The chemical composition of lunar soil

As a lunar geochemist I have been approached many times by people who believe that they have a sample from the Moon. Common stories are (something like) “This dust was given to my late grandfather by astronaut Buzz Lightyear” or “This rock that I found in my petunia pot looks just like lunar meteorite QUE 94281 on your website.” Lately, people have been sending me reports that they have obtained of chemical analyses from labs or one of those hand-held x-ray “guns.” So, here is what you need to know in order to interpret those reports.

Major Elements – In lunar rocks and soils 99% of the mass
consists of the following 7 chemical elements

Oxygen (41-45%) | Silicon (Si) | Aluminum (Al) | Calcium (Ca) 
 Iron (Fe) | Magnesium (Mg) | Titanium (Ti)

Fe/Mn | Ca/Al

Minor Elements – Nearly all of the remaining 1% consists to these 4 chemical elements

Below are charts I have made from data from dozens of literature sources and my own lab for what we geochemists call the “major elements” and “minor elements” in samples from the 6 Apollo mission and 3 Russian Luna missions that brought samples back from the Moon. To make it simple, I have stuck to just soil (regolith) samples. I have also included data for those lunar meteorites that are breccias because many to most of these rocks are composed of lithified soil. The lunar meteorites come from all over the Moon whereas the Apollo and Luna mission all come a small area of the nearside.

In rocks of the Earth and Moon, oxygen is the most abundant chemical element, 41-45% on the Moon. Practically nobody actually measures the concentration of oxygen in rocks anymore. We measure the “metals” like iron and aluminum.

Terrestrial geochemists like to “express” the measured concentration of, say, silicon “as the oxide.” They measure the concentration of Si and state the concentration as the SiO2. So, 10.0 % Si is 21.4% SiO2. Quartz is a form SiO2, but quartz is rare on the Moon. Nearly all (>99%) lunar Si is in the silicate minerals plagioclase, pyroxene, and olivine. Likewise, there is no actual MgO (the mineral periclase) on the Moon; magnesium is carried mostly by the minerals pyroxene and olivine. We express the metal concentrations as oxide concentrations because the sum of 10 major and minor metal oxides above should be 100±1%. If not, something wrong (!) as there are no (= insignificant amounts of) carbonates, sulfates, or hydrous (water-bearing) minerals on the Moon. Lunar meteorites, however, often to contain carbonates, sulfates, or hydrous minerals as a result of weathering on Earth after they land.

So, for geochemists, the bottom and left axes of the plots below are in weight-percent oxide. For scrap-yard dealers and jewelers who might have an x-ray gun set to the “metal” setting, use the top and right axes.

All the plots have aluminum concentrations on the horizontal axis. I do it that way because Al varies over a large range in lunar samples. (To confuse you, elsewhere here I have put FeO+MgO on the horizontal axis, but that is OK because there is a strong anticorrelation between Al2O3 and FeO+MgO in lunar samples.)

Finally, in the plots below, each point for Apollo 11, and the 3 Luna missions represents a chemical analysis. For example, nearly all the Apollo 11 points represent soil sample 10084, which is probably the most well characterized geologic sample ever analyzed. For Apollos 12, 14, 15, 16, and 17, each point represents a numbered soil sample (“surface” and “trench” soils, no cores), e.g., samples 12032, 14163, 15071, 65701, and 76501 (mean of all available analyses for each). The large spread for some of these missions reflect the compositional variation among the various locations at which samples were collected at the site. For the lunar meteorites, each point represents a named stone, e.g., MacAlpine Hills 88105 or Northwest Africa 8046 and its pairs. For reference, each plot also includes an “Earth” point, which is an average of 4 different estimates I found in the literature for the mean composition of upper continental crust of the Earth.

Silicon (Si)

On Earth, SiO2 concentrations in rocks vary from 0% to 100%. The variation on the Moon is much less because the 3 major minerals in lunar rocks, plagioclase feldspar (usually anorthite), pyroxene, and olivine, all have about the same SiO2 concentration.

Iron (Fe)

On Earth, iron exists in the 2+ (ferrous) and 3+ (ferric) oxidation states so in chemical analysis of rocks, Fe concentrations are usually stated as % Fe2O3 because the ferric oxidation state is more common than ferrous oxidation state. On the Moon there is (effectively) no oxygen-bearing atmosphere so there are no iron 3+ iron minerals. The iron in pyroxene, olivine, and iron-titanium minerals like ilmenite is all in the ferrous (2+) oxidation state. To complicate the issue, some of the iron in every lunar soil exists as metal. Up to 10% of the iron in some of these sample is metallic, usually as iron-nickel metal derived from meteorites. So, in analyses of lunar samples, results for iron are usually stated as “total Fe as FeO” or FeOT. The anticorrelation in this plot occurs because soils on the left (basaltic) are dominated by the Al-poor, Fe-rich minerals pyroxene, olivine, and ilmenite whereas those on the right (feldspathic) are dominated by the Al-rich, Fe-poor mineral plagioclase.

Manganese (Mn)

On the Moon, all the Mn is in the 2+ oxidation state so it “behaves” just like 2+ Fe.

Iron/Manganese (Fe/Mn)

On the Moon, all the Mn is in the 2+ oxidation state so out “behaves” just like 2+ Fe. As a result, Fe/Mn ratios of lunar samples are rather constant in the 60-90 range. This characteristic is useful for distinguishing lunar meteorites from other types of meteorites but is often not useful for distinguishing lunar meteorites from terrestrial rocks.

Magnesium (Mg)

Most of what is said above for 2+ Fe is also true for magnesium. In lunar rocks, nearly all the Mg is in pyroxene and olivine.

Calcium (Ca)

For Al-poor rocks, some of the Ca is in clinopyroxene but on the Moon most of the Ca is in plagioclase (anorthite), which is also the main host for aluminum. Thus, the two elements strongly correlate.

Calcium Aluminum (Ca/Al)

The Ca/Al ratio in lunar samples varies by only a factor of 2. The few high-Ca meteorites are contaminated with terrestrial calcite.

Titanium (Ti)

Concentrations of Ti vary by a factor of 10 in basaltic lunar soils.

Chromium (Cr)

Cr concentrations in lunar samples are much higher than they are in nearly all terrestrial samples. Cr is one of the best elements for distinguishing between lunar and terrestrial samples.

Sodium (Na)

Na concentrations in lunar samples are much lower than they are in most terrestrial samples. Na is an element that is often good for distinguishing between lunar and terrestrial samples.

Potassium (K)

Like Na, K concentrations in lunar samples are much lower than they are in most terrestrial samples. Potassium is an element that is often good for distinguishing between lunar and terrestrial samples.

Phosphorus (P)

Phosphorus in not particularly useful for distinguishing between lunar and terrestrial samples.