Mineralization is the rare process by which organic remains are transformed into rock. This transformation is a form of fossilization where organic materials are replaced or filled by inorganic minerals from the surrounding environment. The study of this journey, from death to discovery, falls under taphonomy. Taphonomy investigates the physical, chemical, and biological changes that occur as matter passes from the biosphere into the lithosphere.
The Necessary Prerequisites for Preservation
Fossilization is an improbable event, requiring specific circumstances to halt decay. The primary challenge is preventing the organic remains from being consumed by scavengers or broken down by weathering. This is often accomplished by rapid burial, which isolates the remains from the destructive surface environment.
Rapid burial quickly removes the organism from the zone of biological activity. Burying the remains in fine sediments, such as mud, silt, or volcanic ash, further seals the tissue. This material acts as a barrier, preventing access by scavengers and minimizing damage from water currents or wind erosion.
The sediment overburden also helps create anaerobic conditions, meaning the environment lacks free oxygen. Most decomposing bacteria require oxygen to thrive, so an anoxic environment drastically slows the rate of decay. This reduction in biological decomposition provides the necessary window for the slower, chemical process of mineralization to begin.
The Core Process Permineralization
Permineralization, often called petrifaction, is a common process for turning organic structures into stone. Mineral-rich groundwater seeps into porous spaces within the remains, such as bone cavities or wood channels. The original organic material is not initially replaced, but rather augmented.
As the water moves through the buried material, changes in temperature, pressure, or evaporation cause its chemical properties to change. This change causes the dissolved minerals, commonly silica (silicon dioxide) or calcite (calcium carbonate), to precipitate out of the solution. The precipitated minerals then crystallize, filling the empty voids and hardening the structure from the inside out.
The resulting fossil is denser and retains the original microscopic structure of the organism. For example, in fossilized wood, the mineral infilling the cell walls preserves the detail of the wood grain and internal cellular structure. The original organic material may still exist within the mineral framework, but the structure’s integrity is maintained by the new mineral deposit.
Complete Replacement of Organic Structures
Distinct from permineralization is the process of replacement, where the original organic material is completely dissolved and substituted by a different mineral. This is a far more intricate chemical exchange that can preserve exceptional detail, sometimes even of soft tissues that rarely fossilize. Replacement often occurs when the remains are submerged in a highly saturated mineral solution for an extended period.
Replacement happens on an atomic or molecular scale. Organic molecules of the original structure are dissolved away one by one, while an ion of a dissolved mineral takes the exact place of the former molecule. This molecule-for-molecule substitution results in a mineral structure that is a perfect cast of the original tissue’s form.
Because the mineral takes the precise shape of the former organic material, replacement can capture features as delicate as the fine hairs on a trilobite or the internal structure of a plant’s stem. This process requires a specific chemical equilibrium to be maintained, ensuring dissolution and precipitation occur at roughly the same rate.
Geological Factors Determining Mineral Type
The final mineral composition of the fossil is dictated by the specific geological and chemical environment surrounding the buried organism. The type of mineral that infiltrates or replaces the organic material is directly dependent on the availability of ions in the local groundwater and sediment. This explains why some fossils are preserved in silica while others are found in iron compounds.
Silicification is common where volcanic ash releases vast amounts of silica into the groundwater, leading to petrified wood and bone composed of quartz. Conversely, in marine environments rich in iron and sulfur, pyritization occurs, where the organic material is replaced by pyrite (iron sulfide), often facilitated by sulfur-reducing bacteria.
Other factors, including the acidity (pH) of the water, ambient temperature, and pressure from overlying sediment, also influence the chemical reactions. Calcite mineralization is prevalent in limestone-rich areas, while phosphatization (replacement by calcium phosphate) can preserve delicate soft tissues. The resulting fossil is a direct reflection of the Earth’s chemistry at the time and place of its preservation.