Crust oxygen content

Aluminum is the most abundant metallic element in the Earth s crust and, after oxygen and silicon, the third most abundant element (see Fig. 14.1). However, the aluminum content in most minerals is low, and the commercial source of aluminum, bauxite, is a hydrated, impure oxide, Al203-xH20, where x can range from 1 to 3. Bauxite ore, which is red from the iron oxides that it contains (Fig. 14.23), is processed to obtain alumina, A1203, in the Bayer process. In this process, the ore is first treated with aqueous sodium hydroxide, which dissolves the amphoteric alumina as the aluminate ion, Al(OH)4 (aq). Carbon dioxide is then bubbled through the solution to remove OH ions as HCO and to convert some of the aluminate ions into aluminum hydroxide, which precipitates. The aluminum hydroxide is removed and dehydrated to the oxide by heating to 1200°C. 

Titanium is a reasonably common element which has been known for over 170 years. The average titanium content of the earth s crust is 0.63% by weight, which makes it the ninth most abundant element in the earth s crust, 20 times more abundant than carbon, and only outranked by oxygen, silicon, aluminum, iron, magnesium, calcium, sodium and potassium. It is only really in this century that elemental titanium has developed any industrial potential, partly because of difficulties associated with its refinement. 

Aluminum is the most abundant metallic element in the Earth s crust and, following oxygen and silicon, the third most abundant element. However, the aluminum content in most minerals is low, and the commercial source of aluminum, bauxite, is a hydrated, impure oxide, Al203-xH20, where x can range up to 3. The bauxite ore, which is red from the iron oxides it contains, is processed to obtain alumina, Al2Q3. 

Silicon is next to oxygen the most abundant element in the lithosphere the average content amounts about 30% by weight. Inorganic silicon compounds such as silica and silicates form the basis of most of the rocks forming the earth s crust. In the atmosphere, there is no silicon present, except as dust of cosmic and terrestrial origin. The silicon content in the hydrosphere, mainly in form of dissolved silica (silicic acid), is also very small. 

Oxygen is the most common and most important chemical element of the Earth s crust, but study of its geochemical evolution did not become possible until the development of methods of isotopic analysis. Numerous determinations of 8 0, i.e., the relative content of the heavy oxygen isotope compared to a generally accepted standard, have indicated the existence of considerable variations, which carry geologic information. Substantial enrichment of... 

Fig. 11.16 Eh, pH diagram for Cu calculated for the chemical conditions prevailing in the deep sea (after Glasby and Schulz 1999). Note that, at pH 8, the metastable form of Cu is Cu(OH)2 at an Eg, > +0.48 V. However, if the Eh, drops to +0.4 V corresponding to the Eh of seawater, the anionic species, CuC, becomes the more stable species. It is believed that the dominance of CuCl, in seawater in the oxygen minimum zone accounts for the low Cu contents in Co-rich Mn crusts.

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