Another substance used in replacement is CaCO3 (Calcium Carbonate). Ground water washes into the porous organism and degrades it, leaving the minerals behind in its place. This takes many years to complete the process, but the end product yields a good internal mold.
"Replacement of Mollusk Shells." Past Life. Web. 2 Mar 2011. <http://www.msnucleus.org/membership/html/jh/earth/pastlife/lesson2/pastlife2e.html>.