Smythite, Fe_8+xS₄, and associated minerals from the Silverfields Mine, Cobalt, Ontario

Smythite was found at the Silverfields Mine, Cobalt, Ontario, where it occurs in two distinctly different sulfide assemblages. Microprobe analyses of these smythites result in a composition of Fe₉S₁₁ (≈Fe_3·25S₄), with a measured density of 4.33 g/cm³ (4.32 g/cm³ calculated). Unit-cell dimensions referred to the cell described by Erd et al. in 1957, apparently not the true cell, at 25°C are a = 3.4651 ± 0.0005 Å, c = 34.34 ± 0.02 Å. X-ray powder diffraction study at elevated temperatures indicates the presence of a polymorphic transition at approximately 65°C. Precession photographs taken at room temperature reveal a cell with a doubling of the a axis; the symmetry of this larger cell is not R3m but possibly primitive hexagonal. Smythite from Cobalt is universally twinned (twin law: 180° rotation about [00.1]). It lacks the perfect basal cleavage typical of Indiana smythite. Its appearance is very similar to that of pyrrhotite, and only X-ray diffraction study enables one to tell them apart; smythite is, therefore, undoubtedly more common in nature than the four reported occurrences suggest.