Many persons have approached us over the years wanting to know if a rock that they possess is a Moon rock. The most common story we hear is that the rock 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 rocks and none has been a Moon rock. 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.

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Lunar meteorite QUE (Queen Alexandra Range Antarctica) 94281 – An unattractive rock that could pass for a cinder or piece of slag. It weighed 23 grams, just less than an ounce. The cube is 1 cm on each side. Image credit: NASA photo S95-14590No 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.
Weathered pebbles of lunar meteorite Northwest Africa 11788, 1-cm cube on right. If I found a few of these in my driveway, I would not give them a second look. Photo credit: Rob WeselHere I discuss some aspects of lunar geology, mineralogy, and chemistry that guide us in our attempts to identify lunar material.

Lunar Mineralogy

Only four minerals – plagioclasefeldspar,pyroxene,olivine,andilmenite – 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 primarymineral 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 (seeDhofar 025).Secondaryminerals 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 .

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 .

feldspar– A group ofalumino-silicate minerals; common in the crusts of the Earth and Moon.

plagioclase– A form of feldspar; a calcium-sodiumalumino-silicate <(CaAl,NaSi)AlSi2O8>.

anorthite– Amineral; the calcium-rich extreme of the plagioclase feldspar; the most common mineral of the lunar crust, but not so common on Earth.

anorthosite– Arockconsisting mainly of anorthite.

Lunar Rocks – Breccias

Pieces of Apollo 16 anorthosite sample 60025. This particular sample is nearly pure anorthite; there are no dark iron-bearing minerals. Photo credit: Randy KorotevMost of the lunar crust, that part called theFeldspathic Highlands Terraneor simply thefeldspathic highlands,consists of rocks that are rich in a particular variety of plagioclase feldspar known asanorthite. As a consequence, rocks of the lunar crust are said to beanorthositicbecause 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.

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rock namemineralogy
anorthosite>90% plagioclase
noritic anorthosite and anorthositic norite60-90% plagioclase, the rest mostly orthopyroxene
gabbroic anorthosite and anorthositic gabbro60-90% plagioclase, the rest mostly clinopyroxene
troctolitic anorthosite and anorthositic troctolite60-90% plagioclase, the rest mostly olivine
norite10-60% plagioclase, the rest mostly orthopyroxene
gabbro10-60% plagioclase, the rest mostly clinopyroxene
troctolite10-60% plagioclase, the rest mostly olivine
In much of the northwest quadrant of the nearside of the Moon, in the region known as theProcellarum 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. Inimpact-meltandglassybreccias, rock fragments calledclastsare suspended in a solidified (crystalline or glassy) melt matrix formed by meteorite impact.