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.
Compression fossils, such as those of fossil ferns, are the result of chemical reduction of the complex organic molecules composing the organism's tissues.
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.
Some fossils are biochemical and are called chemofossils or biosignatures.