Silicification

In geology, silicification is a process in which silica-rich fluids seep into the voids of Earth materials, e.g., rocks, wood, bones, shells, and replace the original materials with silica (SiO2). Silica is a naturally existing and abundant compound found in organic and inorganic materials, including Earth's crust and mantle. There are a variety of silicification mechanisms. In silicification of wood, silica permeates into and occupies cracks and voids in wood such as vessels and cell walls. The original organic matter is retained throughout the process and will gradually decay through time. In the silicification of carbonates, silica replaces carbonates by the same volume. Replacement is accomplished through the dissolution of original rock minerals and the precipitation of silica. This leads to a removal of original materials out of the system. Depending on the structures and composition of the original rock, silica might replace only specific mineral components of the rock. Silicic acid (H4SiO4) in the silica-enriched fluids forms lenticular, nodular, fibrous, or aggregated quartz, opal, or chalcedony that grows within the rock. Silicification happens when rocks or organic materials are in contact with silica-rich surface water, buried under sediments and susceptible to groundwater flow, or buried under volcanic ashes. Silicification is often associated with hydrothermal processes. Temperature for silicification ranges in various conditions: in burial or surface water conditions, temperature for silicification can be around 25°−50°; whereas temperatures for siliceous fluid inclusions can be up to 150°−190°. Silicification could occur during a syn-depositional or a post-depositional stage, commonly along layers marking changes in sedimentation such as unconformities or bedding planes.

Sources of silica

The sources of silica can be divided into two categories: silica in organic and inorganic materials. The former category is also known as biogenic silica, which is a ubiquitous material in animals and plants. The latter category is the second most abundant element in Earth's crust. Silicate minerals are the major components of 95% of presently identified rocks.

Biology Biogenic silica is the major source of silica for diagenesis. One of the prominent examples is the presence of silica in phytoliths in the leaves of plants, i.e. grasses, and Equisetaceae. Some suggested that silica present in phytoliths can serve as a defense mechanism against the herbivores, where the presence of silica in leaves increases the difficulty in digestion, harming the fitness of herbivores. However, evidence on the effects of silica on the wellbeing of animals and plants is still insufficient. Besides, sponges are another biogenic source of naturally occurring silica in animals. They belong to the phylum Porifera in the classification system. Silicious sponges are commonly found with silicified sedimentary layers, for example in the Yanjiahe Formation in South China. Some of them occur as sponge spicules and are associated with microcrystalline quartz or other carbonates after silicification. It could also be the main source of precipitative beds such as cherts beds or cherts in petrified woods. Diatoms, an important group of microalgae living in marine environments, contribute significantly to the source of diagenetic silica. They have cell walls made of silica, also known as diatom frustules. In some silicified sedimentary rocks, fossils of diatoms are unearthed. This suggests that diatoms frustules were sources of silica for silicification. Some examples are silicified limestones of Miocene Astoria Formation in Washington, silicified ignimbrite in El Tatio Geyser Field in Chile, and Tertiary siliceous sedimentary rocks in western pacific deep sea drills. The presence of biogenic silica in various species creates a large-scale marine silica cycle that circulates silica through the ocean. Silica content is therefore high in active silica upwelling areas in the deep-marine sediments. Besides, carbonate shells that deposited in shallow marine environments enrich silica contents at continental shelf areas.

Geology The major component of the Earth's upper mantle is silica (SiO2), which makes it the primary source of silica in hydrothermal fluids. SiO2 is a stable component. It often appears as quartz in volcanic rocks. Some quartz that is derived from pre-existing rocks, appear in the form of sand and detrital quartz that interact with seawater to produce siliceous fluids. In some cases, silica in siliceous rocks are subjected to hydrothermal alteration and react with seawater at certain temperatures, forming an acidic solution for silicification of nearby materials. In the rock cycle, the chemical weathering of rocks also releases silica in the form of silicic acid as by-products. Silica from weathered rocks is washed into waters and deposit into shallow-marine environments.

Mechanisms of silicification

The presence of hydrothermal fluids is essential as a medium for geochemical reactions during silicification. In the silicification of different materials, different mechanisms are involved. In the silicification of rock materials like carbonates, replacement of minerals through hydrothermal alteration is common; while the silicification of organic materials such as woods is solely a process of permeation.

Replacement The replacement of silica involves two processes: 1) Dissolution of rock minerals 2) Precipitation of silica It could be explained through the carbonate-silica replacement. Hydrothermal fluids are undersaturated with carbonates and supersaturated with silica. When carbonate rocks get in contact with hydrothermal fluids, due to the difference in gradient, carbonates from the original rock dissolve into the fluid whereas silica precipitate out of it. The carbonate that dissolved is therefore pulled out from the system while the silica precipitated recrystallizes into various silicate minerals, depending on the silica phase. The solubility of silica strongly depends on the temperature and pH value of the environment where pH9 is the controlling value. Under a condition of pH lower than 9, silica precipitates out of the fluid; when the pH value is above 9, silica becomes highly soluble.

Permeation

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