Silicification

Silicification is the process by which silica-bearing fluids permeate rocks or organic remains, leading to the replacement of original materials with silica or to the infilling of spaces by silica. It includes replacement silicification, where minerals are dissolved and replaced by silica, and silicification as cementation or impregnation, where silica precipitates as a cement or infill. A common manifestation is petrification of wood, where lignin and cellulose are replaced by opaline silica, preserving cellular structures. Silica in diagenetic fluids often originates from the dissolution of silicate minerals or from volcanic glass and is transported in solution before precipitating as opal, chalcedony, or quartz.

Silicification occurs under a range of temperatures and is common in sedimentary basins; conditions control the silica polymorph, with opal forming at relatively low temperatures and quartz forming with higher temperatures or longer diagenetic histories. Silica can form cement that reduces porosity or can replace organic matter to preserve fine morphology, including fossils of plants and invertebrates. Biogenic silica from organisms such as diatoms, radiolarians, and siliceous sponges is a major natural source of silica in many environments.

In paleontology, silicified fossils are valued for preserving intricate details that may be lost in other fossilization modes. Examples include petrified wood and numerous silica-embedded fossils from various geological periods. In economic geology, silicification can influence reservoir quality in sandstones by creating silica cements or nodules, affecting porosity and permeability. Analytical methods for studying silicification include petrographic microscopy, X-ray diffraction, and spectroscopy to identify silica phases and diagenetic history.