Moon rocks collected by the Apollo astronauts and unmanned Soviet missions include anorthosites, gabbros, and basalts. Most are basalts. Although these rocks indicate an active volcanic history, significant eruptions ceased on the moon about 3.2 billion years ago. Petrologists classify the ancient basalts exposed at the Moon's surface into several groups: olivine basalts, silica-rich basalts, aluminum-rich basalts, titanium-rich basalts, and so on. All have equivalents on the Earth, but lunar basalts are generally richer in TiO₂, FeO, MgO, and Cr₂O3, and poorer in volatiles (H₂O, CO₂, O₂) terrestrial basalts.
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Lunar basalts and other moon rocks contain the same minerals found in terrestrial rocks, including pyroxenes, plagioclase, olivine, ilmenite, and small amounts of K-feldspar. However, the chemical differences between Earth and Moon rocks result in slight differences in mineral chemistries. For example, lunar anorthosites typically consist of very calcium-rich plagioclase (An97-98Ab2-3) with lesser amounts of orthopyroxene, olivine, clinopyroxene, pigeonite, and magnesium-rich spinel. In contrast, terrestrial anorthosites tend to have more sodium in plagioclase (An34-77AB23-66), and spinels are normally rich in iron. Study of lunar rocks also disclosed some previously undiscovered minerals resulting from excess titanium and other chemical differences. Two new silicates include pyroxferroite, CaFe6 (SiO3)7, and tranquillityite, Fe8(Zr,Y)₂Ti3Si3O24, which is named after the Sea of Tranquility, the site of the first Moon landing. Armalcolite, (Fe, Mg)Ti₂O5, is named after the three Apollo 11 astronauts, Neil Armstrong, Ed Aldrin, Jr., and Michael Collins.