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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.
Compression fossils, such as those of fossil ferns, are the result of chemical reduction of the complex organic molecules composing the organism's tissues.If this happens rapidly before significant decay to the organic tissue, very fine three-dimensional morphological detail can be preserved.Nodules from the Carboniferous Mazon Creek fossil beds of Illinois, USA, are among the best documented examples of such mineralization.Often what remains is a carbonaceous film known as a phytoleim, in which case the fossil is known as a compression.Often, however, the phytoleim is lost and all that remains is an impression of the organism in the rock—an impression fossil.
For permineralization to occur, the organism must become covered by sediment soon after death or soon after the initial decay process.