High resolution transmission electron microscopic study of synthetic nanocrystalline mackinawite

Direct observation using high resolution transmission electron microscopy reveals that precipitated iron(II) monosulfide, FeS, consists of nanocrystalline mackinawite particles. The individual nanocrystals are laminar rectilinear prisms displaying a continuum of particle sizes from 2 to 5.7 nm in thickness (the direction parallel to the c axis) and from 3 to 10.8 nm in length. The corresponding mean specific surface area is estimated to be to 380 ± 10 m2/g. The d001 of mackinawite nanocrystals obtained from precipitated FeS and freeze-dried FeS by electron diffraction are 5.19 and 5.08 Å, respectively. The effect of water on the nanoparticle structures is indicated by the formation of curved structures and infrequent dislocations in an anhydrous environment. The apparent disorder suggested by absent or weaker lattice spacings is symptomatic of the breakdown of Braggian systematics at these particle sizes. The results confirm that nanoparticulate materials do not behave simply as small fragments of their bulk crystalline counterparts. The results contribute to understanding the behavior of nanoparticulate materials on planetary surfaces and in the biosphere in general and the nature and properties of FeS in anoxic aqueous environments in particular.