Surface alteration of pentlandite and spectroscopic evidence for secondary violarite formation

Abstract

Synthetic pentlandite surfaces were subjected to oxidation by a range of inorganic oxidants, and the resultant alteration of the surface studied by a range of surface-sensitive spectroscopic techniques. The oxidants used were air during heating to relatively low temperatures (150~ steam, ammonium hydroxide, hydrogen peroxide, and sulphuric acid. Electrochemical oxidation was also undertaken. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), conversion electron M6ssbauer spectroscopy (CEMS), and spectral reflectance measurements were used to characterize the surface compositions. New data for the binding energies of core electrons in pentlandite and violarite, based on the fitted XPS spectra, are proposed. For pentlandite and violarite respectively, values of 707.3 eV and 708.4 eV for the Fe 2p 3/2, 853.0 eV and 853.2 eV for the Ni 2p 3/2, and 161.2 eV for the S 2p in both sulphides, were obtained. After oxidation the pentlandite surfaces indicated nickel enrichment in the subsurface, with the formation of violarite. The immediate oxidized surface, of approximately 10A thickness, indicated a range of iron oxides and hydroxides (Fe304, Fe203 and FeOOH, with possible Fe1_xO and Fe(OH)3), nickel oxide (NiO), and iron sulphates (FeSO4, Fe2(SO4)3). The proportions of the phases present in the surface layer are inferred to be a consequence of both the strength of the oxidant employed, and the thermodynamic stability of the phases, as can be illustrated using partial pressure and Eh/pH diagrams. A sequence of oxidation is proposed, accounting for the sub-surface enrichment in violarite, and the development of the oxidized surface, which is inferred to have a major affect on the rates of oxidation.

High Temperature Pressure Oxidation of a Low-Grade Nickel Sulfide Concentrate with Control of the Residue Composition

“…Equations (3) and (4) are specifically consistent with previous conclusions [11][12][13]. Nickel hexahydrite formation suggests oxidation of nickel sulfide minerals, and based upon its decrease in abundance, from violarite, which is also consistent with published data [14]. The following equation is representative only as iron(II) can be oxidized to iron(III) and either form a sulfate salt at low pH or hydrolyse to goethite at higher pH.…”

Chemical composition of weathering products in neutral and acidic mine tailings from stibnite exploitation in Slovakia

“…Our results confirm the previously published conclusions that pyrite is the slowest mineral to oxidize, followed by arsenopyrite (Buckley and Walker 1988;Richardson and Vaughan 1989;Nes-As ( As already observed (Vink 1996;Maruška et al 2000;Ashley et al 2003;Wilson et al 2004), stibnite oxidizes and decomposes quickly and is only rarely observed in oxidation zones. The most frequent secondary phases in the Dúbrava impoundments are oxidation rims around pyrite and arsenopyrite, with relics of the primary sulphides frequently preserved.…”

“…Similarly, oxidation processes involving arsenopyrite (FeAsS) are relatively well known (Buckley and Walker 1988;Richardson and Vaughan 1989;Nesbitt et al 1995;Nesbitt and Muir 1998;Corkhill and Vaughan 2009). The weathering products in tailings rich in pyrite or pyrrhotite (Fe 1-x S) have been described in detail (Jambor 1994;Blowes et al 2003).…”

Mineralogy and texture of Fe-Ni sulfides in CI1 chondrites: Clues to the extent of aqueous alteration on the CI1 parent body

“…Another study, carried out by Richardson and Vaughan (1989) used a variety of oxidizing agents to alter synthetic pentlandite. Their experiments produced quite different results, with iron removed preferentially to nickel from pentlandite, producing a suite of iron oxides and iron hydroxides, with a minor amount of nickel-oxide also forming.…”