Permineralization, also known as petrifaction, involves the crystallization of minerals in the fossil's pore spaces. A well known form is silification, in which silica-drenched solutions penetrate the cells of a plant, according to the Cartage website. Over time, the cells crystallize and become as hard as stone.
Yellowstone's petrified forests are an example of this process. Some petrified trees contain 1,000 rings or more. Petrifaction also takes place when iron, calcite or silica flows into the empty spaces of animal bones or shells and is allowed to crystallize.
Recystallization occurs when the minerals of animal shells transform into another type of crystal or grow into a different crystal type. Most shells consist of aragonite, which is a form of calcium carbonate. Over millions of years, this material decays into a more stable form known as calcite.
Although this process annihilates the internal details of the animal shell, its shape is preserved. According to the Cartage website, bivalve and snail shells that date back to the Jurassic Period, or approximately 200 to 145 million years ago, are still primarily made of aragonite. Older fossilized shells have turned into calcite.
Replacement happens when the mineral in the fossil has dissolved and a different material seeps in and replaces it. In these cases, silica or sulfide (pyrite) replaces the original mineral, which is either aragonite or calcite, in an organism trapped in mud. Although soft tissue is replaced, the organism's anatomical details are preserved. In rare cases, the replacement occurs atom by atom.
According to the Cartage website, German shells that date back 410 to 360 million years (Devonian Period) were x-rayed. The scans showed a cephalopod's tentacles and a trilobite's antennae, both of which had been replaced by pyrite. Thus paleontologists were able to compare the skeletal structures of these prehistoric animals with the anatomies of modern day organisms.