Crystalline quartz, structure

In principle, there is no difference in the surface groups on quartz and on amorphous silica. The most important question discussed in the literature is whether the structure of crystalline quartz is represented in its surface, too. Many investigators (282-287) reported that there is a disturbed layer of amorphous character present on the quartz surface. It is more readily dissolved by water or by hydrofluoric acid. Holt and King (288) claimed that only a monomolecular layer of silicic acid was adsorbed on quartz surfaces. 

Transmission electron microscopy (tern) is used to analyze the structure of crystals, such as distinguishing between amorphous silicon dioxide and crystalline quartz. The technique is based on the phenomenon that crystalline materials are ordered arrays that scatter waves coherently. A crystalline material diffracts a beam in such a way that discrete spots can be detected on a photographic plate, whereas an amorphous substrate produces diffuse rings. Tern is also used in an imaging mode to produce images of substrate grain structures. Tern requires samples that are very thin (10—50 nm) sections, and is a destructive as well as time-consuming method of analysis. 

Figure 7.8). It means that the point symmetry of surface (=29Si -0)3Sia PC is closed to C3v group and symmetry axis must be to orient along the normal to the surface. This structure is shown on Figure 7.7c. The spatial arrangement of the substituents (=Si ) in the coordination sphere of the silicon atom Sia resembles the structure of the silicon coordination sphere in the open modifications of crystalline quartz (tridimite and cristobalite) [46],...

A characteristic of the early neutron reflectivity studies of nonionic surfactant adsorption was some variability in the pattern of adsorption. This was investigated in more detail and more systematically by McDermott et al. [55], who compared the adsorption of Ci2E6 onto a range of different substrates, amorphous silica, crystalline quartz, and the oxide layer on a silicon single crystal. The adsorbed surfactant was found to form a bilayer with an overall thickness 49 4 A, with a structure similar to that determined in the previous studies (see Fig. 4). 

The integral heats and entropies of adsorption for water on fused quartz powder have been obtained as a function of particle size. In the light of previous studies, this is indicative of identical surface structure for all samples studied. The results further indicate that many crystalline quartz powders are amorphous in their surface layers. 

The surface structure of fused quartz powders obtained by grinding is independent of specific surface area. The surface structure is similar to a medium area crystalline quartz but differs from that of gels, nonporous silicas of very high area, and coarse crystalline quartz powders. 

Figure 5.3 The amount of order in silicates can vary dramatically. A. The crystalline backbone structures for olivine, pyroxene, and quartz. The charge of the silicon tetrahedra is neutralized by metal cations in olivine and pyroxene. B. Silicate melts contain a mix of unaligned crystalline structures with metal cations randomly distributed in the melt. C. Chaotic condensates have not formed silicate tetrahedra rather, they appear more like a frozen gas state. These materials are typically under-oxygenated and contain more metals than a glass. Annealing supplies the chaotic silicate with the energy needed to rearrange into the more stable silicate tetrahedra. D. The gas phase largely consists of SiO. Metals are typically present as atoms or simple monoxides while excess oxygen can be found as OH (Nuth et al. 2002).

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). 

As with crystalline quartz, feldspar crystals also transform to diaplectic glass when, subjected to shock compression. Studies of diaplectic feldspar glasses liave been carried out by many investigators using many techniques (TEM, X-ray. IR, and Ramiui spectroscopy, and thermal analysis [11,15,19,22,25,28-30.44-47] However, tire number of atomic scale structural studies is limited [25,30,33,45]. In this section, our group s recent results on the structural analysis of some diaplectic feldspar glasses by X-ray and Raman spectroscopy will be presented, with comparison to many previously obtained data. 

According to Wyckoff [7], the crystalline forms of silica are the largest group of tetrahedral structures. Each of the three main polymorphs of silica formed at atmospheric pressure in nature (quartz, tridymite, and cristobalite) has a low and high temperature modification. The unit cell of low (a) quartz has three molecules and similar dimensions as the high ((3) quartz structure. The difference between low and high forms of quartz arises from small shifts of atom positions. Table 1 lists structural data for both quartz structures. 

PON formed under high pressure is crystalline and has the -quartz structure [212], PON is isoelectronic with Si02. 

FIGURE 11.31 Two-dimensional representation of (a) crystalline quartz and (bj noncrystalline quartz glass. The small spheres represent silicon. In reality, the structure of quartz is three-dimensional. Each Si atom is tetrahedraHy bonded to four O atoms. 

Crystalline quartz is one of those materials whose fundamental molecular units are achiral, but which are forced to adopt a chiral configuration by virtue of the crystal structure. The proof of this lies in the observation that quartz, when dissolved or melted, loses its ability to rotate the plane of linearly polarized light. The fusion/solubilization process... 

The absorption spectra of the thermally formed amorphous Si02 layers are similar to crystalline quartz spectra but can be distinguished by larger full-width-at-half-maximum (FWHM) values [9, 13, 15]. This confirms that the nearest-neighbor structure, which affects IR absorption spectra, is the same for amorphous Si02 and crystalline quartz. 

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