Akimotoite

Akimotoite is a rare silicate mineral ((Mg,Fe)SiO3) in the ilmenite group of minerals.[3] It is polymorphous with pyroxene and with bridgmanite, a natural silicate perovskite that is the most abundant mineral in Earth's silicate mantle.[4][5][6] Akimotoite has a vitreous luster, is colorless, and has a white or colorless streak. It crystallizes in the trigonal crystal system in space group R3. It is the silicon analogue of geikielite (MgTiO3).[1]

Akimotoite
General
CategoryOxide minerals
Ilmenite group
Formula
(repeating unit)		(Mg,Fe)SiO3
Strunz classification4.CB.05[1]
Crystal systemTrigonal
Crystal classRhombohedral (3)
H-M symbol: (3)
Space groupR3
Unit cella = 4.7284, c = 13.5591 [Å]; Z = 6
Identification
ColorColorless
Cleavageperfect (0001)
Lustervitreous
Streakwhite or colorless
DiaphaneityTransparent
Specific gravity3.81(calculated)
References[1][2]

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 R3 (trigonal) with a = 4.7284 Å; c = 13.5591 Å; V = 262.94 Å3; Z = 6.[7]

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 Shun'ichi Akimoto (秋本 俊一)) (1925–2004), University of Tokyo.[1]

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.[1]

Akimotoite is believed to be a significant mineral in the Earth's mantle at depths of 600–800 kilometres (370–500 mi) 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.[8]

See also

References
  1. Akimotoite on Mindat.org
  2. Akimotoite on Webmineral
  3. Tomioka and Fujino 1999. https://pubs.geoscienceworld.org/msa/ammin/article-abstract/84/3/267/43613/akimotoite-mg-fe-sio-3-a-new-silicate-mineral-of
  4. Mindat, http://www.mindat.org/min-6794.html
  5. Tomioka and Fujino 1997, http://science.sciencemag.org/content/277/5329/1084
  6. Tschauner et al. 2014, http://science.sciencemag.org/content/346/6213/1100
  7. Horiuchi, H., Hirano, M., Ito, E., and Matsui, Y. (1982) MgSiO3 (ilmenite-type): single crystal X-ray diffraction study. American Mineralogist, 67, 788-793
  8. 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


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