Many persons have approached us over the years wanting to know if a rock that they possess is a Moon rock or soil sample. The most common story we hear is that the sample was given to a relative in the 1970’s by an astronaut, a military person, or a NASA security guard. We have chemically tested several such samples and none has been from the Moon. Other people suspect that they have found a lunar meteorite. None of the many samples that we have been sent has been a lunar meteorite, except those from meteorite dealers, those persons who bought lunar meteorites from a dealer, or those from experienced meteorite prospectors who found them in the deserts of northern Africa or Oman.
No lunar meteorite has yet been found in North America, South America, or Europe. They undoubtedly exist, but the probability of finding a lunar meteorite in a temperate environment is incredibly low. Many experienced meteorite collectors have been looking and none have yet succeeded. Realistically, the probability that an amateur will find a lunar meteorite is so low that I cannot raise much enthusiasm to examine the thousands of rocks and photos that I have been asked to examine. If I wanted to find a lunar meteorite myself, I would not scour the Mojave Desert. I would look through rock collections at colleges and universities. It is not unreasonable that a lunar meteorite exists in an old drawer somewhere because a sharp-eyed geology student or professor found a funny-looking rock years ago in a place it did not belong. It would not surprise me to learn that some “expert” proclaimed that the rock was not a meteorite because it did not look like an ordinary chondrite, it did not attract a magnet, or it did not contain a high concentration of nickel. Both visually and compositionally, lunar meteorites “look” more like terrestrial (Earth) rocks than do “normal” meteorites (ordinary chondrites). It would be easy to overlook a lunar meteorite. A weathered lunar meteorite would look remarkably unremarkable.
Here I discuss some aspects of lunar geology, mineralogy, and chemistry that guide us in our attempts to identify lunar material.
Only four minerals – plagioclase feldspar, pyroxene, olivine, and ilmenite – account for 98-99% of the crystalline material of the lunar crust. (Material at the lunar surface contains a high proportion of non-crystalline material, but most of this material is glass that formed from melting of rocks containing the four major minerals.) The remaining 1-2% is largely potassium feldspar, oxide minerals such as chromite, pleonaste, and rutile, calcium phosphates, zircon, troilite, and iron metal. Many other minerals have been identified, but most are rare and occur only as very small grains interstitial to the four major minerals and cannot be seen with the naked eye.
Some of the most common minerals at the surface of the Earth are rare or have never been found in lunar samples. These include quartz, calcite, magnetite, hematite, micas, amphiboles, and most sulfide minerals. Many terrestrial minerals contain water as part of their crystal structure. Micas and amphiboles are common examples. Hydrous (water containing) minerals have not been found on the Moon. The simplicity of lunar mineralogy often makes it very easy for me to say with great confidence “This is not a moon rock.” A rock that contains quartz, calcite, or mica as a primary mineral is not from the Moon. Some lunar meteorites do, in fact, contain calcite. However, the calcite was formed on Earth from exposure of the meteorite to air and water after it landed. The calcite occurs as a secondary mineral, one that fills cracks and voids (see Dhofar 025). Secondary minerals are easy to recognize when the meteorite is studied with a microscope.
pyroxene – A group of magnesium-iron-calcium silicates, common on the Earth and Moon.
clinopyroxene – A form of pyroxene; typically contains some calcium; most common in mare basalts [Ca(Mg,Fe)Si2O6].
orthopyroxene – A form of pyroxene; contains little calcium; most common in highlands rocks [(Mg,Fe)SiO3].
olivine – A magnesium-iron(II) silicate; common on the Earth and Moon [(Mg,Fe)2SiO4].
ilmenite – An iron(II)-titanium oxide; more common in lunar basalts than in terrestrial basalts [FeTiO3].
feldspar – A group of alumino-silicate minerals; common in the crusts of the Earth and Moon.
plagioclase – A form of feldspar; a calcium-sodium alumino-silicate [(CaAl,NaSi)AlSi2O8].
anorthite – A mineral; the calcium-rich extreme of the plagioclase feldspar; the most common mineral of the lunar crust, but not so common on Earth.
anorthosite – A rock consisting mainly of anorthite.
