Opals amorphous structure

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... 

Silicon is a major constituent of diatoms, which form a large proportion of marine phytoplankton. Some other algae, fungi and the siliceous sponges also have structural parts consisting of silica. The diatoms and radiolaria can excrete silica in the form of opal (amorphous silica, Si02 WH2O). 

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. 

Silica has 22 polymorphs, although only some of them are of geochemical interest—namely, the crystalline polymorphs quartz, tridymite, cristobahte, coesite, and stishovite (in their structural modifications of low and high T, usually designated, respectively, as a and jS forms) and the amorphous phases chalcedony and opal (hydrated amorphous silica). The crystalline polymorphs of silica are tectosilicates (dimensionality = 3). Table 5.68 reports their structural properties, after the synthesis of Smyth and Bish (1988). Note that the number of formula units per unit cell varies conspicuously from phase to phase. Also noteworthy is the high density of the stishovite polymorph. 

The mechanisms operating in the formation of textures seen in polycrystalline aggregates of the same species have been discussed in Sections 8.1-8.4. This may correspond to the analysis of a mechanism controlling the so-called selforganization or self-assemblage. Other mechanisms are possible for example, tiny spherical particles are assembled and a close-packed structure is formed due to surface tension. The formation of opal consisting of a close-packed structure of minute amorphous silica spheres maybe such a case. 

The best preservations of organic particles are found in quartz grains of syn-sedimentary origin. It is probable that the material was entrapped in amorphous silica which, upon dehydrating to solid opal, formed an incompressible matrix with minimal deformation. The resistance of opal and of the subsequently crystallized quartz provides a physical environment which preserves the structures and reduces the effects of heat and pressure over long periods, which otherwise lead to degassing and coalification of the organic matter. 

The hydrated amorphous mineral silica (opal) is widely used by many plants and animals for structural purposes. Most skeletons are formed by unicellular organisms (diatoms Figure 2(a), radiolarians Figure 2(b)), but silica is also present within multicellular organisms (sponge spicules, plant... 

Opal is related to the very common Si02 mineral species, quartz. Oceans are at present undersaturated with respect to opal (Broecker, 1971) possibly because of the biological formation of animals with silicified skeletons such as the diatoms. These delicate structured creatures, which proliferate in the upper photic zone, dissolve at depth. Therefore, only robust siliceous skeletons such as sponge spicules are retained in sediments that accumulate in deep waters, although some diatoms survive on the continental shelf under zones with high productivity. The initial deposition of the amorphous hydrated silica, opal, converts first to opal-CT and eventually to crystalline quartz (Kastner, 1981). 

Inverse opal structures have been classified into three structures, the so-called residual volume structure , shell structure and skeleton structure . The residual volume structure is a perfect inverse opal structure, which can be produced if the whole space among the opal spheres is completely filled by the product materials. If the space is incompletely filled, the surface of the sphere template is covered by the product materials, and a shell structure is generated. Most amorphous compounds tend to form a shell structure. On the other hand, crystalline compounds tend to form a skeleton structure. 

The structure of precious opal was first described by Jones. Sanders, and Segnit (347) and Sanders (348), who demonstrated that the structure consisted of spheres of amorphous silica 150-350 nm in diameter showing an X-ray pattern devoid of any... 

As a comparison, electrodes made of amorphous macroporous Si in an "inverse opal" structure had initial capacities of 2500 mAh/g using the C/10 rate, which faded to about 1500 mAh/g for C/5,500 mAh/g for C/2, and no capacities for rates 1C and higher. The authors attributed the poor cycling performance at higher currents to the poor electronic conductivity of a-Si. This illustrates the superior electronic transport properties of the SiNW electrode, since each NW is still connected to the current collector even after becoming amorphous. 

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