The rare earth elements (REE) include 15 elements with atomic numbers 57 through 71, and are named: lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. Yttrium (atomic no. 39) and scandium atomic no. 21) are sometimes included in the same group.
The lanthanides elements have similar radii and oxidation states, which allow for number of substitution into various crystal lattices causing a wide dispersion in the earth’s crust and generating multiple occurrences of REE’s.
Within the fifteen chemical elements of the lanthanides, we differentiate the Light Rare Earth (LREE: atomic number 57 to 62) and the ‘Heavies’ (HREE: atomic number 63 to 71). These categories are defined based on their ionic radii and their atomic number, which is known as the 'lanthanide contraction'. This makes the chemistry of lanthanides predominantly ionic and the HREE have a higher affinity to Yttrium, being chemically associated. The rarity of HREE is not explained by their content in rocks but by the difficulty to separate them from Yttrium and to finely get the desired oxide powder.
The elements scandium (21), yttrium (39), lanthanum (57) and thorium (90) usually occur with the rare earths and are commonly classed with them.
The rare earths are not particulary rare, being a family of metals with atomic numbers 58 to 71. More than 100 minerals are known to contain one or more of the rare lanthanides. Only two of these minerals, monazite* and bastnasite**, are commercially important.
The challenge with rare earths is their natural concentration to form a deposit and then the separation of rare earth elements. A mineral like monazite may contain half a dozen or more different rare earth elements.
The most common lanthanides in these minerals are: cerium, lanthanum, neodymium, and praseodymium. Monazite mineral also contains thorium and yttrium
*Monazite: reddish-brown phosphate mineral containing rare earth (Ce, La, Th, Nd, Y)PO4
**Bastnasite : carbonate fluoride minerals with chemical formula (Ce, La, Y)CO3F