rare earth element的定義

rare earth element = 稀土元素 = 稀土金屬

稀土金屬是元素周期表中鈧、釔和鑭系元素等化學元素的共同的俗稱。鈧和釔被列入稀土金屬的原因是它們往往與鑭系元素出現在同一礦床中（地球上自然里沒有的鑭系元素鉕一般不算稀土金屬）。

「稀土」中的「土」字實際上指的是氧化物。這些元素被發現時人們以為它們在地球上分佈非常少。實際上它們在地殼內的含量相當高，最高的鈰是地殼中第25豐富的元素，比鉛還要高。而最低的「稀土金屬」鑥在地殼中的含量比金甚至還要高出200倍。因此國際純粹與應用化學聯合會現在已經廢棄了「稀土金屬」這個稱呼。

稀土金屬的主要來源是氟碳鈰鑭礦、獨居石、鈰鈮鈣鈦礦和吸收離子的粘土。雖然稀土金屬非常豐富，但是比起其它過渡金屬來說它們比較難提煉，因此也比較昂貴。

"Rare earth elements" and "rare earth metals" are trivial names sometimes applied to a collection of sixteen chemical elements in the periodic table, namely scandium, yttrium, and fourteen of the fifteen lanthanides (excluding promethium), which naturally occur on the Earth. The former two are included as they tend to occur with the latter in the same ore deposits. Some definitions additionally include the actinides.

"Earth" is an obsolete term for oxide. At the time of their discovery, earths of these elements were believed to be scarce in abundance as minerals. However, the term "rare earth" is now deprecated by IUPAC, as these elements are in fact relatively abundant in the Earth's crust; the most abundant, cerium, at 68 parts per million, is the 25th most abundant element in the crust, more common than lead, while even the least abundant "rare" earth element, lutetium, is 200 times more abundant than gold.

The principal economic sources of rare earth elements are the minerals bastnasite, monazite, and loparite and the lateritic ion-adsorption clays. Despite their relative abundance, however, these are more difficult to mine and extract than the sources of transition metals (due in part to their very similar chemical properties), making them relatively expensive.

The following abbreviations are often used:

REE = rare earth elements
LREE = light rare earth elements (La-Sm)
HREE = heavy rare earth elements (Eu-Lu)