Major Lunar Minerals
Before we discuss materials processing in Chapter 3, we need to discuss the minerals we are mining.
As always, if this section gets too technical for you, please skip it and continue on with the following one.
In a planetary crust, mines for the base metals like iron and aluminum do not dig out pure iron or aluminum from the ground. For example, for aluminum they dig out minerals whereby atoms of aluminum are bonded to atoms of oxygen and silicon, called "silicates". This material must be processed by heat, chemicals and/or electrical current to separate the metal from its oxide or silicate. The industrial facility to process the material is often called a "smelter".
For example, the lunar highlands mineral "anorthite" is similar to the ore "bauxite" from which aluminum is produced on Earth. Anorthite is a mineral consisting of aluminum (chemical symbol Al), calcium (Ca), silicon (Si) and oxygen (O), with a chemical formula of CaAl2Si2O8. The smelter's job is to split all that up to produce pure aluminum metal, and optionally calcium metal, free oxygen, "silica" glass (SiO2), and perhaps pure silicon. Alternatively, anorthite could be processed to produce ceramics like "calcia" (CaO, aka "lime") and "alumina" (Al2O3) instead of the metals, or silica glasses with various properties depending upon the metal oxides existing in the final glass product and any other impurities added.
Metals are generally found as metal oxides. (When manmade pure iron rusts, it is returning to its natural oxidized state.) These metal oxides usually bond to silica to produce various minerals, though sometimes they can be found in their metal oxide state without silica. For example, magnesium oxide combines with silica to make a greenish mineral called "olivine": MgO + MgO + SiO2 = Mg2SiO4.
Average composition of Apollo and Luna samples:
ppm is parts per million.
A caveat of the above table is that it represents only average compositions for all samples taken at each site. There were significant variations from sample to sample at each site. Some rocks were almost purely of one mineral.
Chapter 3 includes a section on mechanical, electrical and/or magnetic techniques called "beneficiation" to roughly separate bulk lunar regolith into its component minerals. NASA studies into lunar materials utilization using Apollo soils which are then beneficiated have come up with the following feeds for a materials processing facility (after bulk beneficiation).
Beneficiation of two Apollo lunar samples:
Chemical analysis in weight parts per million, extracted from NASA SP-428.
There are many other minerals which occur in lunar material in lower abundance, and some are exotic, but a more expansive coverage of this topic is beyond the scope of this book. The above is intended mainly to give the reader an understanding of the basics of lunar materials so that they understand the main issues related to lunar resources and materials processing.
It has also been speculated that there are probably beds of nearly pure ilmenite, anorthite and other minerals which would not need to be beneficiated to produce the above or better purity of minerals. These are much more likely to exist a few meters down, under the crater splashed surface.
The reader can now understand that oxygen is the most abundant element on the Moon even though there is no air on the Moon. Making air to breathe is no problem (within an enclosed capsule, of course). The reason oxygen is the most abundant element is that it bonds to so many things. Since oxygen-bonded minerals are lightweight, they float up to form the crust of a planet. On the other hand, metals like nickel, gold and platinum stay shiny because they don't like to bond to oxygen. For that reason, they usually sink to the core of a planet and hence they're rare in the crust and precious to surface dwellers.
In some of the tables and in the following text, the names of additional minerals are mentioned. There's no need to memorize anything, as I redundantly remind the reader what the minerals are, and keep clear what the main points of the discussion are.
Aluminum (spelled/pronounced "aluminium" by non-Americans) is a particularly interesting lunar resource. It's a good electrical conductor, indeed the most widely used conductor material on Earth, even more than copper. It's a lightweight structural material, which helps when building large structures rotating for artificial gravity. Aluminum mirrors are good reflectors and could compete with those plated from asteroidal nickel. Atomized aluminum powder also makes a good fuel when burned with oxygen. Indeed, it's the fuel source of the Space Shuttle's solid boosters. On the Moon, it could become the primary fuel source for chemical rocketry of material to and from orbit (though we would need a different kind of rocket since the Space Shuttle solid boosters use aluminum in a kind of rocket we can't make from lunar materials).
The main disadvantage of aluminum is that it expands and contracts with temperature much more than most common metals, which could be an issue with large exposed structures on the Moon which are exposed to the extreme day/nite temperature variations, or equipment which operates over a wide temperature range. Iron (steel) is better used on the Moon and other such places for metal structures.
We are fortunate that the Moon has concentrations of aluminum in an attractive mineral form, anorthite. As McKay and Williams conservatively report:
"Anorthite can be considered to be a potential aluminum ore in the sense that it is a naturally occurring concentration of aluminum from which it may be economically feasible to extract the metal. Bauxite [, a sedimentary Earth ore,] which contains about 25 percent aluminum [compared to 20% in anorthite], is currently the major terrestrial aluminum ore. However, terrestrial anorthite has been used in some countries as a commercial aluminum ore. The United States Bureau of Mines recently studied the economics of extracting aluminum from anorthite ... They concluded that the cost of extracting aluminum from anorthite was within a factor of 2 of the cost of extracting aluminum from bauxite and would become even more competitive as the cost of bauxite increased [with depletion]. The Bureau of Mines is currently planning to build a pilot plant to extract aluminum from anorthite. Alcoa Corp., which recently purchased a large area in Wyoming estimated to contain as much as 30 billion tons of recoverable anorthosite, is developing plans to recover aluminum from this source.
