Smythite occurs as a coherent twinned intergrowth with hexagonal 3C pyrrhotite. Individual single crystals contain about 50% smythite. Reciprocal lattice rows with h-k 3n show continuous diffraction streaks. The available data suggest that smythite forms via a polytypic displacive transformation, by the introduction of stacking faults in the hexagonal close-packed layers of S atoms in high-temperature 1C pyrrhotite. This is analogous to the transformation of 2H wurtzite to intermediate ordered and disordered ZnS layer sequences. The ideal formula of smythite appears to be Fe₁₃S₁₆.

Monoclinic Fe₃S₄ (a=5.93, b=3.42, c=10.64 Å, =91.9°) is present in amounts up to 25% of total sulfides. It has a derivative NiAs-type structure, and is isomorphous with monoclinic Cr₃S₄ and Fe₃Se₄. It occurs as small lenticular lamellae within grains of 3C pyrrhotite, and apparently corresponds to the unidentified lamellar phase of Arnold (1962). The lamellae have a rhombohedral morphology, with a habit plane close to {1011}. In single crystal grains of pyrrhotite, monoclinic Fe₃S₄ in twinned in a manner consistent with transformation from high-temperature 1C pyrrhotite. Although Fe₃S₄ lamellae have the general appearance of plate martensite, they do not represent a diffusionless transformation.