Rare Earth Elements: A Review of Production, Processing ...

Rare Earth Elements Review Section 2 – Introduction to Rare Earth Elements
minerals can contain any of the REEs (i.e., HREEs or LREEs), enrichment of specific REEs is
variable and a function of the temperature and pressure regime in which they formed. Monazite
commonly occurs in placer deposits; xenotime can occur along with monazite, but generally
occurs as a more minor constituent of these types of deposits. Deposits of phosphate rare earth
ores provide the opportunity to produce co-products of phosphates and REEs. Thorium and
uranium may also be taken advantage of and produced as a co-product, or may represent a
significant management challenge. A further description of these two minerals follows:
– Monazite is generally enriched with the LREEs cerium, lanthanum, and neodymium, but can
also contain HREEs, particularly yttrium (Ni et al., 1995). The predominance of LREEs is
due to the lower crystallization temperature and pressures of this mineral; however, it
typically contains more HREEs than bastnasite ore deposits. It occurs in acidic igneous rocks
(primarily pegmatites), metamorphic rocks, and some vein deposits. Monazite is resistant to
weathering and occurs in many placer deposits as the host rocks are eroded. Thorium may
also be associated with monazite in various amounts.
– Xenotime crystallizes under higher temperatures and pressures than those of monazite;
therefore, its crystalline structure more readily accommodates a higher ratio of HREEs
(terbium through lutetium, and yttrium) than is commonly found in monazite. It is primarily a
yttrium phosphate mineral and occurs as a minor constituent of granitic and gneissic rocks.
Although not always present in significant quantities, uranium and thorium can also occur as
constituents of xenotime.
� There are two other important REE-containing minerals in the United States (Long et al., 2010)
including:
– Euxenite which contains yttrium, erbium, and cerium. It is found mostly in placer deposits in
Idaho, and occurs as a tantaloniobates (e.g., minerals where Ta and Nb form the compound)
of titanium, rare earths, thorium, and uranium.
– Allanite is an epidote mineral and contains cerium, lanthanum, and yttrium. It occurs in
igneous, metamorphic, and hydrothermal environments and is disseminated in pegmatite or
occurs in vein deposits.
These five minerals are considered to represent the principal occurrences and the potentially more
significant REE reserves in the United States (USGS, 2010). However, many other minerals containing
REEs do occur, and deposits of these minerals could be found in the United States and prove to be viable
for mining. It is also not uncommon for REEs to be produced as a coproduct or byproduct of other
mineral production. A list of minerals that commonly contain REEs is presented in the USGS report by
Long et al. (2010).
The principal future domestic supply of REEs is one carbonatite formation in Mountain Pass, California.
Other common and potentially viable deposit types containing almost exclusively the two phosphate
REE-bearing minerals (monazite and xenotime) are most common as placer ores that originated from the
erosion of pegmatite granites and related gneisses. The general occurrences of rock types that host REEcontaining
mineral ores in the United States are shown in Figure 2-2.
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