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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Putirka, Keith / Swainson, Ian

12 Issues per year


IMPACT FACTOR 2016: 2.021

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1945-3027
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Volume 99, Issue 5-6 (May 2014)

Issues

Crystal structures of laihunite and intermediate phases between laihunite-1M and fayalite: Z-contrast imaging and ab initio study

Huifang Xu
  • Corresponding author
  • NASA Astrobiology Institute, Department of Geoscience, University of Wisconsin-Madison, Madison, Wisconsin 53706, U.S.A.
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  • De Gruyter OnlineGoogle Scholar
/ Zhizhang Shen
  • NASA Astrobiology Institute, Department of Geoscience, University of Wisconsin-Madison, Madison, Wisconsin 53706, U.S.A
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/ Hiromi Konishi
  • NASA Astrobiology Institute, Department of Geoscience, University of Wisconsin-Madison, Madison, Wisconsin 53706, U.S.A.
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/ Pingqiu Fu
  • Institute of Geochemistry, The Chinese Academy of Sciences, Guiyang, Guizhou 550002, P.R. China
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/ Izabela Szlufarska
  • Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, U.S.A.
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Published Online: 2014-05-15 | DOI: https://doi.org/10.2138/am.2014.4691

Abstract

Crystals of laihunite from Xiaolaihe of Liaoning Province, northeast China, were studied using selected-area electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM), and Z-contrast imaging. Z-contrast images directly reveal ordered vacancies in M1 sites. The results confirm early structural models for 1-layer laihunite (or laihunite-1M) with ideal stoichiometry of □0.5Fe2+0.5Fe3+SiO4. 2-layer laihunite and 3-layer laihunite are found to be chemically different from laihunite-1M. The 2-layer laihunite can be viewed as a periodic intergrowth of laihunite and fayalite in the 1:1 ratio. The 3-layer laihunite can be considered to be a periodic intergrowth of laihunite and fayalite in the 1:0.5 ratio along the c-axis. Ideal stoichiometries for the 2-layer structure and the 3-layer structure are □0.5Fe2+2.5Fe3+[SiO4]2 and □1.0Fe2+ 3.0Fe3+2.0[SiO4]3, respectively. The structural intergrowth of the 3-layer laihunite and the 1-layer lahunite results in chemical compositions falling within the range between the two aforementioned structures, such as the chemical formula of □0.4Fe2+ 0.8Fe3+0.8SiO4, reported earlier in the literature. The crystal structures of the 1-layer laihunite (1M), the 2-layer laihunite (2M), and the 3-layer laihunite (3Or) determined from Z-contrast images and ab initio calculations using the density functional theory (DFT) have space groups of P21/b, P21/b, and Pbnm, respectively. The previously reported monoclinic symmetry for the 3-layer laihunite may be an artifact due to overlapping diffraction spots from both, the laihunite-3Or and the laihunite-1M. Our study demonstrates that the method of combining Z-contrast imaging and ab initio calculation can be effectively used for identifying structures of nano-phases in host crystals. Perhaps more importantly, Z-contrast imaging provides a powerful means for direct observation of vacancies and other defects, and may be utilized to map vacancies in Fe3+-bearing olivines, the alignments of which can greatly affect anisotropic diffusion in such structures.

Keywords : Laihunite; vacancy ordering; Z-contrast imaging; DFT; HRTEM; superstructure; olivine; oxidation

About the article

Published Online: 2014-05-15

Published in Print: 2014-05-01


Citation Information: American Mineralogist, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am.2014.4691.

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© 2014 by Walter de Gruyter Berlin/Boston.

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