Pyroxene

Topic Abstract

Pyroxene is a chemically complex group of silicate minerals with many variations. The generic formula for pyroxene is XY(Si,Al)₂O₆, with X and Y representing many types of elements. Pyroxene rocks can contain a mix of any of 20 different subgroups.

Pyroxene is the most abundant mineral found in Apollo 11, 12, 14, and 15 samples and the Luna-16 and -24 samples [1, pg 123]. However, it's present in significant quantities in the Apollo 16 and 17 samples and the Lunar-20 sample [1 pg123]. Any ISRU design will have to be able to deal with pyroxene. If we're smart, we'll figure out how to use it.

Core Data

Abundance and distribution

• Pyroxene is the most abundant dark mineral at the surface (1, pg 363)
• Mare basalts = 40%-65% (5%-30% for a few samples) pyroxene
• Anorthositic rocks (highland) = 0%-40% pyroxene
• The pyroxene/plagioclase ratio in mature soil increases with decreasing grain size. (6, pg 249)

Physical properties

• Pyroxene is paramagnetic. This is because of the Fe²⁺.

Chemical composition

It is difficult to define pyroxene because there are so many variants. For example, pigeonite -(Ca,Mg,Fe)(Mg,Fe)Si₂O₆ - is referenced in the Apollo 11 and Apollo 15 samples. But pigeonite covers a range of minerals including the two main pyroxene minerals, ferrosilite (FeSiO₃) and enstatite (MgSiO₃). The pyroxene we need for iron will have either Fe⁺²Fe⁺²(Si,Al)₂O₆ or Fe⁺²Y(Si,Al)₂O₆, with Y being some other metal. It can get a bit confusing. The one constant is that all types of pyroxene contain silicon (Si).

One thing to remember is that there are two types of chemical formulas, empirical and molecular. Actually, there are several other types but let’s keep it simple. Empirical formulas show the simplest positive integer ratio of atoms in the compound. This means that Fe₂Si₂O₆ becomes FeSiO₃. This means that there is one Fe and one Si atom for each O atom. The molecular formula specifies exactly how many atoms are in the molecule, but not its structure. Fe₂Si₂O₆ tells you that there is exactly 2 iron, 2 silicon, and 6 oxygen atoms making up that molecule. It can get confusing when the resource you are reading doesn’t specify which formula they are using.

Here’s how it breaks down (as far as I can figure out):

• Pyroxenes are usually represented by a mixing of the possible end members. (5, Chapter X,M)
  • Enstatite (Mg₂Si₂O₆) – Again, usually specified as MgSiO₃.
  • Ferrosilite (Fe₂Si₂O₆) – In most of the literature it’s specified as FeSiO₃. This is the pyroxene we want for iron production.
  • Wollastonite (CaSiO₃) – Pretty rare and not what we’re looking for right now.
• Pyroxenes are often reported as mole % of end members. For example, Wo₂En₈₀Fs₁₈ has 2% wollastonite with 80% enstatite and 18% ferrosilite. (5, Chapter X,M)
• Pyroxenes also have 3 structural forms based on how much CaSiO₃ they contain. (2, pg 19)
  • Orthopyroxene have the lowest concentration of CaSiO₃.
  • Pigeonite [(Ca,Mg,Fe)(Mg,Fe)Si₂O₆] (low-calcium clinopyroxene) has the middle concentration of CaSiO₃.
  • Augite (high-calcium clinopyroxene) has the highest concentration of CaSiO₃.
• All forms display a wide range of enstatite and ferrosilite, and accept large amounts of Al, Ti, Mn, Cr, and Na. (5, Chapter X,M)

Other Lunar pyroxene facts:

• Lunar pyroxenes show substantial subsolidus reduction (chemical changes to a rock while it’s hot but still solid)(1, pg 126). This can result in free iron (also called native iron) being formed in the rock.
• Oxidized iron Fe³⁺ (Fe₂O₃) is not present in Lunar pyroxenes (1, pg 122).  This is due to the more reducing conditions (low oxygen partial pressure) in gases on Luna. This means that iron minerals that are common on Earth are rare or nonexistent on Luna. The reverse is also true.
• Mare pyroxenes are almost exclusively clinopyroxene (augite) derived from mare basalts. Mare pyroxenes show extreme compositional zoning. (6, pg 249).
• Highland pyroxenes are >1/3 orthopyroxene (mostly bronzites En85-En61) and clinopyroxene. (6, pg 249)

Other members of the pyroxene group include:

• Aegrine
• Diopside
• Hedenbergite
• Hypersthene
• Spodumene

Expanded Data (all the data we can find)

Analysis of pyroxene mare basalts (selected samples)(1, pg 157-159)

