# Sources of Extraterrestrial Rare Earth Elements: To the Moon and Beyond
## 5 Lunar Resources
### Lunar REE-Bearing Minerals
While significant proportions of the KREEP REE budget are associated with merrillite, which can constitute up to 3% of a lithologies volume in the PKT, the amount of merrillite is unfortunately too small for economical extraction of metals on the Moon based on current understanding [96]. This of course does not rule out the discovery of future, more concentrated, deposits. From [121] Apollo 14 Quartz (whitlockite (merrillite) rich) Monzodiorite fragments (2-4 mm) in lunar soil has the highest REE concentrations of any lunar material reported to date (excluding merrillite separates). One 18.4-mg merrillite-rich particle (W-QMD 14161,7373) was found to contain 11% phosphates and 1% zircon. This particle exhibited a REE normalized pattern that resembled KREEP, but with a distinctly higher La/Yb ratio (4.78 vs. 3.38 (from 15386)), consistent with the presence of merrillite. The petrogenesis of this highly enriched REE sample was reconciled in a silicate-liquid immiscibility model after fractional crystallization involving merrilite. This model was also used to account for some of the observed geochemical trends in lunar granites [112; 113; 121]. KREEPy geochemical signatures are also present in impact melt breccias (IMBs) collected from the Apollo landing sites (most notably 14, 15, 16, and 17; [122]). These samples have an average (from each landing site) La (ppm) ranging from 21.2 to 87.4, Ce (ppm) from 55 to 224, Sm (ppm) from 9.92 to 38.5, and Lu (ppm) from 0.92 to 3.82 [122] but exhibit similar inter-element ratios: La/Lu: 22.8-23.0; Ce/Sm: 5.5-5.8; Ce/Lu: 58.6-59.8 which attest to their LREE enrichment. One other REE-bearing lunar phase, which is present in lower abundance relative to merrillite and apatite, is monazite. Until 2006 monazite had only been identified as small (10 \(\upmu\)m \(\times\) 3 \(\upmu\)m) inclusions in pyroxene from Apollo 11 basalts (sample 10047,68; Figure 6; [95]). Additional lunar monazites were reported in [119], where particles \(\sim\)0.5-3 \(\upmu\)m in size were identified in regolith from the Luna 24 samples. One striking feature of one of these newly identified monazites was the positive Eu-anomaly (Figure 6) and associated lack (below detection limit) of Th. This was attributed to crystallization from an aqueous solution during metasomatic alteration of Eu-rich plagioclase in primary mare basalts which contained REE-bearing phases (merrillite and apatite for example; [119]). In addition to monazite, merrillite, and apatite, the REE-rich phase yittrebotafite has been found in sample 14321,1494 (a clast-rich, crystalline matrix breccia known as \"Big Bertha\") and contains 17.67 wt. % Nb\({}_{2}\)O\({}_{3}\); 4.33 wt. % UO\({}_{2}\) (38,200 ppm U); 2.38 wt. % Nd\({}_{2}\)O\({}_{3}\) (20,420 ppm Nd); 12.00 wt. % Y\({}_{2}\)O\({}_{3}\) (94,500 ppm Y); and 12.81 wt. % REEs [123]. At the time of its identification in Apollo 11 and Apollo 12 basalts, the mineral tranquillityite was not only a new lunar silicate mineral, it was a new mineral. Initially, it was identified as \"A\" and as an \"unnamed yttrium-zirconium silicate\" in [124; 125] respectively. It is associated with interstitial phases (e.g., troilite), and occurs as thin laths in coarse-grained cristobalite [126]. Geochemically, it is enriched  
Figure 7: (**a**) SEM photograph of Apollo 10 basalt (10047): fluorapatite in pyrox ferrite, in contact with ilmenite. Upper left portion is epoxy; (**b**) SEM photograph of apatite crystal in Apollo 14 breccia vug; (**c**) SEM photograph of doubly terminated apatite crystal in Apollo 14 breccia vug. All images from [95]. in Zr (12-13 wt. %), Y (1.0-4.6 wt. %), and Nd (0.1-0.25 wt. %), and was not discovered on Earth until 2011 (in Australia, [127]). In order for a mineral deposit to be mined as an ore, the concentration of the desired element must be high enough for economic extraction, meaning that its initial abundance must also be high enough so that a differentiation process can concentrate it.