Rare Earth Element

Rare Earth Element minerals (also referred to as “REE minerals”) are minerals that contain at least one of the fifteen chemical elements in the periodic table known as the lanthanides, as well as scandium (Sc) and yttrium (Y). Scandium and yttrium are not lanthanides but are considered to be rare earth elements because they tend to occur in the same ore deposits as the lanthanides and exhibit similar chemical properties. The lanthanide series of chemical elements comprises the fifteen metallic elements with atomic numbers from 57 (lanthanum) through 71 (lutetium). The fifteen lanthanides are (in order of atomic number starting from the lowest): 57 lanthanum (La), 58 cerium (Ce), 59praseodymium (Pr), 60 neodymium (Nd), 61 promethium (Pm), 62 samarium (Sm), 63 europium (Eu), 64 gadolinium(Gd), 65 terbium (Tb), 66 dysprosium (Dy), 67 holmium (Ho), 68 erbium (Er), 69 thulium (Tm), 70 ytterbium (Yb) and 71 lutetium (Lu). Click here for more detailed information on the lanthanides near the bottom of this page.

The term “rare earth element” is somewhat outdated as these elements are not particularily rare nor are they “earths”. “Earths” is an obsolete mineralogical term referring to “water-insoluble, strongly basic oxides of electropositive metals incapable of being smelted into metal” (using late 18th century technology). The REE elements certainly are not rare. Cerium is the most abundant lanthanide and is the 26th most abundant element in the Earth’s crust, about as abundant as copper and more so than tin. Neodymium is more abundant than gold. The scarcest of the lanthanides, thulium, is more abundant than arsenic or mercury and they are not considered to be rare. The “rare” in rare earth elements originally referred to them typically being widely dispersed in very small quantities throughout the Earth’s crust and very difficult to obtain in their pure form. Because lanthanides tend to congregate in the same minerals, the original isolation and identification of the lanthanides was very difficult and took well over a century of scientific development and discovery. It took over 100 years for all of the natrually occuring lanthanides to be discovered and over 150 years before the last lanthanide, the synthetic Promethium, would be “discovered”.

The lanthanides are named after the element lanthanum (La) because it is the first in the group because it has the lowest atomic number, 57, of the group. The name “lanthanide” was introduced by Swiss born mineralogist Victor Moritz Goldschmidt (1888-1947) in 1925. Victor Goldschmidt, together with Russian mineralogist and geochemistVladimir Vernadsky (1863-1945), were considered to be the founders of modern geochemistry and crystal chemistry. Goldschmidt was also the developer of the Goldschmidt Classification of elements. The name lanthanum comes from the Greek word λανθανω (lanthánein) meaning to lie hidden. This description may refer to the discovery of the element lanthanum as it was discovered “lying hidden” within a cerium bearing mineral that had previously been analysed. Many of the lanthanides were later discovered “lying hidden” within minerals that had previously been analysed.

The discovery of the lanthanides began in 1787 with the discovery of the mineral Ytterbite. Swedish chemist Carl Axel Arrhenius (1757-1824) found a dark mineral that he could not identify in a feldspar mine near Ytterby, on the island of Resarön, Sweden. Arrhenius named the black mineral Ytterbite after the nearby town of Ytterby. Arrhenius sent a sample of Ytterbite to Finish chemist Johan Gadolin (1760-1852) at Uppsala University, Finland, where he began a detailed analysis of it in 1794. He found it contained silica, alumina, iron oxide and an unknown “earth”. Gadolin’s results were confirmed in 1797 by Swedish chemist Anders Gustaf Ekeberg (1767-1813). Ekeberg suggested the name yttria (later called yttrium) for the oxide of the new earth metal. Gadolin is credited with the discovery of the element yttrium. The mineral Ytterbite was renamed Gadolinite in 1800 by German chemist Martin Heinrich Klaproth (1743-1817) in honor of Johan Gadolin. Klaproth discovered uranium, zirconium and cerium.

Yttrium was the first of the rare earth elements to be discovered from Ytterbite. But the yttrium discovered by Gadolin also contained several other rare earth metal oxides. Over the next century, several of the remaining lanthanides were extracted from Ytterbite (Gadolinite). The Swedish chemist Carl Gustaf Mosander (1797-1858) was responsible for discovering lanthanum, erbium and terbium. The mineral Mosandrite was named in his honor in 1841.

The remaining lanthanide elements, ytterbium, thulium, holmium, dysprosium, lutetium and promethium, were discovered separately over many years. In 1907 lutetium was the last of the naturally occuring lanthanides to be discovered. However, the existence of an element between neodymium (atomic number 60) and samarium (atomic number 62) was predicted by Czech chemist Bohuslav Brauner (1855-1935) in 1902. This was confirmed by English phyisicist Henry Moseley (1887-1915) in 1914. Finally in 1945, after many attempts by many scientists, firm evidence that element 61 had been isolated was produced by Charles D. Coryell, Lawrence E. Glendenin, Jacob A. Marinsky, and Harold G. Richter at the Oak Ridge National Laboratory, Oak Ridge, Tennessee. They produced element 61 by fission of uranium and neutron bombardment of neodymium in a graphite reactor. They named the new element Promethium after the Greek Titan Prometheus, who, according to Greek mythology, stole fire from Mount Olympus to give to mankind. The name was accepted in 1949 by the International Union of Pure and Applied Chemistry (IUPAC). Promethium is considered a synthetic element and does not occur naturally in the Earth’s crust. Promethium has been identified however, in the spectrum of the star HR465 in the Andromeda Galaxy approximately 780 kiloparsecs (2.5 million light-years; 2.4×10¹⁹ km) from Earth.