Abundance of Elements in Crustal Rocks

Table 13.1 Abundances of elements in crustal rocks (g tonne )...

Sodium, 22 700 ppm (2.27%) is the seventh most abundant element in crustal rocks and the fifth most abundant metal, after Al, Fe, Ca and Mg. Potassium (18 400 ppm) is the next most abundant element after sodium. Vast deposits of both Na and K salts occur in relatively pure form on all continents as a result of evaporation of ancient seas, and this process still continues today in the Great Salt Lake (Utah), the Dead Sea and elsewhere. Sodium occurs as rock-salt (NaCl) and as the carbonate (trona), nitrate (saltpetre), sulfate (mirabilite), borate (borax, kemite), etc. Potassium occurs principally as the simple chloride (sylvite), as the double chloride KCl.MgCl2.6H2O (camallite) and the anhydrous sulfate K2Mg2(S04)3 (langbeinite). There are also unlimited supplies of NaCl in natural brines and oceanic waters ( 30kgm ). Thus, it has been calculated that rock-salt equivalent to the NaCl in the oceans of the world would occupy... 

Chlorine is the twentieth most abundant element in crustal rocks where it occurs to the extent of 126 ppm (cf. nineteenth V, 136 ppm, and twenty-first Cr, 122 ppm). The vast evaporite deposits of NaCl and other chloride minerals have already been described (pp. 69, 73). Dwarfing these, however, are the inconceivably vast reserves in ocean waters (p. 69) where more than half the total average salinity of 3.4 wt% is due to chloride ions (1.9 wt%). Smaller quantities, though at higher concentrations, occur in certain inland seas and in subterranean brine wells, e.g. the Great Salt Lake, Utah (23% NaCl) and the Dead Sea, Israel (8.0% NaCl, 13.0% MgCU, 3.5% CaCU). 

Figure 1.14 shows a typical distribution for the geochemically abundant elements in crustal rocks. It could be seen that the proportion of the volume of material available for exploitation increases in geometrical progression as grade falls in arithmetical progression. In a sense, therefore, there is no finite limit to the availability of such elements, however, dilution with host rock implies that revenue would be insufficient to cover the fixed cost of extraction. 

Chlorine Chlorine is a toxic, reactive, greenish-yellow gas that must be handled with great care. Though chlorine is only the twentieth most abundant element in crustal rocks, there are vast additional amounts of chloride ion in the world s oceans (1.9% by mass). The free element was first prepared in 1774 by oxidation of NaCl with Mn02, although not until 1810 was it recognized that the product of the reaction was indeed a new element rather than a compound. 

Table 7-lb. Average Absolute and Relative Abundances of Major Elements in Crustal Rock, Soil, and Shale-, Relative Abundances of Elements in Fly Ash from Coal and Fuel-Oil Combustion and Relative Abundances of Major Elements in the Remote Continental Aerosol, with Enrichment Factors (Aerosol) EF= (X)/(AI)aeroso,/(X)/(AI)crusta, rock ... 

TABLE III Abundance of Groups 13-16 (IIIA-VIA) Elements in Crustal Rocks... 

Germanium and Sn appear about half-way down the list of elements in order of abundance in crustal rocks, together with several other elements in the region of l-2ppm ... 

Phosphorus is the eleventh element in order of abundance in crustal rocks of the earth and it occurs there to the extent of 1120 ppm (cf. H 1520 ppm, Mn 1060 ppm). All its known terrestrial minerals are orthophosphates though the reduced phosphide mineral schrieber-site (Fe,Ni)3P occurs in most iron meteorites. Some 200 crystalline phosphate minerals have been described, but by far the major amount of P occurs in a single mineral family, the apatites, and these are the only ones of industrial importance, the others being rare curiosities. Apatites (p. 523) have the idealized general formula 3Ca3(P04)2.CaX2, that is Caio(P04)6X2, and common members are fluorapatite Ca5(P04)3p, chloroapatite Ca5(P04)3Cl, and hydroxyapatite Ca5(P04)3(0H). In addition, there are vast deposits of amorphous phosphate rock, phosphorite, which approximates in composition to fluoroapatite. " These deposits are widely... 

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. 

Fluorine is the thirteenth element in order of abundance in crustal rocks of the earth, occurring to the extent of 544 ppm (cf. twelfth Mn, 1060 ppm fourteenth Ba, 390 ppm fifteenth Sr, 384 ppm). The three most important minerals are... 

Estimates of the abundance of zinc in the sun, in meteorites, in the Earth s core and crust, and in the oceans are very difficult to make, but its abundance in the Earth s crustal rocks and soils is of the order of 100 ppm, about 1000 times as abundant as its congeners cadmium and mercury. All three elements are Chalcophiles so that, in the reducing atmosphere that prevailed when the earth s crust solidified, they were deposited in the sulfide phase giving rise to the sulfide ores, their most important source. Eater, as weathering took place, zinc became soluble only to be precipitated as the carbonate, silicate, or phosphate. 

Iron is the most abundant transition element in the Earth s crust and, in general, in all life forms. An outline of the distribution of iron in the Earth s crust is shown in Table 1.2. As can be seen, approximately one-third of the Earth s mass is estimated to be iron. Of course, only the Earth s crust is relevant for life forms, but even there it is the most abundant transition element. Its concentration is relatively high in most crustal rocks (lowest in limestone, which is more or less pure calcium carbonate). In the oceans, which constitute 70 percent of the Earth s surface, the concentration of iron is low but increases with depth, since this iron exists as suspended particulate matter rather than as a soluble species. Iron is a limiting factor in plankton growth, and the rich... 

One third of Earth s mass, most abundant element by weight Distribution in crustal rocks (weight %) igneous 5.6... 

As the twelfth most abundant element in the earth s crust (0.106% in crustal rocks), manganese is found in over 250 different minerals of which some 10-15 are of commercial importance. In primary hydrothermal deposits the metal occurs as silicates, but as it is readily depleted from igneous and metamorphic rocks by weathering, particularly under acid conditions, it is normally found in commercially useful ores as oxides or carbonates, deposited, as are iron and aluminum, under alkaline conditions [9]. 

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