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Synthesis and crystal structure of LiNbO (sub 3) -type Mg (sub 3) Al (sub 2) Si (sub 3) O (sub 12) ; a possible indicator of shock conditions of meteorites

Takayuki Ishii, Ryosuke Sinmyo, Tetsuya Komabayashi, Tiziana Boffa Ballaran, Takaaki Kawazoe, Nobuyoshi Miyajima, Kei Hirose and Tomoo Katsura
Synthesis and crystal structure of LiNbO (sub 3) -type Mg (sub 3) Al (sub 2) Si (sub 3) O (sub 12) ; a possible indicator of shock conditions of meteorites
American Mineralogist (September 2017) 102 (9): 1947-1952

Abstract

LiNbO (sub 3) -type Mg (sub 2.98(2)) Al (sub 1.99(2)) Si (sub 3.02(2)) O (sub 12) (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) A, c = 12.6885(8) A, V = 255.23(3) A (super 3) , with Z = 6 (hexagonal, R3c). The average A-O and B-O distances of the AO (sub 6) and BO (sub 6) 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 A, respectively). The present compound has the B-site cations at the octahedral site largely shifted along the c axis compared with other LiNbO (sub 3) -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 BO (sub 6) 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.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 102
Serial Issue: 9
Title: Synthesis and crystal structure of LiNbO (sub 3) -type Mg (sub 3) Al (sub 2) Si (sub 3) O (sub 12) ; a possible indicator of shock conditions of meteorites
Affiliation: University of Bayreuth, Bayerisches Geoinstitut, Bayreuth, Germany
Pages: 1947-1952
Published: 201709
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 30
Accession Number: 2017-082718
Categories: Geophysics of minerals and rocks
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables
Secondary Affiliation: Ehime University, JPN, JapanTokyo Institute of Technology, JPN, JapanUniversity of Edinburgh, GBR, United Kingdom
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201743
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