There are currently three sources of Moon rocks on Earth: 1) those collected by US Apollo missions; 2) samples returned by the Soviet Union Luna missions; and 3) rocks that were ejected naturally from the lunar surface by cratering events and subsequently fell to Earth as lunar meteorites.
In general, the rocks collected from the Moon are extremely old compared to rocks found on Earth, as measured by radiometric dating techniques. They range in age from about 3.16 billion years old for the basaltic samples drived from the lunar maria, up to about 4.5 billion years old for rocks derived from the highlands. Based on the age dating technique of "crater counting," the youngest basaltic eruptions are believed to have occurred about 1.2 billion years ago, but we do not possess samples of these lavas. In contrast, the oldest ages of rocks from the Earth are about 3.8 billion years old.
In some regards, the rocks possess characteristics very similar to rocks on Earth, particularly in their composition of oxygen isotopes. Nevertheless, in contrast to the Earth, large portions of the lunar crust appear to be composed of rocks with high concentrations of the mineral anorthite, the mare basalts have relatively high iron concentrations, some of the mare basalts have very high concentrations of titanium (in the form of ilmenite), and all rocks are depleted in volatile elements (such as potassium or sodium) and are completely lacking in water. Furthermore, a geochemical component called KREEP, which has high abundances of incompatible elements, has no equivalent on Earth. Among the new minerals found on the Moon was armalcolite, which is named for the three astronauts on the Apollo 11 mission: Armstrong, Aldrin, and Collins.
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