allanite-ce crystal, rare earths, stephan wolfsried
Allanite-(Ce) crystal (credit: Stephan Wolfsried)
monazite-ce crystal, rare earths, stephan wolfsried
Monazite-(Ce) crystal (credit: Stephan Wolfsried)
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Rare Earth Mineral Resources

What distinguishes rare earth mineral deposits from commodities like copper or lead is the fact that the rare earths occur in a large variety of different minerals and geological settings and generally in very low concentrations. Even if their general crustal abundance is not low (a few tens ppm), the often very complex nature of mineralization (more than 200 rare earth bearing minerals known) makes the feasibility of extracting the REE from the ore an extremely challenging task. Since the know-how in this specialized field is scarce and concentrated in very few places the term “rare” in REE refers rather to the restricted availability of these metals in a usable form than to the number of deposits.

In general REE bearing minerals are found in the following primary and secondary geological settings:

  • Carbonatites (almost exclusively LREE)
  • Pegmatites
  • Hydrothermal veins (enriched in HREE)
  • Weathered deposits/Laterites (Ion adsorption clay)
  • Placers (mostly sands of marine origin)

The deposit with the world’s largest known REE resources is situated in Bayan Obo, Inner Mongolia, where three different types of ore are distinguished: Iron-REE ore, REE ore in dolomite and REE ore in silicate rock. Economically exploited minerals are bastnaesite and monazite. The genetic origin of the deposit is still a matter of debate.

Other considerable occurrences of rare earth mineral resources are related to alkaline igneous intrusive complexes and carbonatites such as Mountain Pass and Mount Weld. Main ore minerals in these deposits are bastnaesite, allanite, monazite and apatite and pyrochlore in the latter.

Of economical importance are also the REE laterites which are formed by weathering of REE-rich alkaline complexes. Apatite, pyrochlore and monazite are typical residual and crandallite group minerals typical secondary minerals formed during weathering. The deposits of Xunwu and Longnan in the Jiangxi Province in China, which form the weathered crust of a granitic rock are exploited for their ion adsorption clays which are rich in HREE.