stylistic:style_transfer
Occurrence

Samarskite and similar minerals contain lanthanides in association with the elements such as tantalum, niobium, hafnium, zirconium, vanadium, and titanium, from group 4 and group 5, often in similar oxidation states. Monazite is a phosphate of numerous group 3 + lanthanide + actinide metals and mined especially for the thorium content and specific rare earths, especially lanthanum, yttrium and cerium. Cerium and lanthanum as well as other members of the rare-earth series are often produced as a metal called mischmetal containing a variable mixture of these elements with cerium and lanthanum predominating; it has direct uses such as lighter flints and other spark sources which do not require extensive purification of one of these metals.

There are also lanthanide-bearing minerals based on group-2 elements, such as yttrocalcite, yttrocerite and yttrofluorite, which vary in content of yttrium, cerium, lanthanum and others. Other lanthanide-bearing minerals include bastnäsite, florencite, chernovite, perovskite, xenotime, cerite, gadolinite, lanthanite, fergusonite, polycrase, blomstrandine, håleniusite, miserite, loparite, lepersonnite, euxenite, all of which have a range of relative element concentration and may be denoted by a predominating one, as in monazite-(Ce). Group 3 elements do not occur as native-element minerals in the fashion of gold, silver, tantalum and many others on Earth, but may occur in lunar soil. Very rare halides of cerium, lanthanum, and presumably other lanthanides, feldspars and garnets are also known to exist.

The lanthanide contraction is responsible for the great geochemical divide that splits the lanthanides into light and heavy-lanthanide enriched minerals, the latter being almost inevitably associated with and dominated by yttrium. This divide is reflected in the first two "rare earths" that were discovered: yttria (1794) and ceria (1803). The geochemical divide has put more of the light lanthanides in the Earth's crust, but more of the heavy members in the Earth's mantle. The result is that although large rich ore-bodies are found that are enriched in the light lanthanides, correspondingly large ore-bodies for the heavy members are few. The principal ores are monazite and bastnäsite. Monazite sands usually contain all the lanthanide elements, but the heavier elements are lacking in bastnäsite. The lanthanides obey the Oddo–Harkins rule – odd-numbered elements are less abundant than their even-numbered neighbors.

Three of the lanthanide elements have radioactive isotopes with long half-lives (La, Sm and Lu) that can be used to date minerals and rocks from Earth, the Moon and meteorites. Promethium is effectively a man-made element, as all its isotopes are radioactive with half-lives shorter than 20 years.