<strong>Rare</strong> <strong>Earth</strong> <strong>Elements</strong> <strong>Review</strong> Section 2 – Introduction to <strong>Rare</strong> <strong>Earth</strong> <strong>Elements</strong> minerals can contain any <strong>of</strong> the REEs (i.e., HREEs or LREEs), enrichment <strong>of</strong> specific REEs is variable and a function <strong>of</strong> 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 <strong>of</strong> these types <strong>of</strong> deposits. Deposits <strong>of</strong> phosphate rare earth ores provide the opportunity to produce co-products <strong>of</strong> phosphates and REEs. Thorium and uranium may also be taken advantage <strong>of</strong> and produced as a co-product, or may represent a significant management challenge. A further description <strong>of</strong> 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 <strong>of</strong> LREEs is due to the lower crystallization temperature and pressures <strong>of</strong> 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 <strong>of</strong> monazite; therefore, its crystalline structure more readily accommodates a higher ratio <strong>of</strong> 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 <strong>of</strong> granitic and gneissic rocks. Although not always present in significant quantities, uranium and thorium can also occur as constituents <strong>of</strong> 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) <strong>of</strong> 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 <strong>of</strong> 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 <strong>of</strong> other mineral production. A list <strong>of</strong> minerals that commonly contain REEs is presented in the USGS report by Long et al. (2010). The principal future domestic supply <strong>of</strong> 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 <strong>of</strong> pegmatite granites and related gneisses. The general occurrences <strong>of</strong> rock types that host REEcontaining mineral ores in the United States are shown in Figure 2-2. 2-4
<strong>Rare</strong> <strong>Earth</strong> <strong>Elements</strong> <strong>Review</strong> Section 2 – Introduction to <strong>Rare</strong> <strong>Earth</strong> <strong>Elements</strong> Figure 2-2. Map showing occurrences <strong>of</strong> REEs, by rock type (adapted from multiple sources, see Appendix B). 2-5
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