Biogenic silica, types

The geographic distribution of opal in the surfece sediments is controlled by (1) the local rain rate of biogenic silica, (2) the degree of its preservation in the sediments, and (3) the relative rate of accumulation of other types of particles. Preservation is promoted by rapid burial as this isolates BSi from seawater. But if the BSi is buried by other particle types, the relative contribution of BSi to the sediment is diluted. This dilution effect causes the BSi content of most continental margin sediments to be low despite high rain rates. Preservation efficiency is also dependent on (1) the intensity of bioturbation and suspension feeding and (2) the various factors that control... 

In 1 of these plants, silica is taken up through the roots as dissolved Si(OH)4 and is precipitated within and around the cells as hydrated opaline deposits which often replicate the shapes of the associated cellular structures. Upon death and decomposition of a plant, these siliceous deposits are released to the soil as discrete and generally microscopic structures known as phytoliths or plant opal . Most phytoliths in soils are derived from the aerial parts of plants. However, silicification can be extensive also in the roots and rhizomes of certain grasses, so that soils developed under this kind of vegetation receive significant quantities of biogenic silica from the underground portions of the plants. A rarer type of siliceous deposit in some vascular plants is tabashir (Jones et al., 1966). This material apparently is restricted to the bamboos, where it occurs within the hollow stems as solid, transluscent, opaline masses up to several cm thick. 

Diatoms have been used to look at the history of changing productivity, and hence nutrient inputs, in lake systems. The use of diatoms as palaeoenvironmental indicators in coastal environments is limited because of the preservation problem (Barker et al., 1994) and the paucity of areas which are continuously accreting sediment. Brown (1994) studied a number of sediment cores from the Wash for preservation of diatom material. The biogenic silica content of the sediments decreases with depth (Figure 5.6) which may indicate progressive corrosion with depth and/or increased input of silica to the sediments with time. Gross changes in the type of diatom present in the sediment may support the latter conclusion but the work is of a preliminary nature. Further examination of siliceous palaeoenvironmental indicators in intertidal sediments will be useful. 

The mineral silica, or more specifically hydrated amorphous silica, often referred to as opal, is the second most abundant mineral type formed by organisms, with only the carbonate minerals exceeding it in abundance and distribution. Much of the biogenic silica produced is formed at temperatures of 4 C or lower in the polar oceans. 

Biosilica extracts from Equisetum arvense have been found to accelerate the rate of formation of small oligomers of silicic acid, and more silica was produced that exhibited a narrower distribution of particle sizes than for silica formed in the absence of the biomolecule extracts (Figure 37.3), with the particle size distribution closely mirroring that found for the biogenic silica from which the biosilica extract was taken. Layered, crystalline materials were also produced that could have arisen from the epitaxial matching of initially formed oligomers on a p-sheet type material, the silica structure continuing to develop from the initial biopolymer-controlled nucleation event. 

Two types of enzymes have been identified as playing a role in the formation of biogenous silica. In a sponge, such enzymes are the silicateins that catalyze the formation of Si-O-Si bonds from the corresponding monomer. Mechanistically the reaction... 

The clay minerals of aeolian origin comprise 25 to 75% of the mass of pelagic sediments. The large range in composition reflects the latitudinal nature of the dust belt as well as dilution by other locally important particle types such as clay minerals of volcanogenic origin and biogenic hard parts (calcite and opaline silica). 

As with most groups of compounds, silica gel has generally proved to be the most widely used adsorbent, although cellulose, alumina, and polyamide layers have sometimes had important uses. The main contributions have been summarized in Table 22. The /f/values for alkyl, hydroxyindoles, and a series of 3-substituted derivatives on silica gel are listed in Table 23 with the solvents A to E (197,203), Table 24 records the ) /values for some of the same compounds on cellulose with the solvents A and B shown. The / /values of a group of 3-substituted indoles on silanized silica gel with and without the addition of //-dodecylpyridinum chloride are shown in Table 25. Apart from these types of layers, polyamide has been employed (205). Biogenic amines, including serotinin, have been examined on silica gel in 23 different solvents with detection by means of thiourea (206). Spray... 

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