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South Dakota Ammonite Colors?


Mochaccino

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Hello,

 

I had a question about the coloration of the beautiful South Dakota ammonites from the Fox Hills formation/Pierre Shale. Namely, many Hoploscaphites/Discoscaphites ammonites have a milky white nacre of the shell, whereas others seem to have a vibrant red/brown sheen. For example here are two photos of Hoploscaphites nicoletti I found demonstrating the two types of colorations:

135463675_ScreenShot2022-01-03at10_50_07PM.thumb.png.9accdfb2516b94f4f3069f561c088e16.png

330529921_ScreenShot2022-01-03at10_49_42PM.png.108b4fd5a2ab6820cdf5d6477ae6f8e0.png

 

 

I was wondering what causes the difference in this coloration, and whether one is more rare/valuable than the other? I want a nice South Dakota ammonite for my collection and I'm contemplating options.

 

Thanks!

 

Edited by Mochaccino
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I can't give you a proper scientific explanation, but I think I can explain it to you in my own way. This has to do somehow with the type of preservation, but maybe someone who is more familiar with the processes can explain it better than I can. Cephalopod shells such as these, as with all molluscs, are made up of many very thin layers stacked on top of one another. The ammonite in your first photo shows mostly the outer layers, revealing only a bit of the more colorful lower ones underneath. I believe that they have lost their color pigment and are white because of weathering over time which has leached them out. I think that the colorful lower layers have been chemically altered, which accounts for preservation of the colors. You could remove the white layer to reveal the colorful lower ones, so the two types you are referring to are actually one and the same.

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Greetings from the Lake of Constance. Roger

http://www.steinkern.de/

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Fox Hills Formation covers a very large area.  Ive hunted the Timber Lake area and have found that ammonites closer to the edge of the rock/concretion can be most colorful, whereas more to the inner part of the rock can be found both these milky whites and reddish brown ones.  Ive got a perfect example of this but no pictures yet of that particular piece.  Here is one that was close to the outside edge of the rock. 

 

RB

DSCN1796.JPG

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my answer to questions like this is..."Physics and Chemistry" which code for "I don't know", but I do think Ludwigia's answer is very good.  

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10 hours ago, Ludwigia said:

I can't give you a proper scientific explanation, but I think I can explain it to you in my own way. This has to do somehow with the type of preservation, but maybe someone who is more familiar with the processes can explain it better than I can. Cephalopod shells such as these, as with all molluscs, are made up of many very thin layers stacked on top of one another. The ammonite in your first photo shows mostly the outer layers, revealing only a bit of the more colorful lower ones underneath. I believe that they have lost their color pigment and are white because of weathering over time which has leached them out. I think that the colorful lower layers have been chemically altered, which accounts for preservation of the colors. You could remove the white layer to reveal the colorful lower ones, so the two types you are referring to are actually one and the same.

 

7 hours ago, RJB said:

Fox Hills Formation covers a very large area.  Ive hunted the Timber Lake area and have found that ammonites closer to the edge of the rock/concretion can be most colorful, whereas more to the inner part of the rock can be found both these milky whites and reddish brown ones.  Ive got a perfect example of this but no pictures yet of that particular piece.  Here is one that was close to the outside edge of the rock. 

 

RB

DSCN1796.JPG

 

1 hour ago, jpc said:

my answer to questions like this is..."Physics and Chemistry" which code for "I don't know", but I do think Ludwigia's answer is very good.  


I see, thank you all for your insights! So it seems likely a difference in preservation method and/or location within the Pierre Shale, rather than a variety in species themselves. 
 

Would you happen to know if one variety is considered rarer or more desirable, akin to how purple ammolite is more valuable than red?

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6 minutes ago, Mochaccino said:

Would you happen to know if one variety is considered rarer or more desirable, akin to how purple ammolite is more valuable than red?

 

I would say not really. I think that it boils down to a matter of taste and aesthetics as to how each unique specimen would be valued.

 

Greetings from the Lake of Constance. Roger

http://www.steinkern.de/

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1 hour ago, Ludwigia said:

 

I would say not really. I think that it boils down to a matter of taste and aesthetics as to how each unique specimen would be valued.

I see, thanks!

 

9 hours ago, RJB said:

Fox Hills Formation covers a very large area.  Ive hunted the Timber Lake area and have found that ammonites closer to the edge of the rock/concretion can be most colorful, whereas more to the inner part of the rock can be found both these milky whites and reddish brown ones.  Ive got a perfect example of this but no pictures yet of that particular piece.  Here is one that was close to the outside edge of the rock. 

