Silicon-aluminum distributions

The equivalent isotropic temperature factors, B, are remarkably similar, atom for atom, in the two stmctures, and the equality of the temperature factors for all four tetrahedral cations (0.65, 0.65, 0.62, 0.63) immediately suggests that the aluminum-silicon distribution is identical in all four positions. A careful consideration of the apparent anisotropic thermal motion of the surface oxygens strongly suggests that the arrangement of and Na ions within either stmcture is completely random there is no evidence for segregation either into different layers or to different domains within a layer. 

Laves, F., and S. Hafner, 1956. Order/disorder and infrared absorption. I. (Aluminum, silicon)-distribution in feldspars. Z. Krist. 108 52. 

Thorium(IV), distribution coefficients between aluminum, silicon, and... 

The spectra of the as-synthesized materials show the AFS and USY zeolites prior to calcination. The USY-2 zeolite shows distinct differences in silicon distribution compared to the two AFS samples although both USY and AFS materials have similar framework composition (as evidenced by unit cell size). The kl spectra show that USY-2 contains aluminum in octahedral coordination (VI) whereas the AFS materials contain only Al(IV). 

The framework silicon distribution depends on the mechanism of aluminum removal and silicon replacement during preparation. These differences remain after calcination but disappear upon steaming. Severe steaming results in loss of a large portion of framework aluminum such that only the strongest-bound aluminum species remain in the framework. Consequently, steamed AFS and USY zeolites have similar framework silicon distributions. 

Finally, it is important to mention the widely distributed and abundant elements sodium, potassium, calcium, magnesium, aluminum, silicon, and iron. These are present to some extent in the vast majority of chemical products and, although not an infallible guide, their concentrations indicate the degree of purification of a chemical and the care with which it has been handled, stored, and packaged. 

Traditional adsorbents such as sihca [7631 -86-9] Si02 activated alumina [1318-23-6] AI2O2 and activated carbon [7440-44-0], C, exhibit large surface areas and micropore volumes. The surface chemical properties of these adsorbents make them potentially useful for separations by molecular class. However, the micropore size distribution is fairly broad for these materials (45). This characteristic makes them unsuitable for use in separations in which steric hindrance can potentially be exploited (see Aluminum compounds, aluminum oxide (ALUMINA) Silicon compounds, synthetic inorganic silicates). 

Calcium—Silicon. Calcium—silicon and calcium—barium—siUcon are made in the submerged-arc electric furnace by carbon reduction of lime, sihca rock, and barites. Commercial calcium—silicon contains 28—32% calcium, 60—65% siUcon, and 3% iron (max). Barium-bearing alloys contains 16—20% calcium, 9—12% barium, and 53—59% sihcon. Calcium can also be added as an ahoy containing 10—13% calcium, 14—18% barium, 19—21% aluminum, and 38—40% shicon These ahoys are used to deoxidize and degasify steel. They produce complex calcium shicate inclusions that are minimally harm fill to physical properties and prevent the formation of alumina-type inclusions, a principal source of fatigue failure in highly stressed ahoy steels. As a sulfide former, they promote random distribution of sulfides, thereby minimizing chain-type inclusions. In cast iron, they are used as an inoculant. 

Aluminum distribution in zeolites is also important to the catalytic activity. An inbalance in charge between the silicon atoms in the zeolite framework creates active sites, which determine the predominant reactivity and selectivity of FCC catalyst. Selectivity and octane performance are correlated with unit cell size, which in turn can be correlated with the number of aluminum atoms in the zeolite framework. ... 

Iron is one of the most abundant metals in the upper crust of the earth. It is the fourth mineral-forming element (after silicon, oxygen, and aluminum), constituting about 5% of the earth s crust (see Table 1). Large deposits of its ores are numerous, widely distributed, and easily accessible. 

Since both analyses assumed random occupancy of T sites, the NMR data substantiate the model of random distribution of silicon and aluminum in ZSM-4. 

Zeolites are aluminosilicates characterized by a network of silicon and aluminum tetrahedra with the general formula Mx(A102)x(Si02)Y. The M are cations that are necessary to balance the formal negative charge on the aluminum atoms. The tetrahedra are linked to form repeating cavities or channels of well-defined size and shape. Materials with porous structures similar to zeolites but with other atoms in the framework (P, V, Ti, etc.), as a class are referred to as zeotypes. The structure committee of the International Zeolite Association (IZA http //www.iza-online.org/) has assigned, as of July 1st 2007, 176 framework codes (three capital letters) to these materials. These mnemonic codes do not depend on the composition (i.e. the distribution of different atom types) but only describe the three-dimensional labyrinth of framework atoms. 

The random A1 siting method of reference (7) was used to compute 29Si NMR intensities for comparison with experimental results reported in reference (2). The results in Table II show clearly some discrepancy between the experimental and calculated results. The variance a2 ranges from 35 to 329. The discrepancy is greatest at higher Si/Al ratios where the experimental distribution is much sharper than is expected of the maximum probability distribution of silicon and aluminum atoms. These results imply some ordering of the aluminum atoms in the lattice. 

We have shown that four of the nine possible prisms containing three or four aluminum atoms are sufficient to describe the 29Si MASNMR data. Are all four necessary It is difficult to answer that question since distributions derived from each of the four identified structures can be approximated by combinations of other stuctures. For example there is a combination of Na a1 and N3°Ma3 that can give the same relative distribution of silicon environments as Na a0. 

Aluminum is the third most abundant element found in the Earths crust. It is found in concentrations of 83,200 ppm (parts-per-million) in the crust. Only the nonmetals oxygen and silicon are found in greater abundance. Aluminum oxide (Al Oj) is the fourth most abundant compound found on Earth, with a weight of 69,900 ppm. Another alum-type compound is potassium aluminum sulfate [KA1(S0 )2 12H20]. Although aluminum is not found in its free metalhc state, it is the most widely distributed metal (in compound form) on Earth. Aluminum is also the most abundant element found on the moon. 

The feldspars are widely distributed and comprise almost two-thirds of all igneous rocks. Orthoclase and albite (NaAlSisOg) are feldspars in which one-fourth of the silicon atoms are replaced by aluminum and anorthite (CaAl2Si20g) and has one-half of the silicon atoms replaced by aluminum. Because the ionic radius of Na+ (0.095 nm) and Ca" " (0.1 nm) are the same, solid solutions are often formed between albite and anorthite. Good stones of albite and orthoclase are known as moonstones. 

The Lunar Prospector orbiter carried a gamma-ray/neutron spectrometer (GRS) that made precise measurements of the concentration and distribution of thorium (Lawrence et al., 1998) and hydrogen (Feldman et al., 2001). Subsequent spectral deconvolutions (Prettyman et al., 2006) have produced analyses of iron, titanium, potassium, magnesium, aluminum, calcium, and silicon. The principles of these analytical techniques are explained in Box 13.1. 

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