All comments have been passed on, thank you.

Tom

On 9/27/2018 10:18 AM, David Campbell wrote:
Yes, drying is an important factor.  It also depends on how it's being 
measured and what type of shell - is the measurement based on amounts of 
carbonate, or of calcium, or what?  Different measurement techniques 
have their advantages and disadvantages.  Echinoderms have rather higher 
magnesium levels than mollusks.  Aragonitic mollusks will have more 
stray strontium mixed in, and calcitic mollusks will have more stray 
magnesium mixed in.  Is there periostracum present?  How is the shell 
treated in the preparation for analysis?  etc.  True nacre (mother of 
pearl, not merely smooth and shiny) has more organic matrix than 
standard non-nacreous shell mineralogies.  Corbulids have layers of 
conchiolin the shell, apparently making it harder for drilling predators 
to get through.  Some shells can have a tiny amount of vaterite, a third 
crystal form of calcium carbonate.

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