Lunar Free Metallic Iron

Lunar free metallic iron is the iron fines and chunks from iron-nickel asteroids that impacted the Lunar surface. It’s also the very small particles of pure iron created during meteoroid impacts, solar wind sputtering, and geological processes. It’s “free” because we won’t need to chemically release it from the rocks. All we need to do is pick it up with a magnet and separate it from the other stuff with it.

The sources of Lunar free iron include:

• Iron fines and chunks from iron-nickel asteroids impacting the Lunar surface.
• Iron precipitates formed from Lunar rocks chemically reacting to the heat generated by an impact event.
• Solar wind ions removing atoms from regolith grains.
• Subsolidus reduction of oxides, olivine, and troilite.
• Normal igneous crystallization.

Iron from impacting asteroids is called “meteoritic contamination”. The last four sources, iron formed on Luna, are called “native iron metal”. It’s important to get the terminology right even though it’s often hard to distinguish between the two.

While we haven’t found any yet, there are sure to be large remnants of these asteroids buried under the regolith at some impact sites. And there could be layered ore deposits caused by iron settling out of magma (Lunar Sourcebook, 140). But they are not my concern. Future homesteaders and business enterprises will surely hunt these resources down and mine them. For this project though I’m only interested in the iron particles that are available to anyone with access to mare regolith. Highland regolith works also but contains significantly less iron.

I’m not going to detail EVERY characteristic here. That would be exhausting and unnecessary. There are really only a handful of things that will determine if Lunar free iron is suitable for pressure hull fabrication (the point of this whole exercise).

And just to clarify, this research does NOT include Lunar meteorites. Lunar meteorites, also called luanites, are Lunar rocks that were ejected FROM the Moon by a high-energy impact and landed ON Earth. It’s a bit confusing as you would think a Lunar meteorite would be a rock that hit Luna (which is what we’re after). But it’s not. For more information about luanites check out (www.lpi.usra.edu/lunar/meteorites). On the other hand, regular iron meteorites landing on Earth are very useful because they contain information about what the asteroids that hit Luna could have been made of.

 

Why Lunar free iron is so useful

• It’s everywhere – All Lunar samples had the presence of iron metal grains (Lunar Sourcebook, 152). The amount is different for each site and each sample but it is never more than 1% by volume (Lunar Sourcebook, 152).
• It should be easy to collect – A simple electromagnet should do the trick. Maybe.
• It can be used immediately – It’s already in metal form and doesn’t have to be chemically refined. It is often encased in glassy agglutinates though. We’ll have to find a way to free the iron particles. Some of the particles are very fine and we might not be able to recover them.
• Some of it is already alloyed (meteoritic contamination and native iron metal) – Iron-nickel alloy is stronger than pure iron. Unfortunately, the amount of nickel varies significantly depending on the sample. This may be an issue.
• Some of it is elemental iron (Fe⁰) (native iron metal) – This useful because it’s all chemically the same, regardless of the source.
• It’s been estimated that if Apollo samples are typical of the average Lunar regolith there could be at least 7 BILLION metric tons of accessible FeNi metal in the top 10cm over the entire Lunar surface (Space Resources: Vol 3 Materials, 157).
• The extracted iron could be 99% pure FeNi powder (Space Resources: Vol 3 Materials, 157).
• This metal iron powder may be directly formable by low-power powder techniques and a near-theoretical density may be achievable by powder pressing in high vacuum (Space Resources: Vol 3 Materials, 157).
• Lunar oxides and titanium can be added to the iron powder before pressing to enhance the final product to steel specifications (Space Resources: Vol 3 Materials, 157).
• It is estimated that 100,000 tons of Lunar soil could yield 150-200 tons of iron (Space Resources: Vol 3 Materials, 281).

 

Sources of Lunar free iron

Meteoritic contamination

This is simple. Luna has been hit by a lot of rocks over a very long period of time. Most of these impactors were rocky (although they probably contained some iron as well). But some were made of iron-nickel (FeNi). Most, or all, of the impactor vaporizes when it hits due to its high velocity. But vaporized doesn’t mean disappeared. All that metal has to go somewhere.

• We still don’t have a definitive answer to how much of the FeNi metal found in the regolith is contamination and how much is native. There’s a lot of overlap between the two. Nickel (Ni) and cobalt (Co) contents of native iron metal can vary from 0%-50% for Ni and 0%-8% for Co. This falls within the range of iron-nickel asteroids. (Lunar Sourcebook, 151)
• Impacts spread material over a wide area. It is very difficult to determine if an iron particle came from an impacting body, was already on the surface, or was created due to the impact.
• The composition of meteoric materials suggests chondritic asteroids are the primary contributors (5, pg 252)

 

Iron precipitates

Lunar rocks are briefly melted by the energy of a meteorite impact. Here’s how iron is produced (Lunar Sourcebook, 154).

