Silicification
Silica is a naturally existing and abundant compound found in organic and inorganic materials, including Earth's crust and mantle.
Silicic acid (H4SiO4) in the silica-enriched fluids forms lenticular, nodular, fibrous, or aggregated quartz, opal, or chalcedony that grows within the rock.
[6][7] 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.
Silicious sponges are commonly found with silicified sedimentary layers, for example in the Yanjiahe Formation in South China.
[12] Diatoms, an important group of microalgae living in marine environments, contribute significantly to the source of diagenetic silica.
Besides, carbonate shells that deposited in shallow marine environments enrich silica contents at continental shelf areas.
[12] 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.
[3] In the silicification of woods, silica dissolves in hydrothermal fluid and seeps into lignin in cell walls.
The loss of fluids over time leads to the cementation of silicified woods through late silica addition.
The rate of breakage of original cells controls the development of the mineral framework, hence the replacement of silica.
[1][18][20][21] In the Conception Bay in Newfoundland, Southeastern coast of Canada, a series of Pre-Cambrian to Cambrian-linked volcanic rocks were silicified.
[22] The source of silica near the area was from hot siliceous fluids from rhyolitic flow under a static condition.
The following table shows the replacement of silica at different localities:[22] In the Semail Nappe of Oman in the United Arb Emirates, silicified serpentinite was found.
[24] The silicification of serpentinite was formed under the condition where groundwater flow and carbon dioxide concentration are low.
[17] The Mid-Proterozoic Mescal Limestone from the Apache Group in central Arizona is classic examples of silicified karsts.
On the northern coast of central Japan, the Tateyama hot spring has a high silica content that contributes to the silicification of nearby fallen woods and organic materials.
[1] With a temperature of around 70 °C and a pH value of around 3, the opal deposited is composed of silica spheres of different sizes arranged randomly.
[27] The Mount Goldsworthy in the Pilbara Craton located in Western Australia holds one of the earliest silicification example with an Archean clastic meta-sedimentary rock sequence, revealing the surface environment of the Earth in the early times with evidence from silicification and hydrothermal alteration.
[8] The succession was subjected to a high degree of silicification due to hydrothermal interaction with seawater at low temperatures.
[8] The condition of silicification and the elements that were present suggested that the surface temperature and carbon dioxide contents were high during either or both syn-deposition and post-deposition.
[8] The Barberton Greenstone Belt in South Africa, specifically the Eswatini Supergroup of around 3.5–3.2 Ga, is a suite of well-preserved silicified volcanic-sedimentary rocks.
With the composition ranging from ultramafic to felsic, the silicified volcanic rocks are directly beneath the bedded chert layer.
The silica enriched fluids bring about silicification of rocks through seeping into porous materials in the syn-depositional stage at a low-temperature condition.
