Rare Earth Elements Defined
REE are a group of 17 chemicals that occur together in the periodic table. This group of rare earth metals has similar physical and chemical properties and consist of lanthanide elements plus scandium and yttrium.
Sixteen REE occur in nature, however, promethium (Pm) is highly-unstable and does not occur naturally on earth. Rare earth elements received their name from perceived scarcity during the 18th and 19th centuries and from ‘earth’ as an obsolete term for oxide.
REE are actually relatively common within the earth’s crust. Even the two least abundant REE, thulium (Tm) and lutetium (Lu) are nearly 140 to 180 times more common than gold. However, economically mineable concentrations of REE are not common. REE range in crustal abundances from cerium (25th most abundant of the 78 common elements in the earth's crust at 60 parts per million, or ppm, to thulium and lutetium (least abundant REE at about 0.5 ppm). Estimates of crustal abundances vary depending on the reference source.
REE are subdivided into Light Rare Earth Elements (LREE) and Heavy Rare Earth Elements (HREE) based on their atomic weight (table 2). LREE include the elements lanthanum (atomic number 57, atomic weight 138.91) through europium (atomic no. 63, atomic wt. 151.96) while HREE are gadolinium (atomic no. 64, atomic wt. 157.25) through lutetium (atomic no. 71, atomic wt. 174.97).