To elucidate the relationship between the superstructure and chemical composition of the pyrrhotite group, natural crystals of pyrrhotite, ranging between approximately Fe9S10 and Fe11S12, have been extensively studied by X-ray single crystal methods.

Pyrrhotite crystals from the Kishu mine are mixtures of three 4C, 4.88C, and 5C types. The 5C type is orthorhombic and has the cell dimensions a = 6.8848(14), b = 11.936(6), c = 28.6760(15), and diffraction aspect C*ca. In the Kohmori mine, various orthorhombic pyrrhotites of nonintegral type—the nC pyrrhotites—occur together with the 2C type. The observed n values range between 5.36 and 5.80. The 6C type from the Makimine Mine is metrically orthorhombic with a = 6.8950(2), b = 11.9536(4) and c = 34.518(2) but is really monoclinic with diffraction aspect F*/d.

The relationship between the cell dimensions and the n value for the nC pyrrhotites, including the 5C and 6C types, from various localities indicates that the change of the n values depends mainly on the chemical composition. The relationship between the composition and the mean d value of 112 and 022. which correspond to 102 by the hexagonal subcell, for the nC pyrrhotite has been obtained by the least squares method assuming a linear equation as follows:


where dmean represents the mean d value of 112 and 022 in Å, and x, the chemical composition of iron in atom percent. The 5C and 6C types are considered as special cases of the nC pyrrhotites, which are equivalent to the intermediate pyrrhotites.

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