Haley of the Mining Engineering Journal reported that critical rare-earth element (REE) data on Earth and the Moon, including the geopolitics of developing national reserves from potential deposits versus developing such resources from the Moon in the Oceanus, which is at least two decades into the future.
Rare Earths and 21st Century Technology
REEs have developed new importance, particularly in the last 20 years, because they are vital for their [existing and] evolving functionality. The REEs have special properties used in a broad range of technical applications, hence they are presently extremely sought after. Once purified, REE are processed into oxides ( cerium, Ce), phosphors (europium, Eu; terbium, Tb; yttrium, Y), metals, alloys, catalyzer magnets (Ce; lanthanum, La) and optical display screens that take advantage of these elements' unique magnetic, luminescent, or electrochemical properties.
Recycling REEs has just begun to take hold [albeit attempting to recycle], but it is not yet profitable worldwide. Furthermore, rare earths are perceived to be a supply risk, as China controls more than 86 percent of mined and produced REEs on Earth as of 2020.
Rare Earths Abundance
It is [or may be] helpful to examine several indicators for REEs:
1) abundance in chlorine (Cl)-type carbonaceous chondrites (meteorites), a measurement of REE abundance in the solar system (Lodders, 2003),
2) crustal abundance on Earth and
3) abundance in bulk silicate Earth (BSE).
Abundances for comparative non-REE elements, lithium (Li) and uranium (U), two commonly mined on Earth, are other instructive data points, as well as estimated abundance for the lunar Procellarum KREEP terrane (PKT). low numbers for all REEs ( under 1.0 ppm), except scandium at 5.83 and yttrium at 1.53.
Comparatively, lithium is 1.46 and uranium is scarce at 0.01. Crustal abundance measures the concentration of elements in the Earth's continental crust
in milligrams per kilogram (mg/kg) or ppm. Of all the 17 REEs, cerium is the most abundant in the Earth's crust at 66.5 ppm, followed by neodymium at 41.5 ppm, lanthanum at 39 ppm, yttrium at 33 ppm and scandium at 22 ppm. All were under 100 ppm, with the lowest abundances for lutetium at 0.80 ppm and thulium at 0.52 ppm. In comparison, the two non-REEs, lithium and uranium, are measured at approximately 50 ppm and 2.73 ppm.
The estimated average abundance of all REEs in the Earth's crust, which ranges from about 150 to 220 ppm, exceeds that of other metals mined on an industrial scale, such as copper at 55 ppm and zinc at 70 ppm. Unlike most commercially mined base and precious metals, however, REEs [as indidual deposits] rarely concentrate into mineable ore deposits.
Another indicator of REE abundance is bulk silicate Earth, silicon (Si)/oxygen (0) minerology, also measured in ppm. BSE is the present-day mantle; Earth's mantle composition after subtracting the crust. For lanthanum, the concentration is 469 ppm; other REEs in the series have the highest abundances, such
as cerium at 1,274 ppm and neodymium at 953 ppm. Gadolinium measured 426 ppm, samarium (Sm) 317 ppm, praseodymium 194 ppm and europium, lowest at 121 ppm. REEs are far more abundant in the BSE than the comparative non-REEs. For example, lithium measures 1.6 ppm, while uranium measures 15 ppm.
The author asked whether it is wise to plan on rare-earth mineral mining on the Moon in the PKT? It is unlikely that REE extraction will happen in the near future - a projected timeline could involve decades. Since transportation of materials payloads to the Moon from Earth would cost $20,000 to $30,000/kg, [pure speculation to make such guesses of what costs could be decades away....] REE resource mining would only be economically beneficial if extraction and utilization occurred on the Moon, for the lunar technology and energy required to go beyond an outpost in Oceanus Procellarum to a functioning base.[Again, short-sighted rhetoic becasue advances will be made as we settled in to actual exploration on the Moon] Bringing REEs [aka REE concentrates?] back to Earth is not a viable option while there are still so many untapped potential deposits worldwide. [More hypobolic statements without foundation]. Increasing demand for REE-dependent technology, supply-risk criticality and geopolitics will continue to raise REE prices until scarcity is replaced by economically productive new reserves and efficient global REE recycling.
Mining operations on the Moon would certainly be far riskier than on Earth [Even more hperbolic statements from author, who is not thinking of the future, only now, which, of course, it is currently riskier]. Extremely difficult technology challenges include energy-intensive approaches, robotic labor, heavy-equipment damage, a lunar environment filled with glass-rich dust, the vacuum of space, temperatures averaging between -170 °C at night and 125 °C by day, low gravity at 1.6 m/s2 versus 9.81 m/s2 on Earth, exposure to radiation from cosmic rays and solar flux, and the cumulative effects of micrometeoroid hits [Of course these issues have to be confronted before mining of any kind can be performed on the Moon].
Modeling shows that over an area of about 150 m2, one micrometeoroid greater than 0.5 mm in diameter per year with an impact velocity of about 13 km/s would excavate a crater about 1.8 mm in diameter at a depth greater than 1 mm. The problems of such dangerous operating conditions on the lunar surface must be solved before any mining can begin [The authors is harping on the obvious].
These significant hazards to lunar structures, base supports, processing plants and instruments pose daunting problems for REE mining [Of course !].The risks should be assessed, quantified and mitigated [Of course !] Only ground-truth sampling via landers and rovers can give space resource scientists the additional data required to accelerate REE use in the PKT [Of Course !] Similarly, it is imperative that extraction of lunar rare earth elements in situ be used in situ [A process to minimize dust and other risks would be required, of course!].
[See earlier, less hyperbolic publications:
https://www.i2mconsultants.com/downloads/corp/SpaceRace04172010.pdf
2013 AAPG Energy Minerals Division Memoir 101: http://www.i2mconsultants.com/downloads/corp/Memoir101-T0fC2016.pdf
Of sprecial interest, Chapter 9: http://www.i2mconsultants.com/downloads/corp/Memoir101-CHAPTER09Rev.pdf
See Summary of REE activities on Earth: https://i2mportal.com/search_resource.php?search_value=Rare+Earth&sort=relevance#page=1
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