akimotoite
{{Short description|Rare silicate mineral in the ilmenite group of minerals}}
{{Infobox mineral
| name = Akimotoite
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| category = Oxide minerals
Ilmenite group
| formula = {{chem2|(Mg,Fe)SiO3}}
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| system = Trigonal
| class = Rhombohedral ({{overline|3}})
H-M symbol: ({{overline|3}})
| symmetry = R{{overline|3}}
| unit cell = a = 4.7284, c = 13.5591 [Å]; Z = 6
| color = Colorless
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| cleavage = perfect (0001)
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| luster = vitreous
| streak = white or colorless
| diaphaneity = Transparent
| gravity = 3.81(calculated)
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| references = {{cite web|url=http://www.mindat.org/min-6794.html|title=Akimotoite|work=Mindat.org}}{{cite web|url=http://webmineral.com/data/Akimotoite.shtml|title=Akimotoite|work=Webmineral}}
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Akimotoite is a rare silicate mineral in the ilmenite group of minerals, with the chemical formula {{chem2|(Mg,Fe)SiO3}}.{{cite web|last1=Tomioka|last2=Fujino|year=1999|url=https://pubs.geoscienceworld.org/msa/ammin/article-abstract/84/3/267/43613/akimotoite-mg-fe-sio-3-a-new-silicate-mineral-of|title=Akimotoite, (Mg,Fe)SiO 3, a new silicate mineral of the ilmenite group in the Tenham chondrite}} It is polymorphous with pyroxene and with bridgmanite, a natural silicate perovskite that is the most abundant mineral in Earth's silicate mantle.{{cite journal|last1=Tomioka|last2=Fujino|year=1997|url=https://www.science.org/doi/10.1126/science.277.5329.1084|title=Natural (Mg,Fe)SiO3-Ilmenite and -Perovskite in the Tenham Meteorite|journal=Science |volume=277 |issue=5329 |pages=1084–1086 |doi=10.1126/science.277.5329.1084 }}{{cite journal|first=Oliver|last=Tschauner|display-authors=etal|year=2014|url=https://www.science.org/doi/10.1126/science.1259369|title=Discovery of bridgmanite, the most abundant mineral in Earth, in a shocked meteorite|journal=Science |volume=346 |issue=6213 |pages=1100–1102 |doi=10.1126/science.1259369 |pmid=25430766 |s2cid=20999417 }} Akimotoite has a vitreous luster, is colorless, and has a white or colorless streak. It crystallizes in the trigonal crystal system in space group R{{overline|3}}. It is the silicon analogue of geikielite (MgTiO3).
Crystal structure
The crystal structure is similar to that of ilmenite (FeTiO3) with Si and Mg in regular octahedral coordination with oxygen. The Si and Mg octahedra align in discrete layers alternating up the c-axis. The space group is R{{overline|3}} (trigonal) with a = 4.7284 Å; c = 13.5591 Å; V = 262.94 Å3; Z = 6.Horiuchi, H., Hirano, M., Ito, E., and Matsui, Y. (1982) MgSiO3 (ilmenite-type): single crystal X-ray diffraction study. American Mineralogist, 67, 788-793
Occurrence
Akimotoite was found in the Tenham meteorites in Queensland, Australia. It is believed to have
formed as the result of an extraterrestrial shock event. It is the silicon analogue of geikielite (MgTiO3). It was named after physicist Syun-iti Akimoto (also known as {{Nihongo|Shun'ichi Akimoto|秋本 俊一}}) (1925–2004), University of Tokyo.
It has also been reported from the Sixiangkou meteorite in the Gaogang District, Jiangsu Province, Taizhou Prefecture, China; the Zagami Martian meteorite, Katsina State, Nigeria and from the Umbarger meteorite, Randall County, Texas.
Akimotoite is believed to be a significant mineral in the Earth's mantle at depths of {{convert|600|-|800|km}} in cooler regions of the mantle such as where a subducted slab enters into the lower mantle. Akimotoite is elastically anisotropic and has been suggested as a cause of seismic anisotropy in the lower transition zone and uppermost lower mantle.Shiraishi, R., Ohtani, E., Kanagawa, K., Shimojuku, A., and Zhao, D. (2008) Crystallographic preferred orientation of akimotoite and seismic anisotropy of Tonga slab. Nature, 455, 657-660