• Apollo 17 (High-Ti)(>9% TiO₂)
  • Iron oxide (FeO) by weight percent range: 8.10% - 19.10%
  • Average FeO by percent weight = 14.16%
  • Ratio of iron to magnesium [Fe/(Fe+Mg)] range = 0.25 – 0.50
• Apollo 11 (High-Ti)(>9% TiO₂)(High-K)(>0.3% K₂O)
  • Iron oxide (FeO) by weight percent range: 12.00% - 45.80%
  • Average FeO by percent weight = 2.90%
  • Ratio of iron to magnesium [Fe/(Fe+Mg)] range = 0.28 – 0.95
• Apollo 11 (High-Ti)(>9% TiO₂)(Low-K)(<0.11% K₂O)
  • Iron oxide (FeO) by weight percent range: 9.60% - 41.07%
  • Average FeO by percent weight = 22.41%
  • Ratio of iron to magnesium [Fe/(Fe+Mg)] range = 0.27 – 0.69
• Apollo 12 ilmenite (<10%MgO, >5% TiO₂) (Low-Ti)(1.5-9% TiO₂)
  • Iron oxide (FeO) by weight percent range: 13.31% - 30.28%
  • Average FeO by percent weight = 21.78%
  • Ratio of iron to magnesium [Fe/(Fe+Mg)] range = 0.32 – 0.87
• Apollo 12 pigeonite [a form of pyroxene] (<10% MgO, <5% TiO₂) (Low-Ti)(1.5-9% TiO₂)
  • Iron oxide (FeO) by weight percent range: 16.90% - 46.54%
  • Average FeO by percent weight = 29.69%
  • Ratio of iron to magnesium [Fe/(Fe+Mg)] range = 0.28 – 0.99
• Apollo 12 olivine (>10% MgO, <5% TiO₂)(Low-Ti)(1.5-9% TiO₂)
  • Iron oxide (FeO) by weight percent range: 16.34% - 41.90%
  • Average FeO by percent weight = 25.79%
  • Ratio of iron to magnesium [Fe/(Fe+Mg)] range = 0.33 – 0.99
• Apollo 15 pigeonite [a form of pyroxene] (<10% MgO, <5% TiO₂) (Low-Ti)(1.5-9% TiO₂)
  • Iron oxide (FeO) by weight percent range: 14.50% - 30.20%
  • Average FeO by percent weight = 20.86%
  • Ratio of iron to magnesium [Fe/(Fe+Mg)] range = 0.25 – 0.80
• Apollo 15 olivine (>10% MgO, <5% TiO₂)(Low-Ti)(1.5-9% TiO₂)
  • Iron oxide (FeO) by weight percent range: 13.60% - 36.60%
  • Average FeO by percent weight = 22.63%
  • Ratio of iron to magnesium [Fe/(Fe+Mg)] range = 0.31 – 0.79
• Luna 16 (Aluminous, Low-Ti basalts)(>10% Al₂O₃, 2-5% TiO₂)
  • Iron oxide (FeO) by weight percent range: 13.27% - 38.67%
  • Average FeO by percent weight = 25.97%
  • Ratio of iron to magnesium [Fe/(Fe+Mg)] range = 0.36 – 0.87
• Apollo 14 (Aluminous, Low-Ti basalts)(>10% Al₂O₃, 2-5% TiO₂)
  • Iron oxide (FeO) by weight percent range: 18.60% - 30.32%
  • Average FeO by percent weight = 24.46%
  • Ratio of iron to magnesium [Fe/(Fe+Mg)] range = 0.33 – 0.97
• Apollo 14 VHK (Very High Potassium)(>0.6% K₂O)(Aluminous, Low-Ti basalts) (>10% Al₂O₃, 2-5% TiO₂)
  • Iron oxide (FeO) by weight percent range: 15.17% - 19.07%
  • Average FeO by percent weight = 17.39%
  • Ratio of iron to magnesium [Fe/(Fe+Mg)] range = 0.35 – 0.42
• Luna 24 (Very Low-Ti basalts)(<1.5% TiO2)
  • Iron oxide (FeO) by weight percent range: 16.95% - 43.35%
  • Average FeO by percent weight = 30.15%
  • Ratio of iron to magnesium [Fe/(Fe+Mg)] range = 0.45 – 0.99
• Apollo 17 (Very Low-Ti basalts)(<1.5% TiO2)
  • Iron oxide (FeO) by weight percent range: 17.42% - 42.54%
  • Average FeO by percent weight = 27.31%
  • Ratio of iron to magnesium [Fe/(Fe+Mg)] range = 0.29 – 0.97

Abundance of Lunar pyroxene (percent by volume) (2, pg 20)

• Mare basalts = 40%-65% (5%-30% for a few samples)
• Anorthositic rocks (highland) = 0%-40%
• Fragmental breccias (>25 micrometers across) = 5%-30%
• Soils = 5%-20% (composition and amount resembles local rocks)

Analysis of Lunar pyroxenes for iron (percent by weight)(2, pg 20)

• Mare = 8.97%
• Highland = 15.42%

FeO concentrations in pyroxene (percent weight)(3, pg 29)

• High-Ti mare basalts = 8.1% - 45.8%
• Low-Ti mare basalts = 13.1% - 45.5%
• Highland rocks = 8.20% – 24.0%

Lunar samples % pyroxene (randomly selected) (7)

• Sample 10003 (Apollo 11 mare basalt) = 48.6-51.7%
• Sample 12015 (Apollo 12 mare basalt) = 13.6-43%
• Sample 14078 (Apollo 14 KREEP basalt) = 25.5%
• Sample 15261 (Apollo 15 soil) = 14%
• Sample 67747 (Apollo 16 highland basalt) = 8%
• Sample 70215 (Apollo 17 high-Ti mare basalt) = 41-58%

Contributors

Ben Smith at Lunar Homestead

Sources

1. Lunar Sourcebook -
2. Handbook of Lunar Materials -
3. Space Resources: Vol 3 Materials -
4. Wikipedia - Pyroxene -
5. Extraterrestrial Materials Processing and Construction -
6. Lunar Stratigraphy and Sedimentology -
7. LPI Lunar Sample Atlas -