Silica sediments structure

Self-assembly of highly charged colloidal spheres can, under the correct conditions, lead to 3D crystalline structures. The highly charged spheres used are either polystyrene beads or silica spheres, which are laid down to give the ordered structures by evaporation from a solvent, by sedimentation or by electrostatic repulsion (Figure 5.34). The structures created with these materials do not show full photonic band gap, due to their comparitively low relative permittivity, although the voids can be in-filled with other materials to modify the relative permittivity. 

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 most important character of all of the berthierine compositions is their low silica content. The variation of compositions found for pellets from the recent sediments appears to be the result of the crystallization of a chlorite structure with full octahedral occupancy. The meta-berthierines fall within the limits deduced for synthetic magnesian 7 X chlorites, limits which are also near full octahedral occupancy. 

A source of error in chemical analyses of montmorillonites (and in other clays) that is not commonly checked is the presence of amorphous material, particularly Si and Al. Table XXXII lists structural formulas given by Osthaus (1955) for montmorillonites which were purified by size fraction and by extraction with 0.5 N NaOH to remove amorphous Si and Al. In six analyses dissolved silica ranged from 3.6 to 8.4% and alumina from 0.6 to 2.25%. Amorphous silicon dioxide should be expected in most montmorillonites derived from volcanic material. The source glass has more Si than is required for the 2 1 layer and the excess must be leached from the glass. Much of the Si is deposited in the sediments underlying the bentonite bed in the form of chert but it is to be expected that the extraction would not be complete and a portion of the colloidal Si would remain in the bentonite bed. 

Compared to bone material, the composition and structure of flint is much simpler. Flint takes part in the big family of silicon-containing sedimentary rocks. It is basically composed of homogeneous microcrystalline silica grains, which give it a homogeneous macroscopic aspect. The name flint especially defines rocks where the cortex is thin and the heart of the stone is characterised by the absence of calcite. The majority of flint is found dispersed in sediments or as subcontinuous... 

Special experimental investigations of the properties, particulars of structure, and variations as a function of time, temperature and pressure have not been made so far for the iron cherts. However, the main components of such sediments—iron hydroxide and silica—have been rather well studied, and some data which are of interest to understanding the diagenetic processes are considered in this section. For the other components—magnetite, siderite, and sulfides—the very limited experimental data were examined in our previous work (Mel nik, 1972b). 

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). 

Thermodynamic values at 5° and 25°C for each of the three forms of silica are given in Table I. The large differences in these values suggest that the changes in these properties as a function of the stability of the crystal structure should help to identify form changes in the sediment. 

A macroporous silica can be synthesized through a slow sedimentation of colloidal particles onto a template. In a fast, single-step reaction (see Figure 8.37), the monomeric alkoxide precursors permeate the array of bulk polystyrene spheres and condense in air at room temperature. Close-packed, open-pore structures with 320-360 nm voids are obtained after calcination of the organic component at high temperatures. 

The presence of fine hydrophobic particles dispersed into the oil phase helps to prevent the formation of very small oil drops and the spreading of the oil at the air—water interface. The particle—particle interactions inside the oil phase were estimated by measuring the settling volume of particles in oil (98). Particles settling under gravity formed a three-dimensional gel-type structure. With small particles, the sediment contained a very low amount of particles, and the final settling volume depended on the type of oil. For example, with 0.2-fim sized, hydrophobic silica particles, the sediment contained 1.2 vol% particles in decane and 1.45 vol% particles in DMPS-V silicone oil. These results show that... 

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