Lunar materials: Their mineralogy, petrology and chemistry

Abstract

The manned Apollo 11, 12, 14 and 15 and the automated Luna 16 lunar missions have provided us with lunar rock and regolith (soil) samples from a number of geologically distinct sites. The mare regions were sampled by Apollo 11, 12 and Luna 16, whereas Apollo 14 landed on a terrain with more relief, the Fra Mauro Formation which represents an ejecta blanket from the Imbrian Basin, and Apollo 15 touched down near the lunar highlands. The samples collected consist of a mixture, mainly of basalt, breccia and regolith (soil-particulate matter, generally < 1 cm in size). The basalts show considerable variation in texture, mineralogy and chemistry and probably represent fragments from various parts of relatively thin and extensive lava flows in the maria. The breccias represent regolith material which was indurated to varying degrees by impact events. The regolith is a product of the breakdown, again by impact, of coherent rock masses of basalt and breccia. The composition (major as well as trace elements) of the regolith at a specific site does not generally represent the average composition of the basaltic rock types collected at that same site. The regolith contains small amounts of anorthositic and noritic fragments, a component rich in KREEP (potassium, rare earths and phosphorus) and a small meteoritic component which appears generally to be chondritic or a mixture of chondrite and iron meteorite in composition. The chemistry of the lunar basalts shows a depletion in the generally more volatile elements (e.g., alkalis) with respect to terrestrial basalts. Some refractory elements such as titanium and rare earths, however, show a considerably higher concentration in lunar basalts than in their average terrestrial counterparts. Three new minerals have been recognized in the basalts, breccias and regolith. These are: pyroxferroite (Ca 0.15 Fe 0.85 Sio 3), armalcolite [Fe,Mg)Ti 2O 5] and tranquillityite [Fe 8 2+ (Zr + Y) 2Ti 3Si 3O 24]. New compositional varieties of several terrestrially known minerals have also been found. H 2O and Fe 3+ are completely lacking and organic compounds are also absent from the lunar materials. Impact craters ranging from several hundred kilometers to several microns in diameter (microcraters) attest to the importance of meteorite impact as an erosional mechanism on the Moon. The age of the Moon appears to be approximately 4.6 aeons (1 aeon= 1,000 million years) and the ages of the basalts indicate that they originated between 3.9 and 3.3 aeons ago. This age span may represent the period during which the mare were flooded with basaltic flows or the specific dates may represent the last major flooding at each site. Regolith ages from all sites lie between 4.3 and 4.6 aeons.