Sudoites exhibit different crystal-chemical and textural properties which may be related to the structural and valence state of Fe. Mössbauer spectroscopic analysis shows that all Fe previously analyzed using a microprobe (from 1 wt.% to 7.2 wt.% total Fe as Fe2O3) is structural and occurs in both oxidation states (40% Fe2+ and ~60% Fe3+). Electron microprobe analyses from ~200 sudoites indicate that Fe occurs in both octahedral sheets according to three main types of substitution: Fe3+ = octahedral Al; Fe2+ = Mg; and Fe3+ + Fe2++ □ = 3Mg. Decreasing tetrahedral substitution balances Fe3+ substitution in the trioctahedral sheet. Increasing octahedral Fe results in a more dioctahedral character of sudoite.
X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, differential thermal analysis (DTA) and scanning electron microscope-transmission electron microscope (SEM-TEM) analyses showed that increasing octahedral Fe is associated with decreased stacking order and thermal stability due to the greater number of defects. In addition, with increasing octahedral Fe in sudoite, particles became smaller and more anhedral and consequently less stable with increasing Fe content. These structural and textural variations are interpreted as a result of the distortion of the sudoite structure by substitutions of Fe3+ with larger ionic radii for Al and Mg octahedral cations and by the formation of octahedral vacancies.