Afossil leaf that is made only of carbon preserved between two rock layers is called

A fossil leaf that is made only of carbon preserved between two rock layers is called carbon film fossil.
Characteristic of the carbon film fossils is that they are primarily composed of carbon. The fossil leaf has usually lost internal components (cell walls and internal cell structures ).

When a leaf gets fossilized between two layers of rock it forms a carbon film.
These fossils are called carbon films because unlike other fossils they are rich in carbon because all the other elements that made the organisms' body have slowly disappeared over time leaving only carbon between the layers of rock.