Skip navigation
125 Views 3 Replies Last post: Jun 8, 2018 9:08 AM by Dr T RSS
Currently Being Moderated

Jun 7, 2018 11:55 AM

False colours?

I found an ammonite at Folkestone yesterday that appears to have a purple stripe! This prompts me to ask whether any of the colours we see are actually true or just artifacts of subsequent mineralisation? I understand, for example, that chalk fossils are often orange because of the replacement of the shell by rusty limonite. But what of the shells that show surface colour differentiation? Is this a consequence of the structures that produced original pigmentation, or random colourisation due to vagaries of thickness or method of mineralisation? Hence the surprise at the clarity of the Gault stripe. I have also attached a couple of pictures of iridescent colourful finds from the Blue Lias where the colour distribution is more random, and a cross section of another Folkestone find yesterday (Hamites) where the interior appears to have filled with crystals like a geode to produce false colours by refraction and reflection. So are any of the colours/patterns we see in ammonites true?  Thanks Rocktapper.

  • Currently Being Moderated
    Jun 7, 2018 5:18 PM (in response to Rocktapper)
    Re: False colours?

    I think the ammonite colours are the original nacreous aragonite - mother of pearl

    • Report Abuse
  • Currently Being Moderated
    Jun 8, 2018 9:08 AM (in response to Rocktapper)
    Re: False colours?

    According to this link the ammonite shell comprised three aragonite layers (+ organic matter): the thin inner and outer layers were prismatic aragonite crystals and the thicker middle layer tabular (nacreous)

     

    http://www.bgs.ac.uk/discoveringGeology/time/Fossilfocus/ammonite.html

     

    The closest analogue is the Nautilus - I expect you have seen examples for sale with the outer layer stripped off:-

    Capture.JPG

    If you search "Google Scholar" you should find some more erudite articles...

    • Report Abuse

More Like This

  • Retrieving data ...

Bookmarked by (0)

What the symbols mean

  • "correct" answer available
  • "helpful" answer