Akimotoite

Akimotoite
General
Category Oxide minerals
Ilmenite group
Formula
(repeating unit)		 (Mg,Fe)SiO3
Strunz classification 04.CB.05[1]
09.DA.05[2]
Crystal system Trigonal
Unit cell a = 4.7284 Å; c = 13.5591 Å; Z=6
Identification
Color Colorless
Crystal symmetry Trigonal rhombohedral
H-M symbol: (3)
Space group: R3
Cleavage perfect (0001)
Luster vitreous
Streak white or colorless
Diaphaneity Transparent
Specific gravity 3.81(calculated)
References [1][2]

Akimotoite is a rare silicate mineral ((Mg,Fe)SiO3) in the ilmenite group of minerals. It is polymorphous with bridgmanite, a natural silicate perovskite, which, on the contrary, is regarded as the most common mineral on Earth.[3][4] 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]

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.[5]

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 (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.[6]

See also

References

  1. 1 2 3 4 5 Akimotoite on Mindat.org
  2. 1 2 Akimotoite on Webmineral
  3. ↑ Mindat, http://www.mindat.org/min-6794.html
  4. ↑ Tschautner et al. 2014, http://science.sciencemag.org/content/346/6213/1100
  5. ↑ Horiuchi, H., Hirano, M., Ito, E., and Matsui, Y. (1982) MgSiO3 (ilmenite-type): single crystal X-ray diffraction study. American Mineralogist, 67, 788-793
  6. ↑ 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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