LiNbO3-type Mg2.98(2)Al1.99(2)Si3.02(2)O12 (py-LN) was synthesized by recovering a run product from 2000 K and 45 GPa to ambient conditions using a large volume press. Rietveld structural refinements were carried out using the one-dimensional synchrotron XRD pattern collected at ambient conditions. The unit-cell lattice parameters were determined to be a = 4.8194(3) Å, c = 12.6885(8) Å, V = 255.23(3) Å3, with Z = 6 (hexagonal, R3c). The average A-O and B-O distances of the AO6 and BO6 octahedra have values similar to those that can be obtained from the sum of the ionic radii of the averaged A- and B-site cations and oxygen (2.073 and 1.833 Å, respectively). The present compound has the B-site cations at the octahedral site largely shifted along the c axis compared with other LiNbO3-type phases formed by back-transition from perovskite (Pv)-structure, and as a result, the coordination number of this site is better described as 3+3. It appears therefore that the B-site cation in the octahedral position cannot be completely preserved during the back-transition because of the small size of Si and Al, which occupy usually a tetrahedral site at ambient conditions. The formation of py-LN can be explained by the tilting of BO6 octahedra of the perovskite structure having the pyrope composition and formed at high P-T conditions. The tilting is driven by the decrease in ionic radius ratio between the A-site cation and oxygen during decompression. This also explains why there is no back-transition from the Pv-structure to the ilmenite-structure during decompression, since this is a reconstructive phase transition whose activation energy cannot be overcome at room temperature. Py-LN may be formed in shocked meteorites by the back-transformation after the garnet-bridgmanite transition, and will indicate shock conditions around 45 GPa and 2000 K.
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September 01, 2017
Synthesis and crystal structure of LiNbO3-type Mg3Al2Si3O12: A possible indicator of shock conditions of meteorites
Takayuki Ishii;
Takayuki Ishii
*
1
Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany2
Geodynamics Research Center, Ehime University, Matsuyama, Ehime 790-8577, Japan*
E-mail: [email protected]
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Ryosuke Sinmyo;
Ryosuke Sinmyo
3
Earth-Life Science Institute, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan
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Tetsuya Komabayashi;
Tetsuya Komabayashi
4
School of GeoSciences and Centre for Science at Extreme Conditions, University of Edinburgh, Edinburgh EH9 3FE, U.K.
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Tiziana Boffa Ballaran;
Tiziana Boffa Ballaran
1
Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany
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Takaaki Kawazoe;
Takaaki Kawazoe
1
Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany
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Nobuyoshi Miyajima;
Nobuyoshi Miyajima
1
Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany
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Kei Hirose;
Kei Hirose
3
Earth-Life Science Institute, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan
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Tomoo Katsura
Tomoo Katsura
1
Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany
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Takayuki Ishii
*
1
Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany2
Geodynamics Research Center, Ehime University, Matsuyama, Ehime 790-8577, Japan
Ryosuke Sinmyo
3
Earth-Life Science Institute, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan
Tetsuya Komabayashi
4
School of GeoSciences and Centre for Science at Extreme Conditions, University of Edinburgh, Edinburgh EH9 3FE, U.K.
Tiziana Boffa Ballaran
1
Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany
Takaaki Kawazoe
1
Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany
Nobuyoshi Miyajima
1
Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany
Kei Hirose
3
Earth-Life Science Institute, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan
Tomoo Katsura
1
Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany*
E-mail: [email protected]
Publisher: Mineralogical Society of America
Received:
26 Nov 2016
Accepted:
04 May 2017
First Online:
20 Sep 2017
Online ISSN: 1945-3027
Print ISSN: 0003-004X
Copyright © 2017 by the Mineralogical Society of America
Mineralogical Society of America
American Mineralogist (2017) 102 (9): 1947–1952.
Article history
Received:
26 Nov 2016
Accepted:
04 May 2017
First Online:
20 Sep 2017
Citation
Takayuki Ishii, Ryosuke Sinmyo, Tetsuya Komabayashi, Tiziana Boffa Ballaran, Takaaki Kawazoe, Nobuyoshi Miyajima, Kei Hirose, Tomoo Katsura; Synthesis and crystal structure of LiNbO3-type Mg3Al2Si3O12: A possible indicator of shock conditions of meteorites. American Mineralogist 2017;; 102 (9): 1947–1952. doi: https://doi.org/10.2138/am-2017-6027
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Index Terms/Descriptors
- alkali metals
- crystal structure
- decompression
- garnet group
- high pressure
- indicators
- lattice parameters
- lithium
- metals
- metamorphism
- meteorites
- nesosilicates
- niobates
- orthosilicates
- oxides
- perovskite structure
- phase transitions
- pressure
- pyrope
- Rietveld refinement
- shock metamorphism
- silicates
- synthesis
- ultrahigh pressure
- unit cell
- X-ray diffraction data
- bridgmanite
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