) variants of chalcedony which may or may not be banded and form the replacement mineral for organic material in sedimentary environments and where there is light metamorphism. Whether or not the resultant fossil could be regarded as having geodised would depend on it (having the capability of) meeting the structural criteria, not its mineral composition.

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Absolutley. Does the fossil 'create' the geode...no, ground water and minerals create the geode. Fossils provide the empty space for the geode to form in.

I agree

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hello there all ppl.

mud cracks can make geodes like septaria .coprolite can make geodes.

post-1185-0-46507200-1422656100_thumb.jpg

post-1185-0-26601400-1422656111_thumb.jpeg

post-1185-0-07203500-1422656120_thumb.jpg

post-1185-0-94831900-1422656131_thumb.jpg

post-1185-0-96359300-1422656142_thumb.jpg

Edited by hssain
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Absolutley. Does the fossil 'create' the geode...no, ground water and minerals create the geode. Fossils provide the empty space for the geode to form in.

I agree

Partially true. A geode (by definition) needs a discrete mineral exterior as a boundary for the cavity and the fossil frequently provides at least some of that boundary via diagenesis of its own components. Even if not, the physical structure of the fossil may (and often does) determine where external mineral sources replace it, while retaining the dictated shape. Similarly, the crystal lining of that dictated shape may well - at least in part - be derived from original constituents of the fossil. All of these things are more true when we are talking about carbonates in shells which are naturally hollow to begin with.

Semantics, schemantics.

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  • 2 weeks later...

there are lots of geodized brachiopods in the Ordovician. I would call each chamber in the ammonite a geode. There are lots of geodes in sedimentary rocks,

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