1. Iron bearing soil particles are sitting on the Lunar surface. Let’s say they are ilmenite (FeTiO₃).
2. Over time, these particles become saturated by hydrogen nuclei from the solar wind. It is estimated that it takes approximately 100 years for exposed soil to become saturated (Lunar Sourcebook, 315).
3. A small micrometeorite impacts the particles and releases a large amount of heat, melting the ilmenite particles.
4. The presence of the hydrogen creates a reducing environment, causing the hydrogen to strip an oxygen atom from the ilmenite. This also precipitates the iron out.
5. The end result is titanium dioxide (TiO₂), water (which evaporates into space), and very tiny spheres of elemental iron (called nanophase iron).
6. The iron spheres are trapped in the rapidly cooling agglutinate (chunks of rocks and minerals welded together by glass).

 

Subsolidus reduction

Subsolidus reduction simply means that a rock continues to chemically change when it’s solid but still hot (substantially below its melting point). According to Lunar Sourcebook, evidence for this is pretty common (Lunar Sourcebook, 147). The end product we’re interested in is the native iron  and iron minerals left behind.

 

Normal igneous crystallization

This just means that molten rocks change their chemical structure as they cool. It’s been a long time since magma has flowed on the Lunar surface.

 

Nanophase-iron

Agglutinates

Iron fines

 

Types of native iron metal minerals

• Kamacite – (0%-6% Ni) – Most abundant (Lunar Sourcebook, 151)
• Taenite – (6%-50% Ni) – Second most abundant (Lunar Sourcebook, 151)
• Tetrataenite – (48%-52% Ni) – Rare and most likely meteorite contamination (Lunar Sourcebook, 151)
• Schreibensite [(Fe,Ni)₃P] – Found in some samples but rare. (5, pg 252)
• Troilite (FeS) – Found everywhere in very small amounts (5, pg 252)

 

Data

Analysis of Fe metal chemical composition in Lunar rocks and soils (selected samples)(Lunar Sourcebook, 180)

• Apollo 11 (High-Ti)(>9% TiO₂)(Low-K)(<0.11% K₂O)(mare)
  • Iron = 98.95%
  • Nickel = 0%
  • Cobalt = 0.8%

• Apollo 11 soil (unknown rock types)(mare)
  • Iron = 85.82% – 99.2%
  • Nickel = 0.1% – 13.4%
  • Cobalt = 0.7% – 0.48

• Apollo 12 pigeonite (<10% MgO, <5% TiO₂) (Low-Ti)(1.5-9% TiO₂)(mare)
  • Iron = 97.54%
  • Nickel = 0.61%
  • Cobalt = 1.24%

• Apollo 12 ilmenite (<10%MgO, >5% TiO₂)(Low-Ti)(1.5-9% TiO₂)(mare)
  • Iron = 95.7%
  • Nickel = 1.72%
  • Cobalt = 2.59%

• Apollo 12 olivine (>10% MgO, <5% TiO₂)(Low-Ti)(1.5-9% TiO₂)(mare)
  • Iron = 67.4%
  • Nickel = 26.7%
  • Cobalt = 2.37%

• Apollo 15 soil (unknown rock types)(unknown highlands or mare)
  • Iron = 37.8% – 88.4%
  • Nickel = 4.7% – 60.0%
  • Cobalt = 1.3% – 11.8%

 

Percent (by weight) of FeNi metal in Lunar soil (Space Resources: Vol 3 Materials, 160).

• Less than 1% (mostly contained in agglutinates)

 

Extraterrestrial Materials Processing and Construction (chapter X, M)

• Free metal is 0.1% – 0.5% (by weight) in Lunar soil
• Mostly iron with minor amounts of Ni and Co
• Strongly ferromagnetic and readily separable from other soil constituents

 

Extraterrestrial Materials Processing and Construction (chapter X,M)

• 05-0.20% by weight in soil
• Contains 5% Ni and 0.2% Co
• Minor energy requirement to excavate, process, and extract

 

Primary literature

1. Lunar Sourcebook
2. A Global Lunar Landing Site Study to Provide the Scientific Context for Exploration of the Moon
3. Space Resources: Vol 3 Materials
4. Extraterrestrial Materials Processing and Construction
5. Lunar Stratigraphy and Sedimentology