Permineralization is the process in which minerals precipitate out of groundwater and fill empty pore spaces in organic materials like plants or bones. In this fossil, mineral-rich fluid has infiltrated the cell space of the fern and precipitates have replaced the cellulose as it disintegrates over time. These fossils are distinguishable because the original shape of the plant has not changed, but the composition is now of mineral and not organic materials. Minerals commonly found in permineralized fossils are calcite, iron and silica.