Fossil plants preserved in detail by mineral infiltration are especially valued by paleobotanists because they preserve details of uncompacted form and cellular detail not found in plant impressions and compressions. The most informative kind of fossil is permineralization, in which the minerals fill cell cavities and conserve the original cell walls of the plant. Less useful are remains that are petrified, literally turned into stone, because even the cell walls are replaced by mineral. In Oregon, permineralized and petrified wood is generally preserved by silica derived from the alteration of volcanic rocks. In Ohio through Iowa another form of permineralized plant is found within coal seams, preserved by calcite in nodules colloquially called "coal balls". In black shales, plant material is permineralized by pyrite, again with preservation of microscopic details. These specimens were collected by Professor Retallack during his career as a paleobotanist.
Images © UO Museum of Natural and Cultural History. Production of this gallery was generously supported by The Ford Family Foundation.