Rare Earth Element, yang diterjemahkan menjadi unsur tanah jarang adalah 17 unsur yang menyusun sistem periodik. Unsur ini tersusun atas Scandium (Sc)-Yttrrium (Y) dan 15 unsur lain dari grup lantanida, secara berturut-turut: Lanthanum (La)-Cerium (Ce)-Praseodymium (Pr)-Neodymium (Nd)-Promethium (Pm)-Samarium (Sm)-Europium (Eu)-Gadolinium (Gd)-Terbium (Tb)-Dysprosium (Dy)-Holmium (Ho)-Erbium (Er)-Thulium (Tm)-Ytterbium (Yb)-Lutetium (Lu). Pada tahun 1869, Mendeleev sudah berhasil menghitung masa atom dari unsur La-Ce, yang sebelumnya sudah diklasifikan sebagai logam tanah jarang.
La, Ce, Pr, Nd, Sm dan Eu umum disebut sebagai light REE (LREE), sedangkan sisanya disebut sebagai heavy REE (HREE), berdasarkan kenaikan massa atom pada tabel periodik dari kiri ke kanan. REE mempunyai karakteristik yang spesial karena kesemuanya memiliki kemiripan sifat, dimana seluruh konfigurasi elektron terluarnya sama. Prometium tidak ditemukan dalam batuan karena tidak stabil.
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REE occur as 3-valent ions (REE3+) with the exception of Ce which occurs as Ce4+ and Ce3+ and Eu as Eu2+ and Eu3+. While the 3-valent ions (such as Eu3+) are in general more difficult to incorporate into minerals, Eu2+ can be easily incorporated into plagioclase substituting Ca2+ of the same valence. Eu2+ occurs preferentially in magmas under reducing conditions. The ionic radii of the three-valent REE slightly decrease from Ce3+ (LREE) of ca. 1.02 Å to Lu3+ (HREE) of 0.80 Å. Thus, depending on the available space within a crystal structure, some minerals prefer incorporation of LREE whereas others are also able to build in HREE.
REE concentrations in rocks are compared to values characteristic of meteorites (values measured are multiplied by factor), which are called chondrite-normalized REE values. If the concentrations of REE in minerals and rocks are measured and chondrite normalized, occasionally a so-called Eu anomaly can be detected. It means that the Eu concentration is much different to the trend displayed by the other REE’s. Eu anomaly is called “negative” if Eu is depleted relative to the other REE’s. Depletion is generally attributed to Eu’s tendency to be incorporated preferably into plagioclase over other minerals. If a magma crystallizes plagioclase, most of the Eu will be incorporated into this mineral. If the rest of the magma gets separated from its plagioclase crystals and subsequently solidifies, the chemical composition of the final rock will display a negative Eu anomaly.
Eu anomalies can also be modified by fluid-rock interactions due to changes in oxidation state (such as oxygen fugacity of the present fluids) and change in formation conditions (such as increasing temperature) where trivalent Eu3+ is reduced to Eu2+.
SIMPLIFIED MODEL FOR THE EXPLANATION OF EU-ANOMALIES IN THE CONTINENTAL CRUST
1. REE-bearing rock forms magma (no Eu anomaly)
2. Accumulation of plagioclase and formation of anorthosite (positive Eu anomaly in rock)
3 Removal of remaining magma to higher levels of the Earth’s crust. Pegmatite derived from anorthosite is depleted in plagioclase (negative Eu anomaly)
4 Further recrystallization of minerals in the pegmatite distributes the remaining REE preferable into some minerals containing Ca2+.
sumber: http://www.gemresearch.ch/journal/No5/page34.htm
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