Aluminosilicates occurrence

In order to interpret the results of the following correctly, we must consider that they obey Loewenstein s rule [16] which forbids the occurrence of Al-O-Al linkages in tetrahedrally bonded aluminosilicates. As a result ... 

There are three traditional structures usually adopted as probable BASs in amorphous aluminosilicates a water molecule coordinated by an electron-acceptor center (I), a bridged OH group (II), and a surface H30 + ion (III) (125,126). The catalytic activity of these sites is obviously determined by their properties and surface concentrations. Pelmenshchikov et al. (127) have attempted to compare these characteristics for the above types of BAS in aluminosilicates in terms of the cluster approach. For this purpose they considered a sequence of states of the model fragment of a dehydroxylated surface plus two water molecules (Fig. 15). State S0 corresponds to a dehydroxylated surface, states S, Sn, and Sm represent the sites of the I, II, and III types and states Sla and SIla correspond to centers I and II at a higher coverage. The relative energies of these structures obtained using the CTP scheme and the CNDO/BW technique are presented in Fig. 15. The relative surface density of the sites, og(nJnf), was estimated as the relative probability of their occurrence ... 

It has been reported by F.A. Mumpton (1978) that more than 1000 occurrences of zeolite minerals in over 40 countries have been discovered since 1950. In addition to the hydrated aluminosilicate species, new minerals related to zeolites have been discovered, including the porous clathrasUs such as Melonophlogite (a silica only framework). Species in which the aluminum or silicon has been replaced by other elements (such as phosphorus, iron, and berylhum) have also been discovered as exemplified by viscite, a sihcoaluminophosphate related to analcime. At the present there are 3 8 different natural framework topologies, as shown in Table 25. 

The average composition of marine phosphorites on continental margins is summarized in Table 2, and the enrichment factors E m (phosphorite/shale) for various elements are plotted in Figure 23. (phosphorite/shale) values of 1 (within a factor of 2) for Li, B, Mg, Al, Si, K, Ti, Fe, Ga, and Bi indicate their aluminosilicate origin. The moderate enrichment of Sc, V, Cr, Y, Zr, Nb, REEs, and Th, and high enrichment of Ag, Cd, and U probably result from their replacement of Ca in CFA, facilitated by their similar ionic radii. However, the enrichment of biophile elements S, Se, Cu, Zn, Mo, Sn, Sb, Au, and Hg is also related to the occurrence of sulfides and organic matter in the samples (Li, 2000). 

The characteristic properties of clays are imparted by their main functional components, the clay minerals, which are aluminosilicates containing hydroxyl groups. The common feature of clay minerals is their layered structure and their occurrence as very fine particles. The physical and technological properties of clays depend not only on the composition and structure of the respective clay minerals, but also on the particle size and shape. 

The aluminosilicates constitute an important class of inorganic compounds. The occurrence of aluminium atoms in the second coordination sphere of the silicon, to which they are bonded through oxygen atoms, produce systematic changes in the Si chemical shift in a similar way to the changes which are associated with differences in the... 

The open frameworks of zeolites are slightly less stable than the corresponding condensed structures [15,16] into which they will transform during severe thermal treatment. Nevertheless, the difference in energy between a-quartz, the most stable polymorph of silica, and siliceous faujasite, one of the most open and least stable, is only about 15 kj mol k The extensive occurrence of aluminosilicate zeolites and their widespread utility in industry therefore depend heavily upon both the strengths of their T-O bonds (e.g. Si-O 466 kJ mol ), which render them stable with respect to framework rearrangement. The challenge with many of the newer materials is that their stability with respect to transformation into alternative condensed structures is considerably lower and they frequently collapse on dehydration or other means of activation. It is for this reason that only a small subset of the many open-framework families of materials can be rendered truly nanoporous,... 

Limestone (chiefly calcite, CaCOa) and dolomite rocks (chiefly dolomite, CaMg(C03)2) are exposed at about 20% of Earth s surface. Carbonate detritus, fossil shell materials, and carbonate cements are also common in noncarbonate sedimentary rocks and arid-climate soils. The carbonate minerals found in such occurrences, in decreasing order of importance, are calcite, dolomite, magnesian cal-cites (Cai jMgfCOa where jc is usually <0.2), aragonite (a CaCOa polymorph) and, perhaps, magnesite. As a rule of thumb, when such materials are present in silicate or aluminosilicate rocks or soils at a level of about 1 % or more, they will lend to dominate the chemistry of the soil or ground-water. This fact is extremely important when one is concerned about the ability of a rock to neutralize acid mine waters, other acid wastewaters, or acid rain. 

Rubidium is widely distributed in the earth s crust with no single mineral source. Its most concentrated occurrence is in the lithium mica lepidolite (lithium aluminosilicate) where its concentration may vary from 0 to 3.5% Rb20 (average 0.5%). This source, as a byproduct of the production of lithium, offers the largest and cheapest supply [5,6]. The metal is obtained by electrolysis of the fused chloride out of contact with air [7]. 

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