Introduction

Samples brought back by the lunar missions

Main lunar minerals

One of the major problems encountered by mineralogists studying lunar mineralogy is due to the scarcity of lunar materials likely to be studied.

Two sources of rocks and lunar minerals exist indeed, they consist in:

Samples brought back by the lunar missions:

An overall weight of 382 kilograms (= 842 pounds) of various moon rocks forming 2196 samples were brought back from the Moon by the 6 American missions of the Apollo program having reached the Moon between 1969 and 1972. We must add to that weight approximately 300 grams (3/4 of pound) of moon rocks brought back by the 3 Russian missions of the Luna program.

These samples contributed to a better knowledge of the Moon and about the early formation of the solar system. One of their main interests is to have been dated by radioisotopic methods. The NASA Johnson Space Center located in Houston (Texas, U.S.A.) is in charge of the preservation of the lunar samples brought back by the American missions as to prepare samples which can be studied by scientists. On the whole, 97,000 catalogued samples were prepared by the Johnson Space Center facilities for study and analysis. Even today, more than 25 years after the lunar missions, scientists belonging to more than 60 laboratories through the whole world continue to study lunar samples. This requires for the Johnson Space Center to prepare and send to the researchers more than 1,000 samples every year. Studied samples remaining non destroyed or undamaged by the analyses are send back to NASA and reconditioned for a later re-use.

Because of their age, ranging between 3 and 4,6 billion years, which are seldom never met with terrestrial rocks, lunar rock are an invaluable source to improve our knowledge of the beginning of the history of the solar system. In such a way, studies of lunar and terrestrial isotopes brought more strength to the hypothesis of the Moon being created during the collision between a giant asteroid having the same dimensions than March with the Earth, in the early times of the formation of our solar system.

It was also learned that the lunar crust formed 4,4 billion years ago. All the steps of the formation of the Moon formation:

are recorded in the lunar rocks and can be studied by specialists.

Lunar soils covering the lunar crust since its formation were subjected to irradiation by solar radiations, so they have recorded the evolution of the solar activity with time.

New minerals like tranquilityite or armalcolite were found initially in moon rocks, but since their discovering, those mineral species have also been found on Earth.

Silicates

Oxides

Sulphides

Native metals

Phosphates

Meteoritic minerals of origin

 Silicates

Pyroxenes is one of the groups of minerals the most abundant in the lunar crust.

Studies of exsolution lamellae in lunar pyroxenes, in particular in clinopyroxenes, augite and pigeonite, demonstrated that these minerals had been created during reaction at temperatures lower than their melting point. These same studies proved that lunar pyroxenes cooled slowly after their formation. From those measurement it have been estimated that a basalt flow 6 meters thick located on Apollo 15 landing site cooled at 0.2 to 1.5 degree per hour.

Rare observations were also made revealing, inside lunar pyroxene crystals, of shock lamellae formed long ago at the time of severe meteoritic impacting.

In crystalline rocks such as anorthosites, the chemical composition of pyroxenes indicates a higher content for magnesium than for iron.

Most of lunar feldspars belong to the plagioclases family. The plagioclases feldspars are major constituents of rocks constituting the lunar crust. In our current state of knowledge, mean anorthite contents in plagioclases of various moon rocks are:

Statistically, lunar plagioclases have a lower sodium content and thus are closer to the pole anorthite than those found on Earth. The more sodic plagioclases were found in geological formations belonging to lunar highlands and in particular in rock enriched in potassium (K), in rare earth (= Rare Earth Element or REE) and in phospore (P) for which the name KREEP is often used.

Olivine is one of the major constituents of rocks forming the lunar crust. Inside the basalts of the lunar mare, olivines have compositions with a forsterite (magnesian pole of olivines) content ranging between 30% and 80%. More ferrous olivines (with less than 30% forsterite) are rarer, although crystals of an almost pure fayalite were found in the most recent basalts of the lunar mare which are also richer in iron. The main impurities of lunar olivines are calcium, manganese, chromium and aluminium. It was noted that chromium was more abundant in lunar olivines (up to 0,6% in weight) that in terrestrial olivines. This is probably due to the smaller mean oxidation degree of chromium (divalent chromium) related to the low partial pressure of oxygen at the time of the basaltic flows resulting in the lunar mare formation. In the same manner, one can note abnormally high chromium content inside the pyroxenes crystals of the same basalts.

In moon rocks, silica crystallises in the form of quartz, tridymite or cristobalite. Silica is much rarer in rocks from the lunar crust than in those of the earth's crust. A less evolved crust can explain this on the Moon due to a weaker development of magmatic differentiation. Another reason is the lower water content. It is interesting to note that if some form of high pressure silica polymorph such as coesite and stishovite can be found on Earth in relation to meteorite impacts, these mineral have never been yet identified on the Moon. This is probably related to the scarcity of silica on Moon and with fast evaporation of molten silica in vacuum.

Lunar minerals from the silica group concentrated first inside KREEP rocks. Quartz was found in felsitic clasts in crystals having a needle shape, which seem to result from the transformation of original tridymite crystals.

Quartz was also found in few rare lunar granite samples.

The most common crystallised silica polymorph in basalts from the lunar mare is cristobalite, which can represent up to 5% in certain basalts. The partial inversion of cristobalite into tridymite has been sometimes observed in those basalts.

Although they are rare, small, and difficult to study, lunar zircons proved to be extremely significant to date lunar samples, particularly the oldest lunar rocks, which constitute the lunar mountains. Zircons also record well fission tracks. The main source of lunar zircons is the famous lunar granites with high silica content, which seem particularly rare. Sample 15405 is made of a breccia with a composition of quartz monzodiorite have been proved to have a volume content of zircon reaching 0,6 %.

However, the majority of lunar zircons are found in single grains in lunar soils and breccias. This is due to the scarcity of lunar granites and the longevity of zircon. They were also found inside metamorphised rocks inclusions inside basalts. In that case, they probably result from the transformation of tranquillityite.

Pyroxferroite is the ferrous lunar equivalent of our terrestrial pyroxmangite. The two minerals possess a formula (Mn, Fe) SiO3 however pyroxmangite never contains more than 25% iron whereas pyroxferroite is richer in this metal and was never found in soils.

Pyroxferroite was found in lunar basalts rich in iron and particularly lunar mare basalts.

NASA mentioned garnets been found in brought back samples from the Moon are probably the result of a contamination and in this case it cannot be considered to be of lunar origin.

The name of this mineral derives from Sea of Tranquillity, Apollo 11 landing site. This mineral was found inside basalts from the famous lunar mare. Tranquilityite crystals possess a planar elongated shape with a length lower than 100 micrometers. This mineral is often associated with apatite and pyroxferroïte inside small voids in the rock and seem to have been among the very last minerals been formed. Tranquilityite is translucent and non- pleochroic. It seems to result from an assembly of thin blades and appears with a deep red colour in transmitted light. This colour seems related to the strong titanium content of the mineral.

The pseudobrookite group or "armalcolite group" gather minerals with a general formula X2YO5. Sites X and Y possess an octahedral co-ordination number allowing the substitution in large proportions between divalent and trivalent iron, magnesium, aluminium and titanium. The major mineral species of the group are pseudobrookite (Fe2TiO5), ferropseudobrookite (FeTiO5), and armalcolite ( Mg 0.5 Fe0.5 TiO5 ).

The study of the zonation inside spinels is used as an indicator of the history of crystallisation of basalts from the lunar mare. Chromite was also discovered on the Moon.