Off the wall thought - Could the shell be effectively thin-sectioned and looked
at under a polarizing microscope?  If iron was substituted for calcium in
calcium carbonate wouldn't you get siderite (Fe2CO3 ???) or something between
siderite and aragonite. I would think this would have different optical
properties from pure aragonite.  Maybe it's not possible for an organism to do
that though, I don't know.
If the iron was just a speck in the shell between layers or stuck in holes this
would show up as a black blob or crystal under polarizing light.
I've never seen a thin-sectioned shell, so maybe things wouldn't be that clear
since there are other things in the shell like conchiolin.  Just an idea from
someone who really has no idea.
Nora Bryan
Calgary, Alberta
CANADA
 
Andrew K. Rindsberg wrote:
 
> Delete this if you don't want to read about technicalities...
>
> John Cramer wrote,
> "Andrew Rindsberg's response to Paul Monfils was along the same lines I was
> thinking.  I presume the iron would be in small crystals in the top shell
> layer.  If so, I should be able to spot them as dark inclusions in the
> shell under the SEM.  If iron is distributed at the molecular level, the
> layer would have to be viewed optically so the red color could be detected.
> Anyone willing to guess which it would be?"
>
> Iron substituting for calcium in calcium carbonate would not make the shell
> red. It might make it a bit greenish, because this would have to be iron in
> the +2 valence state. In rust (hematite), iron has a valence of +3.
>
> An SEM would work to spot hematite crystals in situ. X-ray diffraction
> would more easily determine the presence or absence of hematite, but
> couldn't tell you where the hematite was within the shell (holes, layers,
> interstices, etc.). Chemical analysis would be required to tell how much
> iron was present.
>
> Andrew K. Rindsberg
> Geological Survey of Alabama