Pyrrhotite - Crystal Structure

Crystal Structure

Pyrrhotite has a number of polytypes of hexagonal or monoclinic crystal symmetry; several polytypes often occur within the same specimen. Their crystalline structure is based on the NiAs unit cell, where metal occurs in octahedral coordination and anions in trigonal prismatic arrangement. An important feature of this structure is an ability to omit metal atoms with the total fraction up to 1/8, thereby creating iron vacancies. One of such structures is pyrrhotite-4C (Fe₇S₈). Here "4" indicates that iron vacancies form a superlattice which 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 (Fec₁₁S₁₂), 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.