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Huifang Xu - Research Activities


Current Research Activities in Nanogeoscience Group

Left: Z-contrast image showing a thin nano-precipitate. The precipitates are of the Si-magnetite phase, although both the host and precipitates have the same structure.

Right: A new magnetic Fe3S4 nano-phase
High-magnification Z-contrast image (a) and noise-filtered image (b) along b showing positions of Fe columns, S columns and vacancy columns. Projections of atoms based on the proposed structure model are also overlaid on the images. Intensity profiles across Fe2 atoms and Fe1 atoms are illustrated in (c) and (d) respectively. The vacancy sites between neighbouring Fe1 atoms are obvious because of low signal intensity.

♦ Atomic resolution chemical imaging of mineral structures and defects: One of my long-term research goal is to investigate interface structure and defects in minerals and crystalline materials in order to understand formation/evolution history of minerals and physical and chemical properties of materials. A state-of-the-art field emission-gun scanning transmission electron microscope and high-resolution TEM (FEI Titan‑80‑200) equipped with cutting edge technology spherical aberration correction for electromagnetic lens (supported by UW and NSF through NSF’s MRI Program) is one of our main research tools. The new FEG STEM/HRTEM will benefit our research in studying microstructures, reaction fronts, and interface structures in minerals at the atomic scale.

Left: Noise-filtered Z-contrast image showing structural modulation through orientation changes of neighboring Si/Al dumbbells (outlined with yellow ovals) and framework. A section of the structure from structural refinement is also illustrated at the left side of the image.
Right: Modulated structure of a labradorite in 3+1 super space (movie)

♦ Plagioclase feldspars: aperiodic structures, modulations, and subsolidus phase relations
Plagioclase feldspars that commonly occur in igneous and metamorphic rocks are the most abundant minerals in the earth’s crust. Although plagioclase feldspars form complete solid solution at high temperature, their subsolidus phase relations at low temperature are very complicated and not well understood due to coupled ordering between (Al+Ca) and (Si+Na). The crystal structures and their formation mechanism of incommensurately modulated structure in intermediate plagioclase, or e-plagioclase (from ~ An25 to ~An75)have been an enigma for decades beginning with the first discovery in 1940. Early published works are based on centrosymmetricity in e-plagioclase feldspars.  Even very different structure models were proposed based on the exact same set of experimental data because of incorrect symmetry assumption. Recent atomic resolution Z-contrast imaging study reveals Ca-Na ordering and non-centrosymmetricity in both Na-rich and Ca-rich e-plagioclase feldspars. We will utilize these breakthroughs regarding incommensurately modulated structure symmetry to refine the e-plagioclase feldspars that span the entire composition range of intermediate plagioclase minerals. Specifically, we will exploit our newly discovered understanding of a sub cell without inversion center, centering conditions, and super space group characteristics to decipher the intricate nature of e-plagioclase feldspars.
It is proposed to use state-of-the-art Z-contrast imaging with 0.08nm resolution, single crystal X-ray diffraction, and subsolidus phase relation calculation using density functional theory (DFT) to investigate the intermediate plagioclase feldspars that span the entire composition range from ~ An25 to ~An75, in order to understand their modulated structures, formation mechanisms, stabilities, and subsolidus phase relations. The proposed study includes the following 3 tasks: (1) Z-contrast imaging of intermediate plagioclase feldspars using an aberration-corrected scanning transmission electron microscope; (2) Crystal structure determination of e-plagioclase by single crystal X-ray diffraction method; and (3) DFT-based calculation of the subsolidus phase relations of the plagioclase feldspar system.


Left: polyhedral models showing arsenate adsorption on (001) surface of the proto-goethite.

