Crystalline silicates

We also note that similar PIMC studies for Si crystals and crystalline silicates have been carried out recently [292]. 

Crystalline silicates, insoluble, 22 474-475 Crystalline silicon, amorphous silicon versus, 22 134... 

Near-Surface Alkali Diffusion into Glassy and Crystalline Silicates at 25 0 C to 100 0 C... 

Silicon undergoes internal cycling within the crustal-ocean-atmosphere fectory dm-ing which mineral silicates are produced via precipitation from seawater. These include biotic and abiotic precipitates. As compared to the crystalline silicates that ultimately originate from the igneous rocks, the precipitates are amorphous in structme. To distinguish them from the crystalline silicates, the amorphous forms are collectively referred to as opaline silica. 

The fundamental structural unit of all crystalline silicates is a tetrahedron that has an anion at each of its four corners and a Si" cation in the center. The silicate tetrahedra can be arranged in a variety of ways giving rise to a great variety of crystalline silicate minerals. Most of these arrangements involve some degree of sharing of the corner 0 anions between adjacent silicate tetrahedra. 

It has been observed that water dramatically enhances oxygen diffusion in silicate melts as well as in many crystalline silicates. Furthermore, in systems studied in more detail, it has been observed that the diffusivity is roughly proportional to H2O pressure (Farver and Yund, 1990), or H2O content in the phase. This has been... 

Table 1.17 Structural Units Observed in Crystalline Silicates...

In silica and in almost all crystalline silicates, the basic building unit is the Si04 tetrahedron (60). Within the limits of experimental uncertainty, this is also true for glassy silicates (56). It is generally agreed that no do-... 

The chemical state of dissolved silica (if Si02 is supplied as silica sol) must influence the nucleation and growth of crystalline silicates. Cary et al. (213) dissolved isotopically enriched silica in H20/D20 and, using 29Si NMR, concluded that tetrahedral dimers corresponding to pyrosilicic acid, H6Si207, built of two Q1 units were present in addition to monomers (Q°). The former species resonated at —9.26 ppm from TMS and accounted for up to 6 % of total spectral intensity. 

On the other hand, we precipitated a magnesium silicate from sea water by adding sodium metasilicate that gave an approximate K of 10-24 after aging. Siffert s material was freshly precipitated and could be expected to age significantly, like other poorly crystalline silicates. Consequently, the value chosen is open to considerable doubt. 

Silica—Particles or spheres of crystalline silicic acid used in chromatography. Its surface is polar, acidic, and tends to attract water of hydration and polar compounds. 

MgO was necessary to form montmorillonite from a variety of starting materials. When Mg was not present, analcite wasformed and all the A1 was presumably tetrahedrally coordinated. When CaO was added to the starting material, rather than MgO, tober-morite was formed and A1 did not enter into any crystalline silicate structure. Although Hawkins and Roy (1963) and Keller (1964) believe the role of Mg is to precipitate the Si, it is possible that its major contribution is, in some manner, assisting the A1 in assuming six-fold coordination under conditions where it would normally take four-fold coordination. 

Kim, J. B. and Inui, T. Synthesis of metal-incorporated mesoporous crystalline silicates for... 

In the case of the crystalline silicates an approach which takes account of the partly covalent character of the Si-O bond is helpful. The [SiCL]4- tetrahedron is taken as a basic building unit, and in most of the silicates these tetrahedra are linked together in an ordered fashion to form strings as in diopside (MgCa(Si03)2), sheet structures as in clay minerals, or three-dimensional frameworks as in quartz and the feldspars. Within these frameworks isomorphic replacement of one cation type for another is extensive. For example, the replacement of Si4+ by Al3+ is common, with the necessary lattice charge balance being maintained either by the incorporation of interstitial cations such as Na+... 

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