Pyrrhotite and Troilite
Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay
Pyrrhotite has about as simple a structure as any mineral. Sulfur atoms are hexagonally close packed, but unlike hematite, where iron only occupies 2/3 of the octahedral voids, iron occupies every void in pyrrhotite.
For some reason, many descriptions of pyrrhotite fixate on the iron atoms, which form a network of triangular prisms. While it's easy to visualize triangular prisms, such a description completely obscures the overall structure. The diagram below shows four layers of the structure with higher layers to the left and darker. Sulfur atoms are yellow, iron green.
The variety where every octahedral void is occupied is called troilite and is often found in meteorites. It is believed to be a principal phase in the cores of smaller planets. Troilite is non-magnetic.
In pyrrhotite, some of the octahedral voids are vacant, so the formula is Fe1-xS. The vacancies distort the lattice, so pyrrhotite is often not hexagonal. The vacancies also cause pyrrhotite to be magnetic.
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Created 18 April, 2011, Last Update