The nA superstructure is developed in sulfur-rich pyrrhotites and (Fe,Ni)1−xS analogues. Precession photographs show slightly offset ‘c’ reflections and weak 001 and 001 reflections. In electron diffraction patterns ‘b’ and ‘d’ reflections as well as ‘c’ reflections are doublets with relative separation distances: Δhb ≃ 2Δhd ≃ 3 Δhc. Direct lattice repeats on the order of 250-700 Å in the (001) plane are required to explain the observed separation distances of ‘c’ reflections. A regular stacking sequence of full and vacancy-containing metal layers may be inferred from the integral 3C repeat. High resolution transmission electron microscopy is the most promising technique for the ultimate solution of the nA superstructure problem.

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