Lunar Rocks – Breccias
Most of the lunar crust, that part called the Feldspathic Highlands Terrane or simply the feldspathic highlands, consists of rocks that are rich in a particular variety of plagioclase feldspar known as anorthite. As a consequence, rocks of the lunar crust are said to be anorthositic because they are plagioclase-rich rocks with names like anorthosite, noritic anorthosite, or anorthositic troctolite (see table below). The ratio of iron-bearing minerals to plagioclase probably increases with depth in the feldspathic highlands at most places. For example, rocks exposed in the giant South Pole – Aitken impact basin on the far side are richer in pyroxene than typical feldspathic highlands.
|noritic anorthosite and anorthositic norite||60-90% plagioclase, the rest mostly orthopyroxene|
|gabbroic anorthosite and anorthositic gabbro||60-90% plagioclase, the rest mostly clinopyroxene|
|troctolitic anorthosite and anorthositic troctolite||60-90% plagioclase, the rest mostly olivine|
|norite||10-60% plagioclase, the rest mostly orthopyroxene|
|gabbro||10-60% plagioclase, the rest mostly clinopyroxene|
|troctolite||10-60% plagioclase, the rest mostly olivine|
In much of the northwest quadrant of the nearside of the Moon, in the region known as the Procellarum KREEP Terrane, the crust contains less plagioclase and more pyroxene. The original rocks of this anomalous crust were probably mostly norites and gabbros. The feldspathic crust of the Moon began to form about 4.5 billion years ago. While it was forming and for some time afterwards, it experienced intense bombardment from meteoroids and asteroids. The rocks of the lunar crust have been repeatedly broken apart by some impacts and glued back together by other impacts. As a consequence, most rocks from the lunar highlands are breccias (brech’-chee-uz), a word for a rock composed of fragments of older rocks. Breccias occur on Earth, but they are much less common than on the Moon. Also, most terrestrial breccias were not formed by meteoroid impacts but by faulting. Lunar breccias are subdivided into a variety of categories such as impact-melt, granulitic, glassy, fragmental, and regolith breccias. In impact-melt and glassy breccias, rock fragments called clasts are suspended in a solidified (crystalline or glassy) melt matrix formed by meteorite impact.
In fragmental and regolith breccias, there is little or no molten portion, just fragmental debris that was lithified (formed into a rock) by the shock pressure of an impact. Because breccia refers to texture and anorthositic or feldspathic refers to mineralogy, rocks from the lunar highlands are variously called anorthositic breccias, feldspathic breccias, or highlands breccias. Because the lunar crust has been battered so intensely, there were very few hand-sized rocks collected on the Apollo missions that are unbrecciated remnants of the early igneous crust of the Moon. Thus, it is no surprise that all of the lunar meteorites from the Feldspathic Highlands Terrane and the Procellarum KREEP Terrane are breccias.
Lunar Rocks – Mare Basalts
On Earth, volcanoes are often cone-shaped mountains because they are a pile of ash and lava ejected from a vent. The lavas are viscous and solidify before they flow very far. Because of their iron-rich composition and lack of water, lunar lavas were much less viscous, more like motor oil. When lunar lavas erupted onto the surface they did not form volcanoes, they simply flowed and filled low spots. As a result, lunar lava deposits are flat, thin, and cover wide areas. Also, because the Moon has no atmosphere and little gravity, ejected ash dissipated widely instead of piling up near the vent, as on Earth.
Starting about the time of the period of intense meteorite bombardment, the lunar mantle partially melted. The resulting magmas rose through the crust to the surface, ponding in low spots. These low spots were mainly the huge craters, called basins, that were left by impacts of the largest meteorites. Lunar volcanism continued for about 2 billion years.
Pronunciation: The Latin word mare is pronounced mar’-ay in English. The plural of mare is maria, which is pronounced mar’-ee-ah. Basalt is usually pronounced bah-salt’.
On Earth, volcanic rocks solidify from molten lava (magma). The most common type of volcanic rock is basalt. The ancient astronomers called the round, basalt-filled basins on the surface of the Moon seas because they were smooth dark areas surrounded by areas of higher elevation. The features were given Latin names like Mare Serenitatis for Sea of Serenity. We now know that the lunar maria are basalt flows, so we call the rocks of the maria mare basalts. Mare basalts are composed mainly, 50-70%, of pyroxene, but they all also contain 20-40% plagioclase, up to 20% ilmenite and related Ti-rich minerals, and 0-20% olivine. The maria are darker than the highlands because (1) mare basalts are rich in iron-bearing minerals, (2) iron-bearing minerals are dark colored, and (3) plagioclase is light colored. In contrast to the highlands, most of the rocks collected on maria by the Apollo astronauts are actual basalts, not breccias composed of fragments of basalt. This observation is one of several reasons why we know that the basalts mostly formed after the time of intense bombardment. Mare basalts cover about 17% of the surface of the Moon, but it is estimated that they account for only about 1% of the volume of the crust.
Because lunar meteorites are samples from randomly distributed locations on the surface of the Moon and because most of the lunar surface is feldspathic, most of the lunar meteorites are feldspathic breccias. Some are crystalline mare basalts, breccias composed of mare basalt, or breccias composed of both mare and highlands material (like QUE 94281, above). A few are dominated by noritic material of the Procellarum KREEP Terrane.
