**1. Introduction**

The rare earth elements (REE) are the 14 elements comprising the lanthanide series: cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu) [1]. Lanthanum (La) is associated with rare earth elements because of its periodic table position. Frequently, scandium (Sc) and yttrium (Y) are grouped with the lanthanides given their similar chemical reactivity. The lanthanide series are elements characterized as having one or more electrons in the 4f electronic orbitals for their ground state configuration [1, 2]. Promethium is absent in the environment because promethium-145 (145Pm) decays *via* electron capture into neodymium-145 (145Nd), with a half-life of 17.7 years.

The typically trivalent rare earth elements have considerable ionic bonding character [1]. Cerium may have a valency of +3 or + 4, whereas europium may have a valency of +2 or + 3 [1]. The rare earth elements show a decrease in their ionic radii on progression from La to Lu, the so-called "lanthanide contraction". The "lanthanide contraction" supports greater chemical affinity for hydrolysis and chelate/complex stability with an increase in atomic number [1]. The LREE are the light rare earth elements, comprising of the elements from La to Eu, and the HREE are the heavy rare earth elements, comprising of the elements from Gd to Lu.

Rare earth element concentrations in soils, sediments, and other earth materials are dependent on their mineral assemblages, with rare earth element concentrations typically ranging from 0.1 to 100 mg kg−1. Scandium concentrations across the earth's crust are in the range from 16 to 30 mg Sc kg−1, with mafic and argillaceous materials showing greater scandium concentrations. Typical soil scandium concentrations are from 0.8 to 28 mg Sc kg−1. Yttrium concentrations average from 20 to 30 mg Y kg−1 across crustal materials, where yttrium concentrations in soil range from 7 to 200 mg Y kg−1 [3]. The Oddon-Harkins rule states that an element with an even atomic number has a greater concentration than the next element in the periodic table. The rare earth elements typically follow the Oddon-Harkin rule. The Post-Archean Australian Average Shale (PAAS), North American Shale Composite (NASC), and Upper Continental Crust (UCC) reflect the Oddon-Harkin rule (**Table 1**).

Kabata-Pendias [3] has compiled many studies observing REE abundances in mafic and felsic igneous rocks, sedimentary rocks (argillaceous, sandstones, and calcareous), and soils. Kabata-Pendias also compiled reference data on terrestrial plant species. Commonly occurring REE-bearing minerals include: (i) fluorite (Ce), (ii) allanite (Ce), (iii) sphene (REE), (iv) zircon (HREE), (v) apatite (LREE), (vi) monazite (Ce and La), (vii) xenotime (REE), (viii) rhabdophane (Ce and REE), and


*PAAS is Post-Archean Australian Average Shale, NASC is North American Shale Composite, and UCC is the upper continental crust. PAAS and NASC reported in McLennan [4, 5], K-P reported in Kabata-Pendias [3], and UCC reported in Rudnick and Gao [6].*

#### **Table 1.**

*REE abundances for various parent materials.*

#### *Evaluation of Rare Earth Element Mine Sites for Environmental Impact DOI: http://dx.doi.org/10.5772/intechopen.109161*

(ix) bastnaesite (REE) [4]. Mineral and soil assemblies typically show greater LREE concentrations than HREE; however, the LREE/HREE ratio may vary with zircon abundances. In a review, Van Gosen et al. [7] noted that many of the significant rare earth element deposits occur in carbonatites (carbonate igneous rocks). Peralkaline igneous systems, magmatic-magnetite–hematite bodies, and mafic gneiss-bearing xenotime-monazite deposits are also important sources for rare earth element extraction.

Van Gosen et al. [7], U.S. geological survey [8], Ramos et al. [9], Kim and Jariwala [10], and Van Veen and Melton [11] review the rare earth element available supply and known resources, concentrating on nations having substantial reserves: Australia, China, India, Malaysia, Russia, and the United States. China and Brazil have significant rare earth element ore extractions.
