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Cherts are siliceous sedimentary rocks. They contain variety of SiO2 minerals. Varieties of chert are recognized based on the amount of impurities present. They mainly form in marine environments in which silica is supplied by different sources. The solubility of silica depends mainly on temperature and depth of burial. Cherts originate from series of conversions of silica minerals under some conditions related to the temperature mainly; opal-A, opal-CT, microquartz and megaquartz. Cherts could be divided based on gross morphology into bedded and nodular cherts. Bedded cherts are composed of silica-secreting organisms' skeletal remains. Nodular cherts are the result of carbonate dissolution and replacement. In both types, models been proposed in order to explain their formation in the ancient and modern environments.
Chert is considered to be a siliceous sedimentary rock. It is made up mainly of SiO2 minerals, such as quartz, opal-A and opal-CT, and chalcedony. (Ireland et al., 2010) It is characterized as a dense, fine grained sedimentary rock. Cherts might form in a pure form, but usually contain some impurities in it. Impurities include aluminum, iron, magnesium, and calcium. The impurities amount present depend on the source in which the chemical elements are derived from. Cherts have three main textural types. The first type is microquartz. This type is characterized by equidimensional quartz grains that range around 9 microns. The second type is megaquartz. Megaquartx which is characterized by elongated to equant grains that have a size bigger than 20 microns. Finally, chalcedony is characterized by their thin crystal shape; sheaf-like bundles. (Umenda, 2003)
The impurities and inclusions produce variety of cherts. Their names are considered to be synonyms to cherts, but for specific settings and/or composition. For example, flint is used to describe chert nodules that take place in Cretaceous chalks. Jaspar is a red-colored chert due to the presence of hematite impurities. Novaculite is a very dense chert that occurred in the mid-Paleozoic period. Some cherts have unglazed porcelaine textures and fractures, which is called Porcelanite. Finally, cherts that are deposited by hot springs' waters and characterized to be porous are called siliceous sinter. (Umenda, 2003)
Silica Solubility and Sources
The solubility of silica in sea water is affected by temperature and pH. The solubility increases with the increase of temperature. Moreover, the pH slightly affects the silica solubility up to 9.0. Beyond that point, the solubility rises stridently. To be specific, the solubility of quartz and opal-A differs in sea water. (Fig. 1) Quartz has a solubility of 7-10 ppm under normal ocean conditions; 25 C0 and pH around 8. The solubility of quartz is not widely studied due to the slow dissolution rates at 25 C0. Under the same conditions, opal-A has a solubility of 60-120 ppm. Opal-A solubility is studied as monomeric silicic acid, H4SiO4. Siever studies show that solubility of opal-A increases when the pH is above 9 as a result of H4SiO4 ionization in sea water. (Siever, 1962)
Silica mainly forms in marine environments. Thus, the sources of silica supply must be understood first. There are many ways in which silica is supplied to sea water, a few important ones would be listed here. Silica could be supplied to sea water by river waters; have higher concentration of silica. Another source of silica in sea water is the hot volcanic rocks reactions with sea water along mid-ocean ridges. Also, the dissolution of silica-secreting organisms' skeletons after death is a major source of silica. Moreover, the inorganic absorption of silica into clay minerals in sea water also works. (Siever, 1962)
Origin of Cherts
Chert forms as a result of opal-A conversion to opal-CT, and then converts from opal-CT to microquartz. These conversions depend on the temperature and depth. In less extent, the rate of sedimentation and absence of detrital impurities allow faster conversion of opal-A. When silica-secreting organisms die, the dissolution produces opal-A. With burial at depth, the increase in temperature, up to 45 C0, causes the conversion of opal-A to opal-CT. Opal-CT is known to be an intermediate step. Opal-CT is a low temperature cristobalite that is disordered by inter-layered tridymite lattices; cristobalite and tridymite are metastable quartz varieties. What exactly happens is that when opal-A dissolves to generate pore waters filled with silica, opal-CT takes place and precipitate. The later process is known as solution- reprecipitation process. Also, opal-CT might occur as rim cements and overgrowths, along with others. With increasing temperature and burial depth, opal-CT converts to microquartz. This conversion occurs at 80 C0. When the temperature and depth reaches those of metamorphism conditions, microquartz converts to megaquartz. Under special conditions, it is been noted that opal-A could convert directly to megaquartz. (Ireland et al., 2010)
Chert Types and Formations
Based on gross morphology, cherts could be divided to two main types. Bedded chert is the first type and primarily forms by recrystallization of siliceous oozes. The second type is nodular chert that forms by replacement processes. (Maliva and Siever, 1989) Each type is identified by different characteristics. Some problems rise regarding the exact formation models and the sources of which the silica been supplied. Chert originates from biogenic and non-biogenic sources. The two types of cherts are discussed below with their different possible formation mechanisms; theories and models.