Lunar mare basalts, as well as basaltic meteorites from Mars, bear a strong resemblance to basalts from Earth. In the absence of a fusion crust, there is little about a lunar mare basalt that would provoke much interest in a geologist who was handed the rock by someone asking “what is this?” Careful examination under the microscope might reveal some suspicious features – the lack of certain minerals and abundance of others (ilmenite) or the low sodium content of the feldspar. The mineral grains would show signs of shock and fracturing from meteorite impacts. However, chemical tests would be required to prove a lunar or martian origin.
Fragmental and regolith breccias are the closest lunar analogs to terrestrial sedimentary rocks, and they bear some textural resemblance. However, there are numerous differences, nearly all associated with the lack of water and wind on the Moon. As noted above, lunar rocks do not contain carbonate minerals or abundant quartz, as do most terrestrial sedimentary rocks. There is no effective sorting mechanism on the Moon, so the lithic components of lunar breccias come in a wide variety of grain sizes, with no preferred size or orientation. Lunar breccias are largely fractal objects that look similar in cross section regardless of the scale at which they are viewed. (See ALHA 81005.) There is no known lunar rock that has any feature that resembles the layers that are characteristic of terrestrial sedimentary rocks. Terrestrial sedimentary rocks have layers because the Earth has gravity, so particles settle in water or in the atmosphere. The Moon has only weak gravity and no water or atmosphere.
Most small clasts in lunar breccias are fragments of plagioclase or anorthosite. It is rare for the aspect (length to width) ratio of a clast in a lunar breccia to exceed 3. Most clasts are angular, not rounded. (Exceptions: There are volcanic glass spherules in the lunar regolith (soil). Such spherules are sometimes found in regolith breccias, but they are <0.1 mm in diameter and not easily seen with the unaided eye. Impact-produced spherules occur and may be large, but they are not common compared to rock and mineral fragments. Impact-melt breccias may contain clasts that have been partially melted and which are consequently not angular.)
Brecciated lunar meteorites are sufficiently tough and cohesive that they survived the blast off the Moon and the hard landing on Earth. Many terrestrial sedimentary rocks break apart much easier. Unlike some terrestrial conglomerates, which resemble lunar breccias, the matrix of lunar breccias is as hard as the clasts. On broken or exterior surfaces of brecciated lunar meteorites, the clasts do not stand out in either negative or positive relief.
Metal and Magnetism
Meteorite collectors know that most meteorites attract a cheap magnet because they contain iron-nickel metal. The most common type of meteorites, the ordinary chondrites, do indeed contain metal as, of course, do iron meteorites. Lunar mare basalts and the original rocks of the lunar highlands contain essentially no iron metal (much, much less than 1%). Brecciated lunar meteorites, however, contain some metal from the asteroidal meteorites that have bombarded the Moon. Among lunar meteorites that we have studied, Dhofar 1527 contains the most metal, about 1.7%; most contain much less. In other words, lunar meteorites do not attract magnets, as do most other kinds of meteorites.
Because of the simplicity of lunar mineralogy, lunar rocks have predictable chemical compositions. Nearly all the aluminum is in plagioclase and nearly all the iron and magnesium are in pyroxene, olivine, and ilmenite. Thus, on the plot of aluminum concentration (Al2O3 in figure below) versus the concentrations of iron (FeO) plus magnesium (MgO), lunar meteorites (and nearly all Apollo lunar rocks) plot along a line connecting the composition of plagioclase and the average composition of the three iron-bearing minerals because these are the only four major minerals in the rock. If the composition of a rock does not plot along this line, the rock is almost certainly not a lunar rock.
On Earth, the silica (SiO2) concentration of igneous rocks is used as a first-order chemical classification parameter because it varies widely among different kinds of rocks. On the Moon (1) there are no rocks rich in quartz or other silica polymorphs*, (2) in a given rock, particularly breccias, the average concentration of silica in the three main minerals, plagioclase, pyroxene, and olivine, are all about the same, and (3) in highlands rocks ilmenite is usually present only in small amounts (<3%), so silica concentrations of common lunar rocks vary by only a small amount. In lunar meteorites, SiO2 concentrations span the narrow range from 43% to 47%. Because aluminum varies by more than a factor of 3, however, aluminum is more useful as a chemical classification parameter. (Titanium is used in mare basalts.) Similarly, among nearly all common lunar rocks calcium concentrations vary by only a factor of 2, from 10% to 20% as calcium oxide (CaO). This is much less than the range in terrestrial rocks. A rock with silica or calcium oxide concentrations substantially outside these ranges is almost certainly not a lunar rock.
|* Some lunar mare basalts contain up to 5% cristobalite, a silica mineral. There are some rare and small lunar samples with 50-70% SiO2 because they contain tridymite, quartz, or silica glass. These include felsites, granites, and related silica-rich rocks like quartz monzodiorite. There are also rocks that contain <10% CaO because they contain little plagioclase. These include some ultramafic rocks like dunite and some picritic volcanic glasses.|