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 ---------------------------------------------------------------------- [log in to unmask] - a forum for informal discussions on molluscs To leave this list, click on the following web link: http://listserv.uga.edu/cgi-bin/wa?SUBED1=conch-l&A=1 Type your email address and name in the appropriate box and click leave the list. ----------------------------------------------------------------------