<P> As crystallization of the lunar magma ocean proceeded, minerals such as olivine and pyroxene would have precipitated and sank to form the lunar mantle . After crystallization was about three - quarters complete, anorthositic plagioclase would have begun to crystallize, and because of its low density, float, forming an anorthositic crust . Importantly, elements that are incompatible (i.e., those that partition preferentially into the liquid phase) would have been progressively concentrated into the magma as crystallization progressed, forming a KREEP - rich magma that initially should have been sandwiched between the crust and mantle . Evidence for this scenario comes from the highly anorthositic composition of the lunar highland crust, as well as the existence of KREEP - rich materials . </P> <P> The Apollo program brought back 380.05 kilograms (837.87 lb) of lunar surface material, most of which is stored at the Lunar Receiving Laboratory in Houston, Texas, and the uncrewed Soviet Luna programme returned 326 grams (11.5 oz) of lunar material . These rocks have proved to be invaluable in deciphering the geologic evolution of the Moon . Lunar rocks are in large part made of the same common rock forming minerals as found on Earth, such as olivine, pyroxene, and plagioclase feldspar (anorthosite). Plagioclase feldspar is mostly found in the lunar crust, whereas pyroxene and olivine are typically seen in the lunar mantle . The mineral ilmenite is highly abundant in some mare basalts, and a new mineral named armalcolite (named for Armstrong, Aldrin, and Collins, the three members of the Apollo 11 crew) was first discovered in the lunar samples . </P> <P> The maria are composed predominantly of basalt, whereas the highland regions are iron - poor and composed primarily of anorthosite, a rock composed primarily of calcium - rich plagioclase feldspar . Another significant component of the crust are the igneous Mg - suite rocks, such as the troctolites, norites, and KREEP - basalts . These rocks are believed to be genetically related to the petrogenesis of KREEP . </P> <P> Composite rocks on the lunar surface often appear in the form of breccias . Of these, the subcategories are called fragmental, granulitic, and impact - melt breccias, depending on how they were formed . The mafic impact melt breccias, which are typified by the low - K Fra Mauro composition, have a higher proportion of iron and magnesium than typical upper crust anorthositic rocks, as well as higher abundances of KREEP . </P>

The most common type of rock on the lunar surface is