Earth composition

A limited number of rare-earth minerals are mined for large-scale rare-earth production mona2ite, bastnaesite, loparite [12173-83-OJ, xenotime [13817-22-6]. In addition, siace the 1980s rare-earth-containing clays called ionic ore are mined ia China. Table 4 shows the rare-earth composition of typical mineral concentrates. 

The rare earth composition of commercial electrodes is also related to electrode corrosion. This was noted by Sakai et. al. [44], who found that the presence of Nd or Ce inhibited corrosion when substituted in part for La in La, fZt(NiCoAl)5 (Z = Ce or Nd) electrodes. However no explanation for the effect was noted. Willems [22] prepared an electrode of La0XNd02Ni25Co24 Si0l which retained 88% of its storage capacity after 400 cycles. He attributed its long cycle life to a low VH of 2.6 A3. 

Since the early days of Goldschmidt or Vernadsky, geochemistry has become a mature science which now plays a central role in the Earth Sciences. More particularly, it has evolved considerably over the last fifty years. From an analytical approach with a goal of establishing the chemistry of the Earth compositions of rocks, soils, water, crust and mantle, geochemistry has become an explanatory science. 

The LaRochelle capacity for ore cracking is over 5000 tons as rare earths oxides per year and in Table VIII we show the average production capabilities for the heavier rare earths. It should be noted that the above figures are average values as the rare earths composition of monazite varies somewhat depending on the source. 

Wolf R. and Anders E. (1980) Moon and Earth compositional differences inferred from siderophiles, volatiles, and alkalis in basalts. Geochim. Cosmochim. Acta 44, 2111-2124. 

As regards the rock-forming elements, the bulk composition of the Earth is basically chondritic (i.e., solar) with approximately equal abundances of magnesium, sihcon, and iron atoms. In detail, however, there are some variations in chemistry among chondritic meteorites, and from a detailed comparison with meteorites it is concluded that the bulk Earth composition has similarities with the chemical composition group of carbonaceous chondrites. 

Hart S. R. and Zindler A. (1986) In search of a buUc-earth composition. Chem. Geol. 57, 247-267. 

The formation of basalts by partial melting of the upper mantle at mid-oceanic ridges and hot spots provides the opportunity to determine mantle composition. Early studies of radiogenic isotopes in oceanic basalts (e.g., Eaure and Hurley, 1963 Hart et al, 1973 Schilling, 1973) showed fundamental chemical differences between OIBs and MORBs (see Chapter 2.03). This led to the development of the layered mantle model, which consists essentially of three different reservoirs the lower mantle, upper mantle, and continental cmst. The lower mantle is assumed primitive and identical to the bulk silicate earth (BSE), which is the bulk earth composition minus the core (see also Chapters 2.01 and 2.03). The continental cmst is formed by extraction of melt from the primitive upper mantle, which leaves the depleted upper mantle as third reservoir. In this model, MORB is derived from the depleted upper mantle, whereas OIB is formed from reservoirs derived by mixing of the MORB source with primitive mantle (e.g., DePaolo and Wasserburg, 1976 O Nions et al., 1979 Allegre et al., 1979). 

Many elements are present in the core at low-concentration levels (<1 wt.%). Some of the minor and trace elements may hold critical information on the origin, evolution, and current processes in the Earth s interior. A few examples are given below, where experimental data on minor and trace elements provide important constraints on the thermal state of the Earth, bulk Earth composition, and differentiation of the core. 

The compositions of the planets in the solar system and those of chondritic meteorites provide a guide to the bulk Earth composition (see Chapter 2.01). However, the rich compositional diversity of these bodies presents a problem insofar as there is no single meteorite composition that can be used to characterize the Earth. The solar system is compositionally zoned planets with lesser concentrations of volatile elements are closer to the Sun. Thus, as compared to Mercury and Jupiter, the Earth has an intermediate uncompressed density (roughly a proportional measure of metal to rock) and volatile element inventory, and is more depleted in volatile elements than CI-chondrites, the most primitive of all of the meteorites. 

Developing a model for the composition of the Earth and its major reservoirs can be established in a four-step process. The first involves estimating the composition of the silicate Earth (or primitive mantle, which includes the crust plus mantle after core formation). The second step involves defining a volatility curve for the planet, based on the abundances of the moderately volatile and highly volatile lithophile elements in the silicate Earth, assuming that none have been sequestered into the core (i.e., they are truly lithophile). The third step entails calculating a bulk Earth composition using the planetary volatility curve established in step two, chemical data for chondrites, and... 

Data for the content of lithophile elements in the Earth plus knowledge of the iron content of the mantle and core together establish a bulk Earth compositional model (McDonough, 2001). This model assumes chondritic proportions of Fe/Ni in the Earth, given limited Fe/Ni variation in chondritic meteorites (see below). This approach yields... 

The bulk Earth compositions of Rb-Sr and Th-U-Pb are less well known, complicated by the... 

Pb- Pb/ ° Pb space. However, balancing the bulk Earth composition to that line has been problematic. Details of these complications are beyond the scope of this contribution, however, there are extensive discussions in the literature. 

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