 

RB

DSCN1796.JPG

Forgot to address this but that is beautiful, and in great condition too! Is that a discoscaphites? I'm so envious of people who hunt for fossils...It's unfortunately not really an option for me so I must resort to buying online.

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I'm hazarding a wild guess, but the "metallic" ones look like they might be pyritized. In other words, instead of the fossil being mineralized with silica, it's been mineralized with iron sulfide. It has more to do with the chemistry of the mud it was preserved in than the fossil itself; pyrite indicates strongly reducing conditions (an anoxic mud) whereas silica is pretty much indifferent to oxygen levels. 

 

But it's hard to be sure from the photographs.

 

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12 hours ago, Mochaccino said:

Forgot to address this but that is beautiful, and in great condition too! Is that a discoscaphites? I'm so envious of people who hunt for fossils...It's unfortunately not really an option for me so I must resort to buying online.

I believe it is a Disco but forgot the species name.  and you and I are in the same boat.  I buy fossils nowadays.  Had a stroke a bit over 3 years ago and purty much changed my life forever.  Luckily I can still prep and do a few things but i do things a bit slower now. 

 

RB

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21 hours ago, kgbudge said:

I'm hazarding a wild guess, but the "metallic" ones look like they might be pyritized. In other words, instead of the fossil being mineralized with silica, it's been mineralized with iron sulfide. It has more to do with the chemistry of the mud it was preserved in than the fossil itself; pyrite indicates strongly reducing conditions (an anoxic mud) whereas silica is pretty much indifferent to oxygen levels. 

 

But it's hard to be sure from the photographs.

 

That's an interesting theory. It does look quite different from the other pyritized ammonites I've seen, but who knows what chemistry goes on?

 

 

 

15 hours ago, RJB said:

I believe it is a Disco but forgot the species name.  and you and I are in the same boat.  I buy fossils nowadays.  Had a stroke a bit over 3 years ago and purty much changed my life forever.  Luckily I can still prep and do a few things but i do things a bit slower now. 

 

RB

 

Oh I'm so sorry to hear that...hope you take care of yourself. It's a relief that you can still prep though! I always wanted to get into prep but wouldn't know where to get raw materials, as most market material is already sold prepped. On a related note, what vendors do you use for buying fossils? I browse the usual trustworthy online vendors: Fossilera, Indiana9, fossils-uk, but would love to get more insight.

 

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14 hours ago, Mochaccino said:

Oh I'm so sorry to hear that...hope you take care of yourself. It's a relief that you can still prep though! I always wanted to get into prep but wouldn't know where to get raw materials, as most market material is already sold prepped. On a related note, what vendors do you use for buying fossils? I browse the usual trustworthy online vendors: Fossilera, Indiana9, fossils-uk, but would love to get more insight.

I used to be very prolific fossil hunter and made about 7 out of state fossil hunting trips per year and that doesn't count about 50 trips locally.  Even hunted with Matt of Fossilera a few times back in the day.  I buy most of my unprepped material from friends who still hunt but once in awhile I find some material elswhere.  My youngest son, who is also a very proific fossil hunter gives me material from time to time too. I did find what appears to be a giant female Hoploscaphites crassus on one of our favorite web sites the other day.  But buying unprepared material is usually a gamble.  I the fragmacone is not there, then I wasted my moneys.  If it is there, then it will be worth a few hundred bucks.  Always cross your fingers when buying unprepped material and the best of luck to you.

 

RB

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The ammonites found in the Bearpaw formation exhibit a similar preservation of nacre, including the famous multi-coloured ammolite that comes out of Alberta. There are also many examples of ammonites that exhibit the pearly/white preservation as in the first photo. From what I've noticed, is that the ammonites that are better preserved and less crushed tend to preserve the pearly/whitish colour, which I believe is closer to the original appearance of the shell. If you've ever seen photos of those brilliantly coloured ammonites that the ammolite mines like to show off, they're almost always severely crushed and distorted.

 

I can't find it now, but I remember reading an article that explained how the colour of ammolite is caused by the way that light refracts off of the stacked plates of aragonite that make up the shells - in the case of the most colourful varities of ammolite, these plates have been compacted and distorted by diagentic forces, which exposes a broader spectrum of colour (I think there was a diagram that explained this very intuitively - I'll have to look for it later!) Anyway, there is an apparent correlation between the distortion of these aragonite plates and the spectrum of colour that is visible.

Edited by Norki
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