The thermal expansion coefficients for natural troilite, FeS, Ni-rich pyrrhotite, Fe0.84Ni0.11S, and Ni-poor pyrrhotite, Fe0.87Ni0.02S, were measured during cooling by in situ neutron powder diffraction over the temperature range 873–373 K. Between 873 and 573 K, the mean thermal expansion coefficients for the three compositions are 7.4(3)×10−5 {FeS}, 8.0(4)×10−5 {Fe0.84Ni0.11S} and 8.5(4)×10−5 K−1 {Fe0.87Ni0.02S}. Below 573 down to 373 K, the first two increase considerably to 14.1(7)×10−5 {FeS} and 9.3(5)×10−5 {Fe0.84Ni0.11S} while the latter sample shows no significant variation, 8.4(5)×10−5 K−1. Below 573 K, the thermal expansion is highly anisotropic, with Δa/100 K−1 ranging from 0.89(9)% {FeS} to 0.48(12)% {Fe0.87Ni0.02S} while Δc/100 K−1 ranges from −0.39(11)% {FeS} to −0.13(2)% {Fe0.87Ni0.02S}.

Upon cooling through 573 K, troilite and pyrrhotite undergo a transition where the FeS6 octahedra distort and in the case of pyrrhotite, cation-vacancy clustering occurs. The thermal expansion coefficients are bigger for low cation-vacancy concentrations and decrease as the pyrrhotites become less stoichiometric. This indicates that the thermal expansion in these minerals is damped by vacancy ordering or clustering. The thermal expansion coefficients for troilite and pyrrhotite are amongst the largest reported for sulphide minerals and their role in the formation of ore textures is discussed briefly.