Hi all

I am an inorganic chemist, with access to X-ray diffraction (both powder
and single crystal) as well as to spectral techniques such as multinuclear
NMR, IR and UV. Some Raman studies on shell pigments have been also made
apart from typical diffusse reflectance UV-vis. All these techniques would
not explain why calcium carbonate (irrespective of the crystalline
variety) does not react with concentrated HCl, unless the shell in
question has a surface treatment of some kind (natural or artificial). I
do know that some filipino "doctors" polish shells and treat them with
some kind of clear-coat polymeric substance. Is this the case in here?
Even silicone oil or any other polymer-based material migh eventually
prevent direct attack of the acid onto the shell.
Concerning shell composition, I must say that very recently it has been
discovered in deep water thermal vents a new kind of shelled mollusc who
has replaced calcium carbonate in its shell by pyrite, that is iron
sulfide, much more abundant in its natural volcanic environment. This
appeared published in Nature, a couple of years ago, I think. In any case,
iron sulfide also reacts with concentrated HCl yielding in this case FeCl2
(which readily oxidises in air to Fe(III)) plus toxic and smelly hydrogen
sulfide, H2S.
Warmest regards

Manuel Jimenez Tenorio

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