How does a shell become a petrified fossil?
Petrifaction occurs in shells when minerals such as calcite, silica, or iron fill the pores and cavities of fossil shells. Occasionally phosphate minerals, pyrite, hematite, and other less common minerals make up all or part of the petrifaction matrix. Animal shells are composed of chitin, bone and cartilage, or silica, or calcium carbonate, plant shells of differing lignin and cellulose contents. The organic material is fossilized before petrification.
