Silicon crustal abundance

Cs are more depleted in western coals. Silicon is also depleted in coal, probably because of the presence of clay minerals. Most lithophile elements (i.e., those normally associated with the earth s crust) have EF values near one, but it is interesting that the rare earth elements show slightly, but consistently higher enrichments in eastern coal. The apparent depletion of Ta is probably not real, but an artifact resulting from Wedepohl s use of too large a crustal abundance for it (14). 

When the relative composition of the biosphere is compared to that of the lithosphere, a new dimension for iron is noted. Iron now constitutes about 1.3 atoms of every 100 atoms of the lithosphere and, on a weight per cent basis, iron in crustal abundance ranks fourth in number, only behind oxygen, silicon, and aluminium (Table 7.3 in Geochemistry, Wedepohl, 1971, p. 60). On a comparative basis, the lithosphere is a dry metallic aluminium silicate, whereas the biosphere is wet and carbonaceous (Deevey, 1970). Iron is very limited in the hydrosphere and atmosphere. 

Information on the abundance of elements comes from diverse sources. Most elements are obtained from minerals in the Earth s crust. The availability of elements therefore depends on the crustal abundance, which can be estimated by analyzing representative samples of minerals. The abundances of elements vary enormously, from common ones such as oxygen and silicon... 

The nuclei of iron are especially stable, giving it a comparatively high cosmic abundance (Chap. 1, p. 11), and it is thought to be the main constituent of the earth s core (which has a radius of approximately 3500 km, i.e. 2150 miles) as well as being the major component of siderite meteorites. About 0.5% of the lunar soil is now known to be metallic iron and, since on average this soil is 10 m deep, there must be 10 tonnes of iron on the moon s surface. In the earth s crustal rocks (6.2%, i.e. 62000ppm) it is the fourth most abundant element (after oxygen, silicon and aluminium) and the second most abundant metal. It is also widely distributed. 

Silicon constitutes 28% of the elemental composition of the earth s crust and is the most abundant crustal element after oxygen (47%). Silicates, compounds which contain Si04 tetrahedra in the crystal lattice, account for about one-third of all known mineral species and about 95% of the earth s crust. In view of this high abundance of silicate minerals in the near-surface crustal regime, it is perhaps not surprising that organisms (which also inhabit the near-surface environments of the earth) have found numerous ways of interacting with siliceous materials. 

The crust, hydrosphere and atmosphere formed mainly by release of materials from within the upper mantle of the early Earth. Today, ocean crust forms at midocean ridges, accompanied by the release of gases and small amounts of water. Similar processes probably accounted for crustal production on the early Earth, forming a shell of rock less than 0.0001% of the volume of the whole planet (Fig. 1.2). The composition of this shell, which makes up the continents and ocean crust, has evolved over time, essentially distilling elements from the mantle by partial melting at about 100 km depth. The average chemical composition of the present crust (Fig. 1.3) shows that oxygen is the most abundant element, combined in various ways with silicon, aluminium (Al) and other elements to form silicate minerals. 

Wet-chemical analyses of aqueous extracts of aerosol samples have established the presence of anions such as sulfate, nitrate, and the halides, and of cations such as ammonium and the ions of the alkali and alkaline earth elements. Table 7-13 shows selected data to illustrate the abundances of important inorganic components in the urban, continental, arctic, and marine aerosols. Included for comparison are the concentrations of silicon, aluminum, and iron, which are the major elements of crustal origin. They occur in oxidized form, such as in aluminosilicates, which are practically insoluble. Taken together, the elements listed in Table 7-13 account for 90% of all inorganic constituents of the atmospheric aerosol. 

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... 

Elemental silicon is central to the vast industry of solid-state electronics. Appropriately doped with other elements, it forms a variety of semiconductors that constitute most transistors and integrated circuits. Other elements and compounds such as germanium or gallium arsenide have also found a niche as semicondnctors in electronics, but silicon occupies the prime position. How fortunate that it is the second most abundant element in the Earth s crust. Its compounds with the most abundant crustal element, oxygen, are equally central in many different aspects of chromatography. Silica, silica gel, glass, qnartz, fused silica, and silicones all have a remarkable variety of key roles to play in chromatography. Let us familiarize ourselves with some of their relevant properties. 

Oxygen is the most abundant element on the earth s surface. As the free element, it makes up 23% of the atmosphere by weight in combination with hydrogen, it makes up 89% of the hydrosphere and in combination with silicon and numerous other elements, it makes up nearly 50% of the minerals in crustal rocks. Combined with carbon, hydrogen, nitrogen, and lesser amounts of sulfur and... 

Silicon is the second most abundant element in the earth s crust, following only oxygen. Almost 95% of Ihe crustal rocks and materials that make up the earth contain silicon. It has been important to humans ever since flint, a silicon dioxide mineral, was first used to fashion tools and weapons by early civilizations. The Romans took advantage of silicon-containing minerals when they discovered how to use them to make concrete. Today, our computer industry depends on obtaining pure silicon for the manufacture of integrated circuits and semiconductors. 

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