Tranquillityiteissilicate mineral with formula (Fe2+)8Ti3Zr2Si3O24.[1] It is mostly composed of iron, oxygen, silicon, zirconium and titanium with smaller fractions of yttrium and calcium. It is named after the Mare Tranquillitatis (Sea of Tranquility), the place on the Moon where the rock samples were found during the 1969 Apollo 11 mission. It was the last mineral brought from the Moon which was thought to be unique, with no counterpart on Earth, until it was discovered in Australia in 2011.[10]
In 1970, material scientists found a new unnamed Fe, Ti, Zr- silicate mineral containing rare-earths and Y in lunar rock sample 10047.[11][12][13][14] The first detailed analysis of the mineral was published in 1971 and the name "tranquillityite" was proposed and later accepted by the International Mineralogical Association.[1][3][15][16] It was later found in lunar rock samples from all Apollo missions.[17] Samples were dated by Pb/Pb ion probe techniques.[18][19][20][21]
Together with armalcolite and pyroxferroite, it is one of the three minerals which were first discovered on the Moon, before terrestrial occurrences were found.[6][22] Fragments of tranquillityite were later found in Northwest Africa, in the NWA 856 Martian meteorite.[23][24]
Tranquillityite forms thin stripes up to 15 by 65 micrometres in size in basaltic rocks, where it was produced at a late crystallization stage. It is associated with troilite, pyroxferroite, cristobalite and alkali feldspar. The mineral is nearly opaque and appears dark red-brown in thin crystals.[8] The analyzed samples contain less than 10% impurities (Y, Al, Mn, Cr, Nb and other rare-earth element) and up to 0.01% (100 ppm) of uranium.[26] Presence of a significant amount of uranium allowed scientists to estimate the age of tranquillityite and some associated minerals in Apollo 11 samples as 3710 million years using the uranium–lead dating technique.[21]
Irradiation by alpha particles generated by uranium decay is believed to be the origin of the predominantly amorphous metamict structure of tranquillityite. Its crystals were obtained by annealing the samples at 800 °C (1,470 °F) for 30 minutes. Longer annealing did not improve the crystalline quality, and annealing at higher temperatures resulted in spontaneous fracture of samples.[17]
The crystals were initially found to have a hexagonal crystal structure with the lattice parameters, a = 1.169 nm, c = 2.225 nm and three formula units per unit cell,[8] but later reassigned a face-centered cubic structure (fluorite-like).[17] A tranquillityite-like crystalline phase has been synthesized by mixing oxide powders in an appropriate ratio, determined from the chemical analysis of the lunar samples, and annealing the mixture at 1,500 °C (2,730 °F). This phase was not pure, but intergrown with various intermetallic compounds.[17]
Cameron, E. N. (1970). "Opaque minerals in certain lunar rocks from Apollo 11". Proceedings of the Apollo 11 Lunar Science Conference (5–8 January 1970, Houston, TX). Geochimica et Cosmochimica Acta Supplement. 1: Mineralogy and Petrology: 193–206. Bibcode:1970GeCAS...1..221C.
Dence, M. R.; Douglas, J. A. V.; Plant, A. G.; Traill, R. J. (1970). "Petrology, Mineralogy and Deformation of Apollo 11 Samples". Proceedings of the Apollo 11 Lunar Science Conference (5–8 January 1970, Houston, TX). Geochimica et Cosmochimica Acta Supplement. 1: Mineralogy and Petrology: 315–340. Bibcode:1970GeCAS...1..315D.
Fleischer, Michael (1973). "New mineral names"(PDF). American Mineralogist. 58 (1–2): 139–141.
Gatehouse, B. M.; Grey, I. E.; Lovering, J. F.; Wark, D. A. (1977). "Structural studies on tranquillityite and related synthetic phases". Proceedings of the Lunar Science Conference, 8th, Houston, Tex., March 14–18, 1977. 2 (A78-41551 18–91). New York: Pergamon Press, Inc.: 1831–1838. Bibcode:1977LPSC....8.1831G.
Hinthorne, J.R.; Andersen, C.A.; Conrad, R.L; Lovering, J.F. (1979). "Single-grain 207Pb/206Pb and U/Th age determinations with a 10-micron spatial resolution using the ion microprobe mass analyzer (IMMA)". Chem. Geol. 25 (4): 271–303. Bibcode:1979ChGeo..25..271H. doi:10.1016/0009-2541(79)90061-5.
Lovering, J. F.; Wark, D. A.; Reid, A. F.; Ware, N. G.; Keil, K.; Prinz, M.; Bunch, T.E.; El Goresy, A.; Ramdohr, P.; et al. (1971). "Tranquillityite: A new silicate mineral from Apollo 11 and Apollo 12 basaltic rocks". Proceedings of the Lunar Science Conference. 2: 39–45. Bibcode:1971LPSC....2...39L.
Rasmussen, Birger; Fletcher, Ian R.; Muhling, Janet R. (2008). "Pb/Pb Geochronology, Petrography and Chemistry of Zr-rich Accessory Minerals (Zirconolite, Tranquillityite and Baddeleyite) in Mare Basalt 10047". Geochimica et Cosmochimica Acta. 72 (23): 5799–5818. Bibcode:2008GeCoA..72.5799R. doi:10.1016/j.gca.2008.09.010.
Rasmussen, Birger; Fletcher, Ian R.; Gregory, Courtney J.; Muhling, Janet R.; Suvorova, Alexandra A. (2012). "Tranquillityite: The last lunar mineral comes down to Earth". Geology. 40 (1): 83–86. Bibcode:2012Geo....40...83R. doi:10.1130/G32525.1.
