1911 Encyclopædia Britannica/Pyrrhotite

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PYRRHOTITE, a mineral species consisting of iron sulphide and crystallizing in the hexagonal system. The formula is FenSn+1 where n may vary from 5 to 16; usually it is Fe7S8 or Fe11S12, the latter being also the composition of the artificially prepared compound. Small amounts of nickel and cobalt are often present.

Crystals have the form of hexagonal plates bounded at their edges by faces of a hexagonal prism and pyramids, which are deeply striated horizontally. More frequently, however, the mineral is massive, with a laminar or granular structure. The colour is bronze-yellow and the lustre metallic; the streak is greyish-black. The hardness is 4 and the specific gravity 4.58-4.64. The mineral is magnetic, sometimes with polarity, and it is therefore often called “magnetic pyrites.”

Pyrrhotite occurs in metalliferous veins, and as grains and plates disseminated through various rocks. In the gabbros and norites of Norway and Sweden it has been concentrated by magmatic differentiation at the margins of the igneous masses. Large bodies of massive pyrrhotite occur at Bodenmais in Bavaria and in Wheal Jane near Truro in Cornwall. Crystallized specimens are from the metalliferous veins at Morro Velho in Brazil, Kongsberg in Norway, and Andreasberg in the Harz. Crystals of pyrrhotite have also been observed in meteoric stones; but iron sulphide appears more commonly in meteorites, especially in meteoric irons, as troilite (FeS), which, if really distinct from pyrrhotite, has not been met with in terrestrial rocks. (L. J. S.)