Silicon deficiency

Table 5.4 lists analyses of some natural specimens. In some instances, there is a silicon deficiency with respect to the stoichiometric formula and diadochy of Ai3+ jj4+ pg3+ Qjj tetrahedral sites (cases 1, 2, 3, and 4). In other cases, the amount of sihcon is near the theoretical value and only divalent cations are virtually present in the M and Ml positions (cases 5 and 6). 

Agreement is also poor concerning entropy and volume excess terms. Because divalent cations (Mg, Ca, Fe, Mn) occupy only dodecahedral sites whereas octahedral sites are reserved for trivalent cations (Cr, Fe, Al), each cation has only one site at its disposal and permutability is fixed by stoichiometry (cf. section 3.8.1). As regards the occupancy on tetrahedral positions, we have already seen that analyses of natural specimens show silicon deficiencies, compensated by AF ... 

Wanke (1981) and Allegre et al. (1995b) have proposed that a significant fraction of the Earth s silicon is in the core. However, this is not well supported by experimental data. To explain the silicon deficiency this way conditions have to be so reducing that niobium would be siderophile and very little would be left in the Earth s mantle (Wade and Wood, 2001). 

Silicon appears to be required for the normal synthesis of the organic matrix of bone and for its normal calcification. The silicon in the blood serum occurs entirely as silicic acid. Osteoblasts may contain the highest concentrations of silicon of all the cells of the body. Within these cells, silicon occurs mainly in the mitochondria. Silicon deficiency in animals results in bones with abnormal structures. These abnormalities include thinner oc rtical bone, reduced bone flexibility, and flattened cranial bones. Silicon deficiency also affects cartilage, and results in a dramatic reduction in the ividth of the epiphyseal cartilage (Carlisle, 1985). 

Since the Si—O bond length (1.66 A) is shorter than that (1.75 A) of Al—O, the crystal lattice of zeolites is shrunken and the structure is stabilized after dealumination and silicon enrichment, as confirmed as follows. Of course, unavoidably there exist silicon-deficient hydroxyl nests after both dealumination and ultra-stabilization, and some mesopores will be generated in the USY framework. 

Si Al ratio of the outer layers (ca. 10 A) of the zeolite crystals. The extent to which the surface composition differs from the bulk composition appears to depend on preparation conditions, and all three possible situations (silicon rich surface, silicon deficient surface and surface composition equal to bulk composition) have been reported (refs. 12-14). Variations in aluminium distribution have also been probed by high resolution scanning electron microscopy (ref. 15) and energy dispersive X-ray analysis (ref. 16). 

Prolylhydroxylase, the collagen synthesis enzyme that has low activity in silicon-deficient tissue, requires iron that cycles between Fe3+ and Fe2+. The apoenzyme will bind aluminum (less strongly than iron), and the enzyme is then, of course, inactive. An experiment was conducted (6) in which the apoenzyme was presented with iron first and then aluminum, all other essential cofactors being present. Activity, as measured by hydroxy-proline production, was reduced by 20% of the control level. When the... 

As Table 7-16 shows, the relative abundances of the major elements in the aerosol do not differ greatly from those in bulk soil, crustal rock, or average shale—that is, the elements are neither greatly enriched nor seriously depleted. A good match with any of the three reference materials is not obtained, however. The differences must be significant, since they are greater than conceivable analytical errors. Consider silicon as an example. Tables 7-13 and 7-16 indicate an average Si/Al ratio of 2.7, which is lower than that for either bulk soil or crustal rock and is more similar to that in shales. Fly ash exhibits a particularly low Si/Al ratio. It is possible that the low aerosol value in heavily industrialized Tees-side (Table 7-13) is due to a mixture of natural and combustion aerosols, but this explanation cannot be extended to the remote continental aerosol. A more likely explanation for the silicon deficiency is the size distribution of the Si/Al ratio in soil particles. The very coarse quartz particles, which are rich in silicon, are not readily mobilized. Since only the fine fraction of soil particles contributes to aerosol formation, the Si/Al ratio in the aerosol will be determined by that of silts and clays (see Table 7-7 for definitions). Common clay... 

Roessler, P.G. (1988) Effects of silicon deficiency on lipid-composition and metabolism in the diatom cyclotella-cryptica. /. Phycol, 24 (3), 394-400. 

Silicon is known to be essential for the development of connective tissue and the bones, although its function in these processes is not known. The silicon content of blood vessel walls decreases with age and with the development of atherosclerosis. It has been suggested, although the evidence is not convincing, that silicon deficiency may be a factor in the development of atherosclerosis. 

Previous studies have shown that the lipid contents of the the diatoms Navlcula pelllculosa (2) and Cyclotella cryptlca (3) Increase significantly In response to silicon deficiency. The chemical nature of the lipids produced under these conditions was not determined, however. Furthermore, the biochemical mechanisms responsible for this switch to lipid accusiulation have never been elucidated. The research described In this paper was therefore undertaken In an effort to further Investigate the regulation of lipid biosynthesis In Sl-deficient diatoms. 

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