Direct Observation of Iron Vacancies in Polytypes of Pyrrhotite

Abstract

The existence of a variety of closely related superstructures in certain sulphide minerals is usually attributed to ordered and partially ordered arrangements of metal vacancies; pyrrhotite, Fe_1-xS being a particularly good example. Three different types of pyrrhotite have been found in nature (Carpenter et al., 1964; Morimoto et al., 1975 a and b), all of them having superstructures of the NiAs type structure (A ~ 3.45 Å and C ~ 5.8 Å). They are monoclinic pyrrhotite, 4C, intermediate pyrrhotite, nC and hexagonal troilite, 2C. The 4C and 2C types are stoichiometric with compositions Fe₇S₈ and FeS, respectively, while the nC type is non-stoichiometric with a composition from approximately Fe₉S₁₀ to Fe₁₁S₁₂ (Morimoto et al., 1970; Nakazawa and Morimoto, 1971; Morimoto et al., 1975b). A wider composition range of nC is observed above 100° C. The iron-deficient nC transforms to a high-temperature phase, NA, above 220° C (Nakazawa and Morimoto, 1971).