The shiny scales of fossil fish are phosphatic. First image is from display at the University of Georgia, Marine Extension Service, Skidaway Island, GA Hard parts made of silica , such as the skeletons of diatoms and radiolarians, and some types of sponges, may be preserved unaltered in some deposits. Organic hard parts, made of resistant materials such as chitin, cellulose, keratin, sporopollenin, or collagen are present in some groups of organisms. The hard parts of many fossil organisms have been chemically altered by the addition, removal, or rearrangement of chemical constituents. The added mineral matter makes the permineralized fossil much heavier than the original material.
Petrified wood is a common example of permineralization.
Petrified Logs from Petrified Forest. The fine details of shell structures are generally preserved. Minerals which commonly replace hard parts are silica and pyrite . Many modern shells are made of aragonite. With time, the aragonite will alter or recrystallize to calcite, a stable form of CaCO 3 .