Abstract
Variability in the chemical composition, textures and electrical properties of the major sulphide mineral pyrite may be one cause of variation in the flotation and leaching properties of different sulphide ores. This report summarises the results of a review that has been conducted to establish the range in variability of natural pyrite with regard to chemical composition, texture and electrical properties. The S/Fe ratio in pyrite is generally very close to 2, suggesting that stoichiometric variability is small in this mineral. However, minor deviations from the ideal are reported. Pyrite typically contains a host of minor and trace elements, including: Ag, As, Au, Bi, Cd, Co, Cu, Hg, Mo, Ni, Pb, Pd, Ru, Sb, Se, Sb, Sn, Te, Tl and Zn. Minor elements are often present within the mineral lattice at levels up to several percent, these include; As, Co, Ni, Sb and possibly Cu, Ag, Au and Sn. Arsenian pyrites may contain up to 10% As, and such specimens are typically rich in other minor and trace elements, particularly Au. As-rich pyrites typically appear to have formed at relatively low temperatures and often exhibit habits that suggest rapid precipitation; these As-rich pyrites may be metastable and, therefore, relatively reactive. Considerable variation has been reported in the semiconducting properties of pyrite. Natural pyrites are either n- or p-type semiconductors and reported conductivities vary by four orders of magnitude. The rates of galvanic processes in mineral pulps are likely to vary with the mineral conductivities. Typically p-type pyrites exhibit low conductivities and are As-rich and are suggested to have formed at relatively low temperature, whereas relatively highly conducting n-type pyrites are typically As-poor and suggested to have formed at high temperature. Based on comparative measurements of rest potential, iron pyrite is the most electrochemically inert of the common sulphide minerals, with a rest potential of the order of 0.6 (cf. sphalerite and galena with rest potentials of 0.46 and 0.40 V. vs. SHE, respectively). Variability in the rest potential of different samples of this mineral are generally small. However, based on the extent of pyrite reaction during a peroxide dissolution procedure, the chemical reactivity of pyrite samples may differ significantly.
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