STRUCTURES AND DEFECTS OF FERRIHYDRITE: DIRECT OBSERVATION USING Z-CONTRAST IMAGING

Ferrihydrite is a common Fe-oxy(hydroxide) nano-mineral with very reactive surfaces in modern sediments. It also occurs in ferritin, an Fe storage in human and other living organisms. Ferrihydrite is a common precursor of goethite and hematite in low-temperature geological environments, such as in banded iron formations. However, the structure of ferrihydrite is still under debate. Due to the nano-crystallinity, structural disordering of the ferrihydrite, and the uncertainty of whether or not ferrihydrite is single phase, previous studies from indirect measurements using both elastic scattering (diffraction, and nano-diffraction), and inelastic scattering of X-ray were not able to provide agreeable structure models. The issues such as number of structural phases, unit cell parameters, stoichiometry, ordering of Fe , and whether ferrihydrite has tetrahedrally-coordinated Fe are still controversial. With spherical aberration-corrected scanning transmission electron microscopy (STEM), we can obtain locations of Fe atoms and vacancies in ferrihydrite directly from high resolution Z-contrast images. Two polymorphs are confirmed: ferrihydrite-4H (~ 2/3) and ferrihydrite-2H (~1/3). Intergrowth of the two polymorphs can also occur with (001) as interface. The 4H polymorph has ABAC packing for oxygen atoms, and the 2H polymorph has ABAB packing. All Fe atoms occupy octahedral sites. Fe and vacancy ordering in ferrihydrite-4H results in locally doubled and tripled unit cells along a-b directions. Due to structural disordering between Fe and vacancies, ferrihydrite can be considered as a 2-D crystal with two strong (hk0) diffraction peaks. The calculated XRD pattern based on the new structural models fit observed and reported XRD patterns well. Stoichiometry of 6-line ferrihydrite is close to Fe(III)_0.75(O, OH)₂ based on occupancy fitting, i.e., between Fe(OH)₃ and FeOOH, instead of between FeOOH and Fe₂O₃.