Editing Lanthanide (section)

==Occurrence==
{{Main|Rare-earth element#Geological distribution}}
[[Samarskite]] and similar minerals contain lanthanides in association with the  elements such as [[tantalum]], [[niobium]], [[hafnium]], [[zirconium]], [[vanadium]], and [[titanium]], from [[Group 4 element|group 4]] and [[Group 5 element|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.<ref name="autoRefA" />  

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.<ref>Rocks & Minerals, A Guide To Field Identification, Sorrell, St Martin's Press 1972, 1995, pp 118 (Halides), 228 (Carbonates)</ref> Other lanthanide-bearing minerals include [[bastnäsite]], [[florencite]], chernovite, [[perovskite]], [[xenotime]], [[cerite]], [[gadolinite]], [[lanthanite]], [[fergusonite]], [[polycrase]], [[blomstrandine]], [[Håleniusite-(La)|håleniusite]], miserite, [[loparite]], [[Lepersonnite-(Gd)|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|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, [[feldspar]]s and [[garnets]] are also known to exist.<ref>Minearls of the World, Johnsen, 2000</ref>

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: [[Yttrium(III) oxide|yttria]] (1794) and [[Cerium(IV) oxide|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 (<sup>138</sup>La, <sup>147</sup>Sm and <sup>176</sup>Lu) that can be used to date minerals and rocks from Earth, the Moon and meteorites.<ref>There exist other naturally occurred radioactive isotopes of lanthanides with long half-lives (<sup>144</sup>Nd, <sup>150</sup>Nd, <sup>148</sup>Sm, <sup>151</sup>Eu, <sup>152</sup>Gd) but they are not used as chronometers.</ref>  [[Promethium]] is effectively a [[Synthetic element|man-made element]], as all its isotopes are radioactive with half-lives shorter than 20&nbsp;years.