The very definition of a fossil includes objects that another material has replaced. These are called casts. Impressions are also fossils, such as the shape of a prehistoric fern leaf embedded in shale rock or the impression of a shell in limestone. Somewhat standard in Western Pennsylvanian fossils from the Carboniferous are shells that still contain the original material from which they were made.

These shells show up as a white color contrasted with the dark limestone locally. This is said to be preserved aragonite (CaCO3) shell material from the original shell. Over 300+ million years from now, it’s pretty remarkable that the shell material persists. Being a carbonite mineral, aragonite is one of three common carbonate minerals. Calcite and Vaterite are the others.

Aragonite preserved within limestone.
Microscope photo of preserved aragonite shell material exposed while splitting limestone.
Electron microspope image of Aragonite layers.
Scanning electron microscope image of aragonite layers in the nacre of blue mussel (Mytilus edulis). By Glenn Elert.
Brachiopod Umbo, preserved in original Aragonite.
What appears to be the Umbo of a Brachiopod, with aragonite preserved.

More about Aragonite Online