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Concept
Pyrrhotite
Summary
Pyrrhotite (pyrrhos in Greek meaning "flame-coloured") is an iron sulfide mineral with the formula Fe(1-x)S (x = 0 to 0.125). It is a nonstoichiometric variant of FeS, the mineral known as troilite.
Pyrrhotite is also called magnetic pyrite, because the color is similar to pyrite and it is weakly magnetic. The magnetism decreases as the iron content decreases, and troilite is non-magnetic. Pyrrhotite is generally tabular and brassy/bronze in color with a metallic luster. The mineral occurs with mafic igneous rocks like norites, and may form from pyrite during metamorphic processes. Pyrrhotite is associated and mined with other sulfide minerals like pentlandite, pyrite, chalcopyrite, and magnetite, and has been found globally.
Pyrrhotite exists as a number of polytypes of hexagonal or monoclinic crystal symmetry; several polytypes often occur within the same specimen. Their structure is based on the NiAs unit cell. As such, Fe occupies an octahedral site and the sulfide centers occupy trigonal prismatic sites.
Materials with the NiAs structure often are non-stoichiometric because they lack up to 1/8th fraction of the metal ions, creating vacancies. One of such structures is pyrrhotite-4C (Fe7S8). Here "4" indicates that iron vacancies define a superlattice that is 4 times larger than the unit cell in the "C" direction. The C direction is conventionally chosen parallel to the main symmetry axis of the crystal; this direction usually corresponds to the largest lattice spacing. Other polytypes include: pyrrhotite-5C (Fe9S10), 6C (Fe11S12), 7C (Fe9S10) and 11C (Fe10S11). Every polytype can have monoclinic (M) or hexagonal (H) symmetry, and therefore some sources label them, for example, not as 6C, but 6H or 6M depending on the symmetry.
The monoclinic forms are stable at temperatures below 254 °C, whereas the hexagonal forms are stable above that temperature. The exception is for those with high iron content, close to the troilite composition (47 to 50% atomic percent iron) which exhibit hexagonal symmetry.
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