Review on Mackinawite and Valleriite: Formulae, Localities, Associations and Intergrowths of the Minerals, Mode of Formation and Optical Features in Reflected Light

Abstract

Investigations of the two minerals concern their chemical compositions and formulae, their intergrowths with other minerals, typical minerals that occur in association with them, their mode of formation within defined types of ore deposits/occurrences and finally also their optical characteristics under reflected light. Mackinawite was observed at 16 localities distributed all over the world from which more than 50 polished sections were available and more than 70 analyses were carried out. Valleriite was found at six localities. Microscopic work was based on more than 40 polished sections and more than 30 analyses. In the literature, mackinawite is reported as a metal excess phase with the formula (Fe, Ni)1+xS and (x ≤ 0.07). Deduced from analyses of this study, mackinawite has a wider compositional range and the formula: (Fe, Ni, Co)1- xS to (Fe, Ni, Co)1+y S, where x=0.00–0.10 and y=0.00–0.10. Therefore, mackinawite may not only occur as a metal excess phase, but also as a metal deficiency one. The high-temperature (max. about 500°C) mineral mackinawite occurs in various types of sulphidic deposits, and is characteristically accompanied with chalcopyrite, pyrrhotite, sometimes pentlandite, pyrite, sphalerite and cubanite and occurs often in oriented intergrowth within chalcopyrite which is always twinned due to inversion. At lower temperature, mackinawite was also formed by the replacement of chalcopyrite (not twinned) and rarely by that of pentlandite and linneite. The formula of valleriite is (Fe, Cu)2S2 • 1.5 [(Mg, Al)1(OH)2]. The analyses of this study show, that valleriite has a wider compositional range and the proposed formula is: (Fe, Cu)2S2 • [(Mg, Fe)1(OH)2] • y [Al(OH)0] with x=1.20 – 2.10 and y=0 – 0.50. Identical with the latter formula, namely that Al and vacancies are constituents of the OH-layer, the formula inverts to: (Fe, Cu)2S2 • [(Mg2+, Fe2+, Al3+ y, ϒy/2)1(OH)2] where x=1.24–2.25 and y=0.00–0.26. Valleriite, formed under mesothermal up to low katathermal conditions, is always a newly-formed mineral. It occurs together with chromite and/or magnetite, mostly along their cracks and fissures, in ultramafic rocks that are serpentinized, but only in the presence of chalcopyrite. Valleriite was also observed as replacer of chalcopyrite. Valleriite decomposes under high-grade metamorphic conditions (e. g. in Outokumpu, Finland).