Amorphous components silicates

Examination of the infrared spectrum of a pure well-crystallized clay mineral can yield considerable information on its structure and composition in some instances, a species can be more quickly and more fully defined in structure and composition by this technique than by any other single technique. Few soil clays, however, are single species. In general, they are a complex mixture, which may include several layer silicates (either interstratified or as separate species), feldspars, various forms of silica, oxides and hydroxides of iron and aluminum, carbonates, sulphates, and phosphates. These minerals may well be poorly ordered, and associated with varying amounts of amorphous material. Not infrequently amorphous components predominate. As isolated, the clays may contain considerable amounts of organic matter, firmly combined with the inorganic constituents. 

Bi-phasic porous silicates containing amorphous and crystalline components can be prepared as stable pellets even if different crystallization routes known from the conventional crystallization processes are employed. Thus the inorganic template-free crystallization route and the crystallization in presence of propylamine could be applied and optimized for the crystallization on and into porous glasses, the so-called supported crystallization. 

Infrared absorption spectroscopy of interstellar clouds shows that the interstellar dust population varies with the line of sight, yet it maintains a similar character. In particular, submicron-sized amorphous silicate grains are the dominant component in every direction. The absence of crystalline grains is likely the result of rapid amorphization by the interstellar radiation field. 

The depletion in FeO may be understood in at least two ways. First, the crystalline grains may be equilibrium condensates from a hot solar nebular composition gas with iron sequestered to metals or sulfides (see e.g. Chapter 4). In this case the condensed grains either had to condense slowly to form crystal domains, or had been reheated and thermally annealed at a later epoch. The second, alternative explanation is that ferromagnesian amorphous silicate grains were thermally annealed in a reducing environment, e.g. in the presence of carbon. Heating such precursors leads to the formation of metallic spheroids embedded between the forsterite crystals, as the initial FeO component is reduced (see e.g. Fig. 8.3 and Connolly et al. 1994 Jones Danielson 1997 Leroux et al. 2003 Davoisne et al. 2006). Because carbon is ubiquitously present in primitive Solar System materials, this pathway offers a natural explanation to the observed FeO-poor silicate crystals. It is yet to be determined whether low-temperature crystallization processes, discussed in Section 8.1.1, would also lead to FeO depletion. 

Vanadyl phosphates (VPO) and multiple component molybdate (MCM) are good examples of catalysts, and alpha alumina, amorphous silica and alumino-silicates are good examples of catalyst supports that can be fabricated in the form of 45 to 150 im diameter spray dried porous spheres with attrition resistance improved by a relatively thin peripheral layer rich in amorphous silica, amorphous alumina, or phosphorus oxides. The hard phase component or precursor is selected in each case so that it will not interfere with the catalytic performance of the catalyst. 

Fine crystalline quartz or amorphous substances (gel, quartz glass, etc.) are used as one of initial components for the synthesis [19]. Depending on temperature, pressure, pH of the medium and the presence of salts, silica can exist in solution both as simple ions or molecules, and as more complicated polymer particles. Under normal conditions silica passes into solution in monomer form, as silicic acid Si(OH)4 at large pH, silicate ions SiOj - are formed. Monosilicic acid is a very weak acid however, at increased temperature its dissociation constant increases substantially. The amount of monomer form also increases with temperature. The dissolution of Si02 is due to hydration as well as to depolymerization. 

In [27-29], hydrothermal, mechanochemical and solid-phase syntheses of calcium silicate from anhydrous and hydrated oxides were compared. Initial components were taken at Ca/Si ratio equal to 0.8 1.0 1.2. According to X-ray analysis, the interaction in the mixtures of anhydrous oxides under mechanical activation is not completed. The product being formed is X-ray amorphous. When heated at 600-800 C, it is crystallized in the form of a -Ca2Si04 (Fig. 3.6a). At higher temperatures, Ca3Si207 is formed P-CaSiOj is crystallized at 860 C. The observed sequence of stages is similar to those observed in solid-phase synthesis of wollastonite. The amount of p-CaSiOj at 900°C does not exhibit any substantial dependence on the initial fractions of the reagents. 

As much as 15% of CM matrices may be amorphous silicate, which is associated with comparable volumes of very poorly crystalline phyllosilicates (Barber, 1981). In ALH81002, 5 vol.% of the matrix rims are composed of amorphous regions a few micrometers across that are rich in silicon, aluminum, magnesium, iron, and presumably oxygen, and contain 5-40 nm grains of iron-rich olivines and metallic Fe,Ni (Lauretta et al., 2000). Chizmadia and Brearley (2003) found more abundant amorphous material in the CM2 chondrite, Y 791198. (Origins of matrix components are reviewed in Section 5.7.11.)... 

Many different processes were involved in making each chondritic component. Unaltered chondrite matrices may contain at least six different types of micrometer-to-nanometer-sized components, which formed in diverse environments amorphous FeO-rich silicate, forsterite and enstatite grains, refractory grains, presolar grains, carbonaceous material, and iron-rich olivine. Chondrules formed by several nebular processes (closed-system melting, condensation, and possibly evaporation) and at least one asteroidal process (impact melting in regoliths). CAIs may be condensates, residues or processed versions of both. An exception to this preference for complexity is provided by the amoeboid olivine inclusions all AO As could have formed by the same basic process nebular condensation. Aluminum-rich chondrules may provide a second exception, at least within carbonaceous chondrites. 

Figure 12 IR data from the ISO for comet Hale-Bopp compared with a hve-component dust composition model. BBl is a 280 K blackbody, BB2 is a 165 K black body. Cry 01 is 22% forsterite. Cry o-Pyr is 8% orthopyroxene, and Am Pyr is 70% amorphous silicate with pyroxene composition (source Crovisier et al., 2000).

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