Oxygen crustal abundance

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... 

Like the other alkali metals, cesium is a soft, silvery metal, but it appears golden if it has been exposed to small amounts of oxygen. It is not found in its metallic state in nature it is obtained as a byproduct of lithium processing of the mineral lepidolite. Its most significant ore is pollucite, and the world s largest pollucite deposit is found in Bernic Lake, Manitoba, Canada. Cesium s average crustal abundance is about 3 parts per million. Cesium is the most electropositive stable element and will ignite if exposed to air. Cesium burns blue in the flame test. 

In addition to its presence as the free element in the atmosphere and dissolved in surface waters, oxygen occurs in combined form both as water, and a constituent of most rocks, minerals, and soils. The estimated abundance of oxygen in the crustal rocks of the earth is 455 000 ppm (i.e. 45.5% by weight) see silicates, p. 347 aluminosilicates, p. 347 carbonates, p. 109 phosphates, p. 475, etc. 

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. 

Fig. 2. Periodic table indicating relative abundance and biological importance of elements. Highlighted elements are the 30 most abundant crustal elements with concentrations ranging from 46 % for oxygen to 0.1 yg/g for selenium. Bold faced elements have known biological functions with indicating limited data and controversy. Data taken from Wood and Wang, 1983.

A critical factor in metal homeostasis is the bioavaUabifity of the specific metal. The bioavailability of a metal can be distinct from the overall abundance of the metal in the environment. For example, iron is the fourth-most abundant element in the Earth s crust. However, in the current-day oxygen-rich atmosphere of Earth, iron is largely present in the ferric (Fe +) form. In aqueous, aerobic environments at neutral or basic pH, ferric iron forms nearly insoluble iron hydroxides. Because of the insolubility of Ee +, iron is one of the least bioavailable of the essential transition metals. The concentration of iron in seawater (3 X 10 ppm) is nine orders of magnitude lower than the crustal concentration (5 x 10 ppm). In contrast, zinc is... 

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. 

The oxygen isotope geochemistry of relatively primitive Icelandic lavas is more controversial. There is abundant evidence that basalts and basaltic andesites from this setting are often (although not ubiquitously) 0-depleted compared to MORBs (Hemond et al. 1993), but there is no consensus as to whether this is because most such lavas have undergone small amounts of crustal contamination known to produce profound 0-depletions in more evolved lavas or instead because the mantle beneath Iceland contains one or more 0-depleted components. It is this author s prejudice that... 

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. 

---