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Volume 3, Issue 2
ISSN: 2470-9905
Crystallography 2017
October 16-17, 2017
2
nd
International Conference on
October 16-17, 2017 | Chicago, USA
Applied Crystallography
Understanding interface and surface structures of nano-phases in natural Fe-oxyhydroxide and Fe-bearing
olivine minerals
Huifang Xu, Seungyeol Lee
and
Shiyun Jin
University of Wisconsin-Madison, USA
U
nderstanding interface structures, nano-precipitates, vacancies, impurities and adsorbed atoms onmineral surfaces are important
to elucidate formation mechanism and reactions of minerals in the earth environments. Aberration-corrected Z-contrast
imaging can provide chemical images with sub-Å resolution. Z-contrast images are HAADF images with atomic resolution. Multiple
diffraction effects that appear in high-resolution transmission electronmicroscopic (HRTEM) images can be eliminated or minimized
in Z-contrast images, because Z-contrast imaging uses non-coherent elastically scattered electrons at high scattering angle. We can
obtain positions of atoms directly over a large range of thickness with Z-contrast to help distinguish columns of different atoms and
their occupancies along the beam direction. Interface structures and crystal structures of nano-minerals and nano-precipitates can
be solved by combining the Z-contrast imaging and ab
initio
calculation using density functional theory (DFT) methods. Vacancies,
impurities, adsorbed heavy atoms can be also revealed directly. Vacancy ordering in Fe-bearing olivine and Fe-sulfides, adsorbed
heavy metals (e.g., As, Au, U) on Fe-oxyhydroxide minerals are resolved clearly. Z-contrast images of the Fe-oxyhydroxides show
ordered FeOOH proto-goethite nano-domains intergrown with nanophase goethite. The FeOOH nanophase is a precursor to the
goethite. DFT calculations indicate that goethite is more stable than proto-goethite. Our results suggest that ordering between Fe and
vacancies in octahedral sites result in the transformation from feroxyhyte to goethite through a proto-goethite intermediate phase.
Combining Z-contrast images and TEM-EDS reveals that arsenate (AsO
4
3-
) tetrahedra are preferentially adsorbed on the proto-
goethite (001) surface.
Figure-1:
(A) a [001]-zone-axis Z-contrast image of proto-goethite, bright spots are positions of Fe atom columns. Very bright
spots are adsorbed arsenic (As) atoms on the surface right above Fe; (B) the intensity profile of an outlined area from X to Y; (C) the
intensity profile of simulated Z-contrast image showing the peak with As atom above an Fe atom column.
Biography
Huifang Xu has received his Bachelor’s degree from Nanjing University and PhD degree from The John’s Hopkins University in field of Mineralogy and Crystallography. He
has completed his Postdoctoral studies at Arizona State University in area of Electron Crystallography. He is a Faculty Member in the Department of Geoscience and Ma-
terials Science Program at the University of Wisconsin-Madison. His research interests are studies of incommensurately modulated structures and nano-phase structures
using e-beam imaging, X-ray diffraction and neutron scattering methods. He has published more than 100 papers in fields of mineralogy, crystallography and inorganic
materials. He is an elected Fellow of Mineralogical Society of America.
hfxu@geology.wisc.comHuifang Xu, Struct Chem Crystallogr Commun, 3:2
DOI: 10.21767/2470-9905-C1-002