Lanthanoids abundance

Erbium ranks about number 42 in abundance in Earth s crust. It is more common than bromine, uranium, tin, silver, and mercury. It occurs in many different rare earth minerals, naturally occurring lanthanoid mixtures. Some common sources of erbium are xenotime, fergusonite, gado-linite, and euxenite. 

Molybdenum never occurs free in nature. Instead, it is always part of a compound. In addition to molybdenite, it occurs commonly as the mineral wulfenite (PbMo04). Its abundance in Earth s crust is estimated to be about 1 to 1.5 parts per million. That makes it about as common as tungsten and many of the rare earth (lanthanoid) elements. In 2008, the largest producers of molybdenum in the world included the United States, China, Chile, Peru, and Canada. In the United States, molybdenum ores were found primarily in Colorado, Idaho, Nevada, and New Mexico. According to the U.S. Geological Survey (USGS), the value of the molybdenum from U.S. mines was 4.5 billion that year. 

Praseodymium is one of the more common lanthanoids. It is thought to occur with an abundance of about 3.5 to 5.5 parts per million in Earth s crust. It occurs primarily with the other rare earth elements in two minerals, monazite and bastnasite. 

Samarium is regarded as a relatively abundant lanthanoid. It occurs to the extent of about 4.5 to 7 parts per million in Earth s crust. That makes it about as common as boron and two other lanthanoids, thulium and gadolinium. 

Terbium is one of the rarest of the lanthanoids. ft ranks about 55th among the elements in Earth s cmst. It is about as abundant as molybdenum and tungsten, but more abundant than iodine, silver, and gold. 

Thulium is probably the rarest of the lanthanoid elements. Its abundance is estimated at about 0.2 to 1 part per million in Earth s crust. This still makes it more abundant than silver, platinum, mercury, and gold. 

The lanthanoids resemble each other much more closely than do the members of a row of t/-block metals. The chemistry of the actinoids is more complicated, and in addition, only Th and U have naturally occurring isotopes. Studies of the transuranium elements (those with Z > 92) require specialized techniques. The occurrence of artihcial isotopes among the /-block elements can be seen from Appendix 5 all the actinoids are unstable with respect to radioactive decay (see Section 24.9), although the half-lives of the most abundant isotopes of thorium and uranium ( Th and t = 1.4 x lO and 4.5 x 10 yr respectively) are... 

Despite the term traditionally applied to this group of elements, rare earths, their crustal abundance is not particularly low. Cerium ranks around 25 in the listing of all the naturally occurring elements, its abundance being similar to that of Ni or Cu [1]. Even the least abimdant lanthanoid elements, Tb, Tm, and Lu, are more abundant than Ag [2]. Because of their geo-chemical characteristics, however, the rare earth-containing minerals consist of mixtures of the elements with relatively low concentration of them [3]. Accordingly, the number of their exploitable deposits, mainly consisting of phosphates and fluoro-carbonates, is rather small [1,3]. 

Figure 23.1 shows the relative abundances of the second and third row fif-block metals. Compared with the first row metals (Figure 22.1), the abundances of some of the heavier metals are very low, e.g. Os, 1 x 10 ppm and Ir, 6 x 10 ppm Tc does not occur naturally. Yttrium and lanthanum are similar to the lanthanoids and occur with them in nature. The major yttrium and lanthanum ores are monazite (a mixed metal phosphate, (Ce,La,Nd,Pr,Th,Y... )P04) and bastndsite... 

On the other hand, two presumptions are responsible for the late development of organometallic chemistry with lanthanoids and actinoids. One is that rare-earth metals implies sparsity and high expenses as impediments for the use of these metals. However, the element concentrations in the continental crust (Figure 6.1) show that these elements are certainly very seldom compared to iron (abundance 43 200 ppm) yet the rarest, uranium and thulium, are far more common than the precious metals, e.g. silver, platinum, or frequently used transition metals such as palladium, rhodium or iridium. 

The elements from lanthanum (Z = 57) through lutetium (Z = 71) are variously called the lanthanide, lanthanoid, or rare earth elements. The rare earth elements are "rare" only relative to the alkaline earth metals (group 2). Otherwise, they are not particularly rare. Ce, Nd, and La, for example, are more abundant than lead, and Tm is about as abundant as iodine. The lanthanides occur primarily as oxides, and mineral deposits containing them are found in various locations. Large deposits near the California-Nevada border are being developed to provide oxides of the lanthanides for use as phosphors in color monitors and television sets. 

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