1. Bedded Cherts
Bedded cherts (Fig. 2) are pure, centimeters-thick beds of cherts, lack internal structures, and commonly interbedded with siliceous shale. (Barret et al., 1982) They are associated with ophiolitic rocks, such as volcanic flows. They contain some sedimentary structures that indicate their turbidity; due to mechanical transport during the depositional process. Bedded cherts also noted as "Ribbon Cherts". In general, bedded cherts are made up of siliceous organisms' remains. They are subdivided into four main categories based on the abundance and type of siliceous organic components. (Robertson, 1977)
Spicular cherts are siliceous rocks made up of invertebrates' spicules that are well cemented. They form mainly in shallow marine environments due to the invertebrates' existence. Radiolarian cherts. They are microcrystalline radiolarite that are well-bedded and have a well-developed groundmass. They usually form in deep marine environments. They are common in the rock record due to their tendency to survive diagenesis more than the other cherts. (Muttoni and Kent, 2007) Diatomaceous cherts are converted diatomite siliceous rocks into hard cherts by silica cement. They form in marine and non-marine environments. During diagenesis, the conversion of diatoms into quartz results in dissolution and recrystallization of diatom tests. (Muttoni and Kent, 2007) Non-fossiliferous cherts are pure chert beds with few organism trace; some have none. They usually represent cherts in the Phanerozoic and Paleozoic ages. The problem with these cherts is the mechanism in which they formed since most of the siliceous organisms were not evolved yet. It is been debated that chemical extraction of silica from sea water is the main force behind those beds. (Peterson and Von Der Borch, 1965) However, I believe that the precipitation models are not able to justify the massive chert beds from the ancient Eras.
For the biogenic formation of bedded chert, the siliceous-secreting organisms are the bulk source of SiO2. While they are alive, there opal-A skeleton goes through dissolution in very under saturated sea water. When these organisms die, their body goes through complete dissolution. Since the rate of skeletal decomposition is lower than the rate of skeletal formation, siliceous skeletons cannot dissolve as rapid as the forming process. As a result, some siliceous skeletons survive total dissolution and sink to the seafloor as oozes. Then, the oozes get buried and undergo more dissolution until they got trapped in the sediment pores. The pore waters become saturated with respect to silica, in which it leads to chert precipitation. (Siever, 1962)
The non-biogenic sources of bedded cherts are not well-understood and still debated. (Muttoni and Kent, 2007) The main problem is that how could direct precipitation lead to massive bedded cherts in the Precambrian, for example. Another problem is the fact that silica supply at the Phanerozioc and Paleozoic is still poorly understood. Almost all silica-secreting organisms were not present before the Cambrian, which rises the question on what conditions favored silica precipitation. (Siever, 1962) One hypothetical solution is that silica was supplied by weathering of non-marine siliceous outcrops represented by Murray, Jones and Brink. In their study, they suggested a model to encounter this problem in the post-Cambrian. They imply that biogenic silica dissolve and escape from shale, and then reprecipitate to form bedded cherts. (1992) However, this approach does not justify to me all the problems listed previously in the paragraph.
