Structure changes of nanocrystalline mackinawite under hydrothermal conditions. Formation of greigite and its structural properties

Structural changes and transformation of nanocrystalline mackinawite to greigite were investigated using powder X-ray diffraction measurements, Raman spectroscopy, X-ray absorption fine structure, and ab initio quantum chemical calculations. We clarify the formation conditions for greigite. Nanocrystalline mackinawite utilized by Sano et al. (2020) was used as a starting material. By hydrothermal heating at 140 ℃ for 12 h, nanocrystalline mackinawite was transformed into greigite without other iron phases. During the transformation, the crystallite size increased from 5 to 12 nm. After 3 h of hydrothermal heating, the Fe³⁺/ΣFe ratio markedly increased, but the a and c lattice parameters of nanocrystalline mackinawite were almost unchanged. Immediately after the transformation, greigite exhibited a larger lattice parameter, which suggests that excess Fe²⁺ was incorporated into the octahedral site. Thereafter, the greigite gradually shrunk and Fe³⁺/ΣFe progressively approached the ideal value. The changes in the unit cell volume and Fe valence state implied that when greigite initially occurred, the nonstoichiometry allowed to accommodate more Fe²⁺ in greigite to the octahedral site. The hydrothermal heating of nanocrystalline mackinawite is potentially applicable to the synthesis of pure nanocrystalline greigite. (author)