Opal formation

Opal formation in Meadow Steppe jwH of Manjury river watershed 22.03 No 3.53... 

The influence of electrical charges on surfaces is very important to their physical chemistry. The Coulombic interaction between charged colloids is responsible for a myriad of behaviors from the formation of opals to the stability of biological cells. Although this is a broad subject involving both practical application and fundamental physics and chemistry, we must limit our discussion to those areas having direct implications for surface science. 

The porous membrane templates described above do exhibit three-dimensionality, but with limited interconnectedness between the discrete tubelike structures. Porous structures with more integrated pore—solid architectures can be designed using templates assembled from discrete solid objects or su-pramolecular structures. One class of such structures are three-dimensionally ordered macroporous (or 3-DOM) solids, which are a class of inverse opal structures. The design of 3-DOM structures is based on the initial formation of a colloidal crystal composed of monodisperse polymer or silica spheres assembled in a close-packed arrangement. The interconnected void spaces of the template, 26 vol % for a face-centered-cubic array, are subsequently infiltrated with the desired material. 

It is observed that in the case of simultaneous saturation of two or more phases, the phase that forms first is often the least stable, or the most disordered, especially at room temperatures. For example, in aqueous solutions, opal (disordered) often forms but the more stable quartz rarely forms. Over a very long time, opal may "mature" to become quartz. The same is true for the formation of calcite (as compared dolomite), and analbite (as compared to albite). From the vapor phase, phosphorous vapor condenses first to yellow phosphorus (high entropy), instead of the more stable red phosphorous (low entropy)... 

One way to rationalize and remember the rule is to think that "nature is lazy" and hence would like to accomplish a process with the least effort. Because ions and molecules in a liquid are more or less randomly distributed, one may guess that the phases with simple structure and with low degree of order (meaning ions and molecules do not have to be arranged in a specific way) tend to form more easily than the phases with complicated structure and high degree of order. This often means the formation of metastable phases such as opal. 

The formation of ordered sphere-packing structures was observed in certain rheological experiments as just described. Due to the extremely uniform size of the particles, an ordered dense packing structure will develop during sedimentation of the Stober silica particles (see Fig. 2.1.12) when the dispersion is either sterically or electrostatically stabilized. The gemstone opal is essentially based on this principle (80-88). A transmission election replica picture is shown in Figure 2.1.13. The uniform... 

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. 

Most commonly, zeolites are found in series of sedimentary rocks which contain pyroclastic material and are formed during the devitrification of this material. If the rocks are silica-rich, the zeolite species formed seems dependent upon the bulk composition and burial depth or temperature of formation (Hay, 1966). They are most frequently accompanied by silica in an amorphous or cryptocrystalline form (opal, chalcedony). Analcite and all other compositional intermediates up to the silica-rich clinoptilolite are found in this association. The most comifton clay mineral in such tuffs is montmorillonite. Zeolites are sometimes found with glauconite (Brown, et al . 1969) or celadonite (Hay, 1966 Iijima, 1970 Read and Eisenbacher, 1974) in pelitic layers or acidic eruptive rocks... 

Biogenic forsterite and opal as a product of biodeterioration and lichen stromatolite formation in table mountain systems (tepuis) of Venezuela. Geomicrobiology Journal, 18, 117-32. 

The formation of artificial opals was achieved by using a number of methods including sedimentation, filtration, evaporation, and drip method. 

The transformation of opal-A to opal-CT generally begins at 35-50 °C, corresponding to burial depths of hundred of meters. In some environments this temperature may be as low as 17-21 °C (Matheney and Knauth, 1993 Monterey Formation) or even 0-4 °C (Botz and Bohrmann, 1991 Antarctic deep sea). The acoustic properties of the sediment are altered during the transformation to opal-CT, typically providing an acoustic reflector of the diagenetic front (Calvert, 1983 Tribble et al., 1992). Opal-CT, also known as porcelanite, exhibits X-ray characteristics of low cristobalite and tridymite (Figure 4). This mineral exists as... 

JLm diameter spheres, called lepispheres, which are composed of bladed crystals 30-50 nm thick. Kastner et al. (1977) reported that the formation of opal-CT results from dissolution followed by reprecipitation reactions that require a source of magnesium, alkalinity, and hydroxide ion. The presence of aluminosilicate phases commonly retards the formation of opal-CT (Hinman, 1998). 

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). 

Interestingly, the powder calcined at 600°C showed the formation self-assem-bled inverse opals of LiNb03 (Figure 6.7b-d). Usually the reported methods for the preparation of LiNb03 inverse opals use colloidal crystals as templates, but... 

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