Magnesium volatile elements

A method has been reported for the determination of calcium, copper, iron, magnesium, potassium, sodium and zinc in cellulose [169]. The sample (10 g) was air-dried and then ashed at 575°C until all the carbon was removed. Hydrochloric acid (5 ml of 6M) was added to the residue and evaporated to dryness twice before taking up the sample in a third aliquot, diluting to 100 ml and aspiration into an air /acetylene flame. It is likely that volatile elements such as cadmium may be lost at such an elevated ashing temperature and temperatures below 500°C may be preferable. Alternatively wet ashing with nitric acid has been proposed for the determination of aluminium, cadmium, potassium and zinc in pressed boards [170] or sodium in gypsum glass board [171]. For the determination of lead in confection wrappers, the sample may be treated with concentrated nitric acid at 70—80°C and diluted for flame analysis [172]. In the full method, the wrapper was wiped clean with a damp tissue, cut up to 0.5 X 0.5 mm pieces and dried at 110°C (for paper, for plastic 80°C) for 1 h. The sample (0.5 g) was heated with concentrated nitric acid (1ml) at... 

Figure 3 Mean abundances of lithophile elements normalized to Cl chondrites and silicon arranged in order of increasing volatility in seven chondrite groups (Wasson and Kallemeyn, 1988). Refractories (elements condensing above V) are uniformly enriched in CO, CM, and CV chondrites and depleted in H, L,and EH chondrites. Moderately volatile elements, which condense below magnesium and silicon, are all depleted relative to Cl chondrites. These fractionations are related in poorly understood ways to the formation of CAIs and chondrules (reproduced by permission of The Royal Society from Phil. Trans. Roy. Soc. London, 1988, A325, p. 539).

The concentrations of four typical moderately volatile elements—manganese, sodium, selenium, and zinc—in the various classes of chondritic meteorites are shown in Figure 12, where elements are normalized to magnesium and CI-chondrites. Again there is excellent agreement between solar abundances and Cl-meteorites. A characteristic feature of the chemistry of carbonaceous chondrites is the simultaneous depletion of sodium and manganese in all types of carbonaceous chondrites, except Cl. Ordinary and enstatite chondrites are not or only slightly... 

In Figure 13 the Earth s mantle seems to extend the trend of the moderately volatile elements to lower abundances, at least for sodium, manganese, and zinc (zinc behaves as a lithophile element in the Earth s mantle (see Dreibus and Palme, 1996)). The elements lithium, potassium, and rubidium which are not plotted here, show similar trends. The carbonaceous chondrite trend of iron is not extended to the Earth, as most of the iron of the Earth is in the core. The magnesium abundance of the Earth shows a slightly different trend. If the core had 5% silicon (previous section) and if that would be added to the bulk Earth silicon, then the bulk Mg/Si ratio of the Earth would be the same as that of carbonaceous chondrites (Eigure 10) and the silicon abundance of the Earth s mantle in Figure 13 would coincide with the magnesium abundance. 

Figure 15(a) plots Cl and magnesium-normalized abundances of lithophile moderately volatile elements against their condensation temperatures. The trend of decreasing abundance with increasing volatility is clearly visible. As mentioned above only few elements can be used to define this trend most of the moderately volatile elements are siderophile or chalcophile and their abundance in the Earth s mantle is affected by core formation. 

For this review the Earth s composition will be considered to be more similar to carbonaceous chondrites and somewhat less like the high-iron end-members of the ordinary or enstatite chondrites, especially with regard to the most abundant elements (iron, oxygen, silicon, and magnesium) and their ratios. However, before reaching any firm conclusions about this assumption, we need to develop a compositional model for the Earth that can be compared with different chondritic compositions. To do this we need to (i) classify the elements in terms of their properties in the nebula and the Earth and (2) establish the absolute abundances of the refractory and volatile elements in the mantle and bulk Earth. 

As can be seen in Fig. 2-1 (abundance of elements), hydrogen and oxygen (along with carbon, magnesium, silicon, sulfur, and iron) are particularly abundant in the solar system, probably because the common isotopic forms of the latter six elements have nuclear masses that are multiples of the helium (He) nucleus. Oxygen is present in the Earth s crust in an abundance that exceeds the amount required to form oxides of silicon, sulfur, and iron in the crust the excess oxygen occurs mostly as the volatiles CO2 and H2O. The CO2 now resides primarily in carbonate rocks whereas the H2O is almost all in the oceans. 

The high temperatures of coal char oxidation lead to a partial vaporization of the mineral or ash inclusions. Compounds of the alkali metals, the alkaline earth metals, silicon, and iron are volatilized during char combustion. The volatilization of silicon, magnesium, calcium, and iron can be greatly enhanced by reduction of their refractory oxides to more volatile forms (e.g., metal suboxides or elemental metals) in the locally reducing environment of the coal particle. The volatilized suboxides and elemental metals are then reoxidized in the boundary layer around the burning particle, where they subsequently nucleate to form a submicron aerosol. 

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