Van den Boorn, van Bergen, Nijman, and Vroon studied silica sources in order to explain the massive bedded cherts in the Archean Era. In their paper, they stated that "â€¦ chert have originated through massive transformation of precursor material by silica added from sea water."(p. 939, 2007) Moreover, they list two other possible types of silica sources to be produced by chemical precipitation and conduits. Their study was based on silicon isotopes.(2007) Another study was made by Hesse to explain the direct precipitation in the Paleozoic. He argues that silica might precipitate in local basins due to dissolution activities of volcanic ash. Also, he argues that volcanic materials that are loaded with silica could dissolve and escape to sea water. (1989) The last argument by Hesse is the most acceptable idea to me due to the lack of silica-secreting organisms back then, and the difficulty of forming massive bedded cherts out of direct precipitation of silica transported by weathering.
2. Nodular Cherts
Nodular chert (Fig. 3) is described by scholars as irregular, spheroidal bodies form mainly by replacements of carbonate rocks and sediments. ( Maliva and Siever, 1989) However, it is been reported that chert can replace burrows, stromatolites, and algal mounds selectively. The structure and precursor grains are preserved in cherts are evidence to such replacements. Nodular chert forms in different environments; from tidal flats to deep ocean basins. Two of the most important mechanisms that form nodular cherts are the calcium carbonate dissolution and silica precipitation. The silica precipitated could be either quartz or opal-CT. (Maliva and Siever, 1989) The sum process of those two mechanisms is called chertification. (Gao and Land, 1991) Silica precipitation and carbonate dissolution occur, simultaneously, along thin solution films. In order to chertification take place, the rate of silica precipitation has to be equal to the calcite dissolution rate. (Maliva and Siever, 1989) There are different models proposed to explain the formation of the nodular cherts.
The mixing zone model states that nodular cherts form in mixing zone due to the under saturation of calcite and super saturation of quartz and opal-CT. The under saturation is caused by mixing the calcite solution with different CO2 partial pressures. The key is that the mixing zone has to be an effectively closed system with respect to CO2. The super saturation of quartz and opal-CT is thought to be a result of the opal-A dissolution from skeletal material. This is achieved by meteoric water movement through carbonate sediment that contain biogenic opal-A. As the water goes through it, the silica concentration increases, in which replacement occur. (Maliva and Siever, 1989) This model has some problems when applied to some chert nodules' localities. For example, some nodular cherts form in contact with limestone
The organic-matter oxidation model states that the decomposition of organic matter decreases the solubility of silica and increases the solubility of carbonates. Due to the organic decomposition, the pore water CO2 partial pressure would increase the solubility of carbonates as dissolution would take place. As a result, the low soluble silica would replace the carbonates. This process, chertification, would continue until the lack of chert supply or the cease of bacterial activity that produces CO2. (Maliva and Siever, 1989)
The Hydrogen Sulfide Oxidation model.- In this model, Clayton believes that the Upper Cretaceous Chalk in Western Europe has oxic-anoxic boundary of sediment due to the flint nodules. Below this boundary, the excess H2S produced by anaerobic bacteria would be oxidized to sulfate which lowers the pH at the oxic-anoxic boundary. Since the H2S is oxidized to sulfate, the release of hydrogen leads to calcite dissolution and the precipitation of opal-A, as a result, is preferred. (Maliva and Siever, 1989)
Although these models explain the formation of nodular cherts, I believe that they lack some important information and have problems with it. One problem is that these models lack the definite, or at least an approximate range, depth to which chertification takes place. Moreover, if the dissolution occurs and affects some carbonates, the models fail to explain the presence of chert nodules in contact with limestones that were not affected by the dissolution. Is there a special way to localize the dissolution on some part of the limestones? Another problem, discussed by Maliva and Siever, is that all three models fail to show the cause of the precipitation rate of silica to be equal to dissolution rate of calcite. They state that these models only show the possibility of both reactions to occur thermodynamically in the environment. (1989) The formation of nodular cherts is still debated between scholars in the sense of the replacement mechanisms.
Cherts are important in the sense of indicating paleoecology and paleogeography since it contains the skeletal remains of the ancient organisms; form biogenically. However, chert also form by non-biogenic activities; direct precipitation for example. The silica sources in the ancient time are still a debatable subject and poorly understood. The massive bedded chert found from the Precambrian lacks a definite explanation in the sense of their silica supply and their formation mechanism. Although different nodular chert models have been proposed, problems still arise from the fact that many chert nodules form in contact with un-effected limestones with respect to dissolution process. More studies are needed to explore these ideas.