Fossilization processes

Fossilization processes proceed differently according to tissue type and external conditions.
Permineralization is a process of fossilization that occurs when an organism is buried. The empty spaces within an organism (spaces filled with liquid or gas during life) become filled with mineral-rich groundwater. Minerals precipitate from the groundwater, occupying the empty spaces. This process can occur in very small spaces, such as within the cell wall of a plant cell.
A pyritised Echioceras ammonite from Charmouth. 
Casts and molds
In some cases the original remains of the organism completely dissolve or are otherwise destroyed. The remaining organism-shaped hole in the rock is called an external mold. If this hole is later filled with other minerals, it is a cast. An endocast or internal mold is formed when sediments or minerals fill the internal cavity of an organism, such as the inside of a bivalve or snail or the hollow of a skull.
External mold of a bivalve from the Logan Formation, Lower Carboniferous, Ohio

Replacement and recrystallization
Replacement occurs when the shell, bone or other tissue is replaced with another mineral. In some cases mineral replacement of the original shell occurs so gradually and at such fine scales that microstructural features are preserved despite the total loss of original material. A shell is said to be recrystallized when the original skeletal compounds are still present but in a different crystal form, as from aragonite to calcite.

Carbon films
Carbon films are thin film coatings which consist predominantly of the chemical element carbon. The soft tissues of organisms are made largely of organic carbon compounds, leaving a thin film of carbon residue is left, forming a silhouette of the original organism called a carbon film.

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