Nanoparticle Assembly Leads to Mackinawite Formation

Iron sufides are important mineral phases in natural environments where they control global elemental cycles. Fe–S phases have been suggested to form through the transformation of several possible precursors to finally reach stable crystalline structures. Mackinawite is a metastable intermediate, of which a full chemical and structural characteristization of various possible intermediate stages in its formation pathways, or the chemical conditions that affect the transformations to the metastable mackinawite, are well understood. Here we report, the various steps of mackinawite formation via oriented aggregation (OA) from a nanoparticulate precursor. During OA, the formation of aggregates is a crucial stage for self-assembly of primary particles to reach stable structures. The formation occurs in five steps: (1) homogeneous nucleation of primary FeSnano particles; (2 and 3) formation of mass fractal-like aggregates from the FeSnano as precursor toward the transformation to mackinawite; (4) oriented alignment and self-assembly of these mackinawite-like aggregates; and (5) transformation to a still metastable but typical layered mackinawite structure.