Armalcolite

Armalcolite () is a titanium-rich mineral with the chemical formula (Mg,Fe²⁺)Ti₂O₅. It was first found at Tranquility Base on the Moon in 1969 during the Apollo 11 mission, and is named for Armstrong, Aldrin and Collins, the three Apollo 11 astronauts. Together with tranquillityite and pyroxferroite, it is one of three new minerals that were discovered on the Moon. Armalcolite was later identified at various locations on Earth and has been synthesized in the laboratory. (Tranquillityite and pyroxferroite were also later found at various locations on Earth). The synthesis requires low pressures, high temperatures and rapid quenching from about 1,000 °C to the ambient temperature. Armalcolite breaks down to a mixture of magnesium-rich ilmenite and rutile at temperatures below 1,000 °C, but the conversion slows down with cooling. Because of this quenching requirement, armalcolite is relatively rare and is usually found in association with ilmenite and rutile, among other minerals.

Occurrence

thumb|left|The [[Apollo 11 crew, after whom Armalcolite is named. Left to right are Neil Armstrong, Michael Collins, and Buzz Aldrin.]]

Armalcolite was originally found on the Moon, in the Sea of Tranquility at Tranquility Base, and also in the Taurus–Littrow valley and the Descartes Highlands. The largest amounts were provided by the Apollo 11 and 17 missions. It was later identified on Earth from samples of lamproite dikes and plugs taken in Smoky Butte, Garfield County, Montana, US. On the Earth, it also occurs in Germany (Nördlinger Ries impact crater in Bavaria), Greenland (Disko Island), Mexico (El Toro cinder cone, San Luis Potosí), South Africa (Jagersfontein, Bultfontein and Dutoitspan kimberlite mines), Spain (Albacete Province and Jumilla, Murcia), Ukraine (Pripyat Swell), United States (Knippa quarry, Uvalde County, Texas and Smoky

Butte, Jordan, Montana) and Zimbabwe (Mwenezi District).

Armalcolite is a minor mineral found in titanium-rich basalt rocks, volcanic lava and sometimes granite pegmatite, ultramafic rocks, lamproites and kimberlites. It is associated with various mixed iron-titanium oxides, graphite, analcime, diopside, ilmenite, phlogopite and rutile. It forms elongated crystals up to about 0.1–0.3 mm in length embedded in a basalt matrix. The quenching step is required both for laboratory and natural synthesis in order to avoid conversion of armalcolite to a mixture of magnesium-rich ilmenite (Mg-) and rutile (TiO₂) at temperatures below 1,000 °C. Consequently, the relative amount of ilmenite and armalcolite can be used as an indicator of the cooling rate of a mineral during its formation. Armalcolite is a part of the pseudobrookite group which consists of minerals of the general formula X₂YO₅. X and Y are usually Fe (2+ and 3+), Mg, Al, and Ti. End members are armalcolite ((Mg,Fe)Ti₂O₅), pseudobrookite (Fe₂TiO₅), ferropseudobrookite (FeTi₂O₅) and "karrooite" (MgTi₂O₅). They are isostructural and all have orthorhombic crystal structure and occur in lunar and terrestrial rocks.

Chemical composition of most armalcolite samples can be decomposed into a sum of metal oxides as follows: TiO₂ (concentration 71–76%), FeO (10–17%), MgO (5.5–9.4%), Al₂O₃ (1.48–2%), Cr₂O₃ (0.3-2%) and MnO (0–0.83%). Whereas the titanium content is relatively constant, the ratio of magnesium to iron varies and is usually lower than 1.

Most titanium is present in armalcolite in the 4+ state, owing to the reducing synthesis environment, but there is a significant fraction of Ti³⁺ in lunar samples. The Ti³⁺/Ti⁴⁺ ratio in armalcolite can serve as an indicator of fugacity (effective partial pressure) of oxygen during the mineral's formation. It also allows one to distinguish lunar and terrestrial armalcolite, as Ti³⁺/Ti⁴⁺ = 0 for the latter.

Since armalcolite's formula is (Mg,Fe²⁺)Ti₂O₅, it follows the general formula of XY₂O₅ where the X=(Mg and Fe²⁺), Y=Ti, and O is oxygen. Both X and Y sites are octahedrally coordinated and the radius ratio between the cations and the anions in armalcolite are at three to five ratio equaling 0.6 making the structure octahedral. Armalcolite is a titanium-rich mineral that falls under the magnesianferropseudobrookite mineral group with Fe²⁺Ti₂O₅ and MgTi₂O₅ as end members.