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Small rock fragments from the lunar "soil" collected by the Apollo 11 astronauts in 1969. The background grid spacing is 2 mm.
(Photo by Randy Korotev)

Apollo 11 was the first mission to the Moon on which samples were collected and brought to Earth. In July of 1969, the Apollo 11 astronauts landed in Mare Tranquilitatis, a huge impact crater formed in the lunar crust about 4 billion years ago that had been flooded by basaltic lavas 3.6-3.9 billion years ago. Between the time the lavas cooled and the astronauts collected samples, the area was impacted by countless meteoroids, big and small. The impacts both shattered large rocks into small rocks and welded (melting and shock compaction) fine-grained material into coarser-grained material, forming impact breccias.

The medium-gray, crystalline fragments in the photo are the basalts, usually called "mare basalts." There are 2 black, glassy, impact-melt spheroids (one shiny, one not). The other darkest fragments are glassy soil breccias (now usually called regolith breccias) consisting of fine-grained soil that was "glued" into a rock by a meteoroid impact. The light-colored fragments are anorthosites and impact breccias composed mainly of anorthosite.
anorthosite: An igneous or plutonic rock consisting mainly of the mineral anorthite, a feldspar with a high ratio of calcium (Ca) to sodium (Na)

A common mineral ranging in composition from sodium aluminum silicate (albite) to calcium aluminum silicate (anorthite)

[pronounced ba-SALT]: One form of solidified lava or magma from a volcano

breccia [pronounced BRETCH-ee-a]: A rock made up from bits and pieces of older rocks

maria [pronounced MAR-ee-a; Latin for "seas"]: The dark, circular features on the surface of the Moon that are easily visible from Earth

mare [pronounced MAR-ay; Latin for "sea"]: Singular form of mare

The Apollo 11 mission landed in a mare because mare are flatter and, consequently, safer places to land than the surrounding highlands. At the time of the Apollo 11 mission, little was know about the crust of the more rugged lunar highlands, although the unmanned Surveyor VII mission in 1968 had suggested that the highlands were rich in aluminum.

When the material of the photo was first examined in late 1969, a team of scientists from the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, made an important observation and leapt to a conclusion that was to become one of the most significant to be made from the first Apollo samples. In the abstract to the paper* describing their findings, John Wood, John Dickey, Ursula Marvin, and Ben Powell state the following:
"We prepared and studied thin section of 1676 rock fragments (dia. range 1-5 mm) from the Apollo 11 bulk sample. Most are basaltic rocks, glasses, and soil breccias, but about 4 per cent of the fragments are of a totally unanticipated material: anorthositic rocks, breccias, and glasses. The anorthositic rocks are similar to terrestrial anorthosites except for the fine grain-size (typically 20-100 µ) and low content of Na (the feldspar is usually anorthite). Many have what we feel are are true cumulate textures. Because of their light color, low density, and chemical resemblance to the Surveyor VII analysis of material in the lunar highlands, we believe this anorthositic suite is derived from the highlands (portions of which lie only ~50 km from Tranquility Base). The finer-grained soil fraction from the Apollo 11 sample must contain more than 4 per cent of the anorthositic component (about 20 per cent) to account for the bulk composition of soils and breccias.

The highlands would have to be underlain by ~25 km of anorthosite, if their 3 km of mean relief above the maria is isostatically compensated. We propose a lunar model in which such an anorthositic layer "floats" on gabbro, which in early times welled up into major impact craters to form maria. Since basaltic lava contracts upon on solidification, faster cooling and earlier solidification of the material in the maria than in the highlands would promote transfer of additional magma from beneath the highlands into the maria to fill the volume vacated upon solidification, and this would give rise to mascons. The model entails extensive melting and crystals fractionation in the moon, with anorthosite crystals floating to the surface through a denser magma."

We now know that the "belief" expressed in the first paragraph is correct: The typical crust of the Moon consists of anorthosite. The "extensive melting" suggested by the researchers in the second paragraph has become what is now called the "magma ocean" model. In this model, much or all of the Moon was molten very early in the history of the Solar System. The earliest minerals to crystallize from the melt (olivine and pyroxene) were rich in iron and sank to form the lunar mantle. When plagioclase feldspar began to crystallize, it was less dense and floated in the melt to form the anorthositic (plagioclase-rich) crust.

* Wood J. A., Dickey J. S. Jr., Marvin U. B., and Powell B. N. (1970) Lunar anorthosites and a geophysical model of the moon, Proceedings of the Apollo 11 Lunar Science Conference, pages 965–988.

See also: "How Do We Know That It's a Rock from the Moon?" and "Lunar Meteorites"