(Having grown up not far from Bauxite, Arkansas, a sedimentary deposit in the foothills of the Ouachita Mountains, and gone around the mines, I became aware of aluminum issues. This mine was the only source of commercial bauxite in the USA; the USA imports 90% of its aluminum, at considerable cost. The ore is also shipped by rail long distances to places where electricity is cheaper, since electricity is used to make aluminum. In space, solar powered electricity will be plentiful.)
"If anorthite is becoming attractive as a terrestrial aluminum resource, it is even more attractive as a lunar aluminum resource. The lunar crust contains a much higher proportion of anorthite than does the Earth's crust and the lunar highlands are particularly rich in anorthite."
Only one of the Apollo missions landed on the rugged lunar highlands (albeit at the junction between two lowland areas). The average anorthite concentration was 75 to 80%, and varied up to 98%. There are many other highland areas which are thought to have even better anorthite concentrations than the Apollo 16 site.
Raw anorthite is also a good material for making fiberglass and other glass and ceramic products.
Interestingly, production of aluminum from anorthite would create calcium as a byproduct, since anorthite is a calcium-aluminum silicate (CaAl2Si2O8). Calcium is the fourth most abundant element in the lunar highlands. Calcium oxides and calcium silicates are not only useful for ceramics, but pure calcium metal is an excellent electrical conductor.
Calcium metal is not used as a conductor on Earth simply because calcium burns spontaneously when it comes in contact with oxygen (much like the pure magnesium metal in camera flashbulbs). But in vacuum environments in space, calcium becomes attractive.
Calcium is a better electrical conductor than both aluminum and copper. Calcium's conductivity also holds up better against heating. A couple of figures mining engineer David Kuck pulled out of the scientific literature: "At [20C, 68F], calcium will conduct 16.7% more electricity than aluminum, and at [100C, 212F] it will conduct 21.6% more electricity through one centimeter length and one gram mass of the respective metal." Compared to copper, calcium will conduct two and a half times as much electricity at 20C, 68F, and 297% as much at 100C, 212F.
Like copper, calcium metal is easy to work with. It is easily shaped and molded, machined, extruded into wire, pressed, and hammered.
As would be expected of a highland element, calcium is lightweight, roughly half the density of aluminum. However, calcium is not a good construction material because it is not strong. Calcium also sublimes (evaporates) slowly in vacuum, so it may be necessary to coat calcium parts to prevent the calcium from slowly coating other important surfaces like mirrors. In fact, calcium is sometimes used to deoxidize some metal surfaces. Calcium doesn't melt until 845C (1553F).
Utilization of lunar materials will see the introduction of industrial applications of calcium metal in space.
Titanium is a "titan"-like high strength metal, offering more strength per unit weight than aluminum. Titanium is used for military aircraft and missiles.
The Apollo 11 and Apollo 17 sites were surprisingly rich in an economically attractive titanium mineral, ilmenite, FeTiO3 (iron titanium oxide). Note that it's not a silicate. Ilmenite grains of high purity make up more than one fifth of the Apollo 17 mare samples, on average. These ilmenite grains average 53% TiO2, 44% FeO, 2% MgO and 1% of various impurities.
McKay and Williams note: "Although rutile [i.e., the mineral of pure TiO2 found in some places on Earth] is more desirable, ilmenite is also considered to be a commercial ore for producing titanium. Dupont Corp., for example, has used ilmenite ores on a commercial basis and the United States Bureau of Mines has recently reviewed the feasibility of replacing imported rutile with domestic ilmenite as a source for U.S. titanium."
Ilmenite minerals also trap solar wind hydrogen very well, so that processing of ilmenite will also produce hydrogen, a rare element on the Moon.
Ilmenite was found in significant quantities on the surface of two of the five Apollo lowland sites. Since it's not so common in Earth's crust, it's a big lesson in lunar geology and the differences between Earth and the Moon.
Iron is most abundant in lowland minerals, and fairly easy to extract, e.g., from ilmenite (above, same as for titanium).
Small quantities of free iron also exist. In the above section, I stated that the main industrial metals don't exist in free form in planetary crusts. However, free iron metal is abundant in asteroids, and asteroids have impacted the Moon and spread their vaporized material far and wide. With the lack of water and air on the Moon, this metal has not rusted into iron oxide. Small grains of free iron exist in lunar soil.
Free iron averages about half of one percent of average lunar soil. The grain sizes are generally less than a few tenths of a millimeter.
(For the curious, there is also a trace of free iron from solar wind hydrogen atoms stealing the oxygen from iron oxide. This kind of free iron is microscopic.)
The free iron metal is extractable by simple magnets after grinding. This produces a supply of iron powder.
This powder can be easily handled to make parts using a standard technique on Earth called "powder metallurgy". On Earth, the metal handled this way was must be powderized, whereas on the Moon and with many asteroids it's already powder.
Free iron metal from the Moon probably could not compete well with asteroid-derived iron due to the abundance and cheapness of asteroidal iron. However, it would be valuable for use on the Moon's surface. The free iron metal is naturally alloyed with nickel and cobalt.
The main minerals on the Moon are "plagioclase" minerals (aluminum silicates of which anorthite -- calcium aluminum silicate -- is the most common plagioclase mineral), "olivine" (predominantly magnesium and iron silicates -- Mg2SiO4 and Fe2SiO4), ilmenite (discussed above, FeTiO3), and pyroxenes (MgSiO3, CaSiO3, FeSiO3). However, there are many other minerals and glasses mixed in.
Covering lunar geology in detail is beyond the scope of a brief introductory page like this one. Nonetheless, we have provided links to many resources and a database of scientific publications for further information, and a forum to discuss matters.