The phase relations of pentlandite assemblages in the Fe-Ni-S system have been reinvestigated by sealed, silica tube annealing experiments at 600 degrees , 500 degrees , 400 degrees , 300 degrees , and 230 degrees C. The compositions of the coexisting phases in the quenched products have been determined by electron probe analyses, so that the phase boundaries are located with much greater accuracy than in previous work and actual tie lines plotted. The tie lines between pentlandite solid solution (pns) and monosulfide solid solution (mss) change direction at a mss composition of about 33 at. % Ni, correlating with the composition at which the mss field breaks down below 400 degrees C. The mss field, continuous at 400 degrees C, extends only to about 25 at. % Ni at 300 degrees C; the other more S-rich phases coexisting with pns at this temperature being a second mss phase, containing about 33 at. % Ni, and beta (Ni,Fe) S containing about 5 at. % Fe. The mss field withdraws to 17 at. % Ni at 230 degrees C, enabling pyrite-pns tie lines to be established. At 230 degrees C, the limits of solid solution of Ni in pentlandite are 15 at. % when coexisting with troilite and 34 at. % when coexisting with beta (Ni,Fe) S.Electron probe analyses have been made on a variety of natural pentlandite assemblages. A compilation of data from all sources confirms that the composition of pentlandite varies systematically with the assemblage, ranging from 18 at. % Ni in the assemblage troilite-pentlandite to 34 at. % Ni in the assemblage millerite-heazlewoodite-pentlandite. The Ni content of pyrrhotite and the Fe contents of millerite and heazlewoodite are significantly lower than the corresponding data for the synthetic system at 230 degrees C, suggesting that chemical readjustment persisted to very low temperatures in these assemblages. The good agreement among several limiting compositions in comparing the synthetic and natural data indicates that the principal factors controlling the phase chemistry of natural nickel sulfide assemblages are related to crystal-crystal equilibria.

The Chemical Compositions of Synthetic and Natural Pentlandite Assemblages

The wide variety of metal sulfide structures and their accommodation of atomic substitution, non-stoichiometry and metal-metal ( M - M ) and ligand-ligand interactions allows for diverse physical, chemical and electronic properties. The energy band structure of 3 d transition-metal sulfides, in particular, is strongly influenced by the covalence of metal-S bonds, which results in hybridization of S 3 p and metal 3 d bonding states and direct or indirect M - M bonding interactions in favorable cases. Differences in the phase relations of isostructural metal sulfides are often attributable to subtle changes in electronic states. The literature on metal sulfide phase relations relevant to the earth sciences is very extensive and could not possibly be summarized in a single chapter. Therefore, following Craig and Scott (1974), this chapter focuses on the base metal (Fe, Co, Ni, Cu, and Zn) sulfides, with the literature for other metal chalcogenides and pnictides and some sulfosalts summarized in a single table (Table 1⇓). The relevant phase relations of the platinum-group element (PGE) chalcogenides and pnictides have been comprehensively reviewed in Makovicky (2002). A section on the halite structure sulfides (niningerite, alabandite and oldhamite) is also included in this chapter. The material presented here relates closely to that discussed in other chapters, in particular the chapter on sulfide thermochemistry (Sack and Ebel 2006). The importance of understanding phase equilibria in the context of electronic and magnetic properties and, hence, electronic structure is also emphasized, thereby reinforcing material presented in several other chapters (Pearce et al. 2006; Vaughan and Rosso 2006).

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 Table 1. 
Summary of studies on sulfide phase relations relevant to Earth sciences.



Abbreviations used for mineral/phase names include: tr- troilite; po- pyrrhotite; hpo- hexagonal pyrrhotite; mpo- monoclinic pyrrhotite; py- pyrite; pn- pentlandite; hpn- high pentlandite; vs- vaesite; mss - (Fe,Ni) monosulfide solid …

Phase equilibria at high temperatures

Sulfurization of meteoritic metal in H 2 S-H 2 gas produced three different sulfides: monosulfide solid solution [(Fe,Ni) 1− x S], pentlandite [(Fe,Ni) 9− x S 8 ], and a phosphorus-rich sulfide. The composition of the remnant metal was unchanged. These results are contrary to theoretical predictions that sulfide formation in the solar nebula produced troilite (FeS) and enriched the remaining metal in nickel. The experimental sulfides are chemically and morphologically similar to sulfide grains in the matrix of the Alais (class CI) carbonaceous chondrite, suggesting that these meteoritic sulfides may be condensates from the solar nebula.

https://science.sciencemag.org/content/sci/277/5324/358.full.pdf

Experimental simulations of sulfide formation in the solar nebula

This paper is based on 465 new analyses of Ni, Cu, S and PGE from the 19 chromitite horizons between the LG-1 and UMG-2 from 6 sectors around the Bushveld Complex, along with microprobe analyses of representative samples of 41 chromites. Two trends in chromite composition, A and B, are distinguished on a plot of cation% Mg/(Mg + Fe2+) versus Cr/(Cr+Al). Trend A, that has a negative slope, is close to that predicted as the result of the reciprocal exchange substitution of Cr and Fe2+ for Mg and Al between spinel and liquid affecting the Mg-Fe2+ spinel-liquid Kd2. Trend B, that has a positive slope and is defined primarily by the LG-5 to MG-2 chromitites, is the result of the progressive increase in the activity of Al2O3 as a result of the fractional crystallization of orthopyroxene. Overall, the average PGE concentrations in massive chromitite increase upward. The LG-1 to LG-4 chromities have low (Pt+Pd)/(Rh+Ru+Ir+Os) ratios (0·1 to 0·3), above which there is an abrupt jump to higher ratios in the ...

Chromite composition and PGE content of Bushveld chromitites: Part 1 – the Lower and Middle Groups

A thorough review and critical evaluation of phase equilibria and thermodynamic data for phases in the iron-nickel-sulfur ternary system at 1 bar pressure has been made over the entire composition range for temperatures from 25°C to above the liquidus. The Gibbs energies of system phases have been modeled, and optimized model parameters have been obtained which reproduce all data simultaneously within experimental error limits. The modeling is based on the recent evaluations by the authors of the Fe-S and Ni-S binary subsystems. For the liquid phase, the recently extended modified quasichemical model for short-range ordering is applied for the first time to a liquid metal-sulfur phase. Although the binary Fe-S and Ni-S liquid solutions were modeled with the extended modified quasichemical model, while the binary Fe-Ni liquid solution was modeled with a simple random-mixing model, it is shown, for the first time, that these can be combined into a consistent model of the ternary liquid phase. Two-sublattice models, within the framework of the compound-energy formalism, are used for the solid solutions: pyrrhotite, pentlandite, pyrite, vaesite, and high-temperature heazlewoodite. The ternary-solution model for iron-nickel pyrrhotite permits very good prediction of thermodynamic properties solely on the basis of binary-model parameters.

Critical thermodynamic assessment and modeling of the Fe-Ni-S system

A curve relating composition of synthetic pentlandite with interplanar spacing was determined experimentally as follows: d115 = 1.9407 – 0.0023x + 0.0077x2, where x represents the iron to nickel we...

R. W. Shewman

Papers

Pentlandite phase relations in the Fe–Ni–S system and notes on the monosulfide solid solution