♦ Solving structures of nanominerals

We use our designed multiple/complementary methods to better understand the nano-minerals including crystal structure, surface behaviors, and their nano pores. Synchrotron XRD is widely used to study nano-structure in bulk. High-resolution TEM (HRTEM) combined with selected-area electron diffraction (SAED) provides direct phase and structure information. High-angle annular dark-field (HAADF) STEM images, so-called Z-contrast image with the intensity being proportional to the atomic number, can be inferred directly atomic positions. The Z-contrast imaging is often combined with electron energy-loss spectroscopy (EELS) and X-ray EDS to obtain additional chemical and structural information on the atomic scale. The Pair distribution function (PDF) method of X-ray and neutron total scattering can yield ancillary data in constraining the atomic-scale bonding. Furthermore, the complex nature of the nano-mineral problem means that the solution will require the coherence between experimental data and simulated modeling work. The density functional theory (DFT) calculation can determine the allowed energy states in the system.

Left: A bridge shape configuration of tri-mannose lying flat on the dolomite surface (insert at low-right corner). The tri-mannose sugar can lower the energy barrier by about 1 kcal/mole (Shen et al., 2015).
Right: Comparison of the MgCO3 contents of synthetic Ca-Mg carbonates precipitated in control, non-metabolizing H. saccharolyticum (fermentative bacteria) or D. retbaense (SRB) biomass-bearing solutions with different initial Mg:Ca ratios (Zhang et al., 2015).
♦ Sedimentary dolomite formation
The “dolomite problem” is a geological challenge that has endured since its first identification by the French scientist Deodate Dolomieu in 1791. Our UW team has studied the dolomite for 10 years with support from the NASA Astrobiology Institute, National Science Foundation, and our university. There have been four PhD students, two Master's students, and two research scientists involved in the project.
Article: Sweet spot for the formation of sedimentary dolomite. PDF


Above: Size-dependent phase map of nanometric iron(III) oxide (γ → ε → α pathway) based on the TEM observations: blue dots, the maximum size of γ-Fe2O3 crystals; dark brown dots, the maximum size of twinned ε-Fe2O3 crystals; brown circles, the maximum size of ε-Fe2O3 single crystals.

♦ Effects of size and morphology of nano-crystals and nano-precipitates on their reactivity and stability: When crystals decrease in size, contributions from their interface energies increase significantly. This affects the structure, reactivity, and stability fields on their phase diagrams. For both nano-crystals and nano-precipitates, we integrate experimental work with theoretical modeling using density function theory to build relationship between size, morphology, and interfaces with host phases. Nano-precipitates that record evolutionary histories of their host rocks are common in rock-forming minerals of metamorphic and igneous rocks.


Fig. 3

Left: Nanoporous alumina for studying the pore size effect on uranium redox reactions.

♦ Nanopore controlled geochemical processes: The behavior of geological fluids in nanopores is significantly different from those in normal non-nanoporous environments; the nanopore environment affects both dielectric property of solvent water and solvation of dissolved metal ions. We investigate the effect of nanopore surfaces on sorption / desorption, and redox reactions of radionuclide of uranium and other heavy metals.


Fig. 3Left: Platy magnetite (left) and zincite (right) formed in organic-bearing solutions (Nature Materials, 2003)
♦ Minerals as biosignatures: The minerals formed through bioorganic- and enzyme-involved systems will display unique shape, chemistry, and architecture. Features such as these in Fe-sulfides, Fe-oxides, and carbonates / dolomite minerals may be used as potential biosignatures preserved in early earth sediments and Martian rocks. We also investigate the roles of lithotrophic bacteria on mineral transformations in Fe-oxides, Fe-hydroxides, and carbonate minerals in surface and subsurface environments. This is project is supported by NASA Astrobiology Institute.

Fig. 3Left: An Achaean BIF nearby Underground Mine State Park, Soudan, Minnesota

♦ BIF minerals and their formation environments: The research focuses on micro- and nano-phase minerals and their textures preserved in banded iron formation (BIF) to understand their formation conditions and mechanisms of formation. By combining mineralogical study and geochemical modeling, we try to understand mechanism for genesis of BIFs, and to constrain the Earth environment during the BIF formation period. BIF formation requires Fe-Si-rich geological fluids. Such fluids can be generated by hydrothermal alteration of komatiites, the low-Al oceanic rocks generated by deep mantle plumes on early earth. This research is supported by NASA Astrobiology Institute.


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