Silica opaline

Opaline silica (opal) is a form of biologically produced silicon dioxide (Si02. H20) secreted as skeletal material by pelagic phytoplankton (diatoms) and one group of pelagic zooplankton (radiolarians) (Fig. 6.12). Opaline silica-rich sediments cover about one-third of the seabed, mainly in areas where sedimen- 

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Major structural components of hard parts Ca, C, SI, 0, P, F, Sr, S Calcite, aragonite, opaline silica, celestite, apatite, fluoroapatite Components of frustules and tests, bone, teeth... 

Silicon undergoes internal cycling within the crustal-ocean-atmosphere fectory dm-ing which mineral silicates are produced via precipitation from seawater. These include biotic and abiotic precipitates. As compared to the crystalline silicates that ultimately originate from the igneous rocks, the precipitates are amorphous in structme. To distinguish them from the crystalline silicates, the amorphous forms are collectively referred to as opaline silica. 

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 the calcareous tests, BSi dissolution rates depend on (1) the susceptibility of a particular shell type to dissolution and (2) the degree to which a water mass is undersaturated with respect to opaline silica. Susceptibility to dissolution is related to chemical and physical factors. For example, various trace metals lower the solubility of BSi. (See Table 11.6 for the trace metal composition of siliceous shells.) From the physical perspective, denser shells sink fester. They also tend to have thicker walls and lower surface-area-to-volume ratios, all of which contribute to slower dissolution rates. As with calcivun carbonate, the degree of saturation of seawater with respect to BSi decreases with depth. The greater the thermodynamic driving force for dissolution, the fester the dissolution rate. As shown in Table 16.1, vertical and horizontal segregation of DSi does not significantly coimter the effect of pressure in increasing the saturation concentration DSi. Thus, unlike calcite, there is no deep water that is more thermodynamically favorable for BSi preservation they are all corrosive to BSi. 

Biogenic silica A mineral form of silica that is amorphous in structure and deposited by marine organisms such as diatoms and radiolaria. Also called opal or opaline silica. 

Opal (or opaline silica) An amorphous silicate formed through the polymerization of silicic acid molecules. Though most is biogenic in origin, some forms as a result of diagenesis. 

Suspended solid surfaces (particles or colloids) in waters play a prominent role in controlling the concentration of dissolved trace elements. Most of these elements are eliminated by sedimentation after incorporation on to or into particles, generally by complexation with the surface sites. The most common inorganic particles and colloids are non-clay silicates (quartz, potash feldspar, plagioclase, opaline silica (diatoms)) clays (illite, smectite) carbonates (calcite, dolomite) Fe-Mn oxides (goethite, magnetite) phosphates (apatite) sulfides (mackinawite). Particles and colloids in a water body may be classified as a function of their origin ... 

Cosmogenous Aragonite Opaline silica Apatite Barite Organic matter Cosmic spherules Meteoric dusts CaC03 SiOz nHzO CajfF.Cl) (P04)3 BaS04... 

Table 7 Quantitatively important plants and animals that secrete calcite, aragonite, Mg-calcite, and opaline silica...

A factor that complicates the analysis of a clay paste, as noted by some of the previously mentioned workers, is the presence of additional constituents in the clay, like calcite, dolomite, iron oxides, gypsum, opaline silica, and others. These constituents, or their decomposition products, often enter into the chemical reactions to form new crystalline phases. Although their... 

Fig. 55. Diffractograms of opaline silica (/), SiOj-Xj (2), SiOj-Y (i) and quartz (4), after Mitsyuk (1974).

Synthetic opaline silica is X-ray-amorphous (Fig. 55) and similar in its properties to natural opal, which consists of densely packed spherical particles (globules) about 1500-3500 A in diameter. 

Plants take up silica as soluble monosihcic acid (H4Si04) and deposit it inside the cell, in cell walls, and near evaporating surfaces. When a plant dies and decays, these microscopic opaline silica phytoliths are returned to the soil. Soil phytolith accumulations are associated with certain plant types such as bamboo (Meunier et al., 1999). Some phytoliths persist in soils for thousands of years (Lucas, 2001). 

Alexandre et al. (1997) found that the biogenic sihca input into the biogeochemical silica cycle from the dissolution of phytoliths is twice as large as silica input from primary silicate mineral weathering in the tropical Congo rainforest. Biogenic (opaline) silica dissolves faster than sihcate minerals. While most of the phytoliths dissolve rapidly with a mean residence time of 6 months (Alexandre et al., 1994), and the sihca is recycled by the forest, a small part (7.5%) does not dissolve and is preserved in the soil. 

Siliceous oozes are accumulations of opaline silica (opal-A, an amorphous phase of high water content and porosity) in the tests of diatoms, radiolarians, and/or silicoflagellates. Opal-A solubility at 25 °C is 60-130 ppm Si02(aq) (e.g., Williams etal., 1985), and solubility increases with increasing temperature and pressure (Walther and Helgeson, 1977). Adsorption of aluminum and iron on the surfaces of siliceous tests decreases their solubility (Her, 1955 Lewin, 1961). Opal-A is a metastable phase that with burial eventually recrystallizes to quartz, often with another metastable intermediary phase, opal-CT (e.g., Hein et ai, 1978 Williams et ah, 1985 Williams and Crerar, 1985). Opal-CT structurally resembles an inter-layering of the two silica phases, cristobalite... 

Carbonate and opaline-silica-rich sediments are exceptions to these generalizations. They are very low in detrital iron, because calcareous and siliceous skeletal debris is much lower in iron than terrigenous material. In these dominantly biogenic sediments, iron may become limiting the degree of pyritization is very high (>80%) and... 

In terms of ceramic photonic crystals, as noted, opaline silica structures have dominated research activities and the publication and patent literature. In more recent years, significant efforts have developed in the fabrication, modeling, and application of optical fibers in which some degree of dielectric periodicity is... 

GS-fabric Grain supported fabric - skeletal grains (i.e. grains >30 pm diameter) constitute a self-supporting framework. Subdivided by cement type (a) optically continuous quartz overgrowths (b) chalcedonic overgrowths (c) microquartz/cryptocrystalline/opaline silica in-fill... 

Opaline silica (+) Calcite (+++) Vaterite High magnesian Phosphates... 

Mineral composition, as determined by X-ray diffraction, shows a dominance of clay minerals, although quartz and opaline silica are persistent as sub-dominant and locally dominant or co-dominant (Table II). Of the clays, expandable lattice clay minerals, predominantly montmorillonite, occur in all the deposits with kaolinite or illite appearing as accessory or subdominant components. A marked contrast in the dominant clay species occurs between the brown oil shale unit and the two units below it at Condor. In these lower units, kaolinite is in greater abundance than other clays as well as quartz, an aspect already alluded to in the variations in Table I. (Loughnan (8) also noted that the structure of the kaolinite changes from ordered in the lower units to disordered in the brown oil shale unit). 

DEPOSIT SEAM ROCK TYPE m.FA (LTOM) AMORPHOUS MATERIAL QUARTZ OPALINE SILICA EXPAND. LATTICE CLAY MINERALS PREDOMINANTLY MONTMORILLONITE... 

Ion River input Sea-air fluxes Evaporites CEC- clay CaC03 Opaline silica Sulphides MOR... 

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