To study thermal desulfurization of pyrite (FeS2), we conducted in situ neutron diffraction experiments in the temperature range 298–1073 K. On heating, pyrite remained stable up to 773 K, at which it started to decompose into pyrrhotite (Fe1−xS) and S2 gas. Rietveld analysis of the neutron data from 298 to 773 K allowed determination of the thermal expansion coefficient of pyrite (space group Pa$\bar 3$) to be αV = 3.7456 × 10−5 K−1, which largely results from the expansion of the Fe–S bond. With further increase in temperature to 1073 K, all the pyrite transformed to pyrrhotite (Fe1−xS) at 873 K. Unit-cell parameters of Fe1−xS (space group P63/mmc) increase on heating and decrease on cooling. However, the rates in cell expansion are larger than those in contraction. This hysteresis behavior can be attributed to continuous desulfurization of pyrrhotite (i.e., x in Fe1−xS decreases) with increasing temperature until the stoichiometric troilite (FeS) was formed at 1073 K. On cooling, troilite underwent a magnetic transition to an orthorhombic structure (space group Pnma) between 473 and 573 K. In addition, using differential thermal analysis (DTA) and thermogravimetric analysis (TGA) implemented with a differential scanning calorimeter, we performed kinetic measurements of pyrite decomposition. Detailed peak profile and Arrhenius (k = A exp(−Ea/RT)) analyses yielded an activation energy Ea of 302.3 ± 28.6 kJ/mol (based on DTA data) or 302.5 ± 26.4 kJ/mol (based on TGA data) and a ln(A) of 35.3 ± 0.1.

The 3.5 Ga Bon Accord Ni deposit occurs within the lowest serpentinized mafic–ultramafic lavas of the Barberton Greenstone Belt (South Africa). Though now completely mined out, it comprised a suite of rare Ni-rich minerals that led to its interpretation as either an extraterrestrial body or as an oxidized fragment of Fe-Ni alloy originating from the terrestrial core. In this study, we draw on detailed petrographic observation and mineral chemical data, as well as previous work, to re-evaluate these ideas. The balance of evidence, from thin section (<1 mm) to regional (∼10s of km) scales, appears to support an alternative origin for Bon Accord, possibly as an oxidized Ni-sulfide deposit formed in association with ocean floor komatiite eruptions.

Petrographic and mineralogical studies of samples of the Normal (or undisturbed) Merensky Reef from Frank Shaft No.1 at Rustenburg Platinum Mine revealed the presence of a Pt-Fe-alloy, probably isoferroplatinum (58 vol.% of total precious metal minerals), arsenides (21 vol.%), bismuthotellurides (10 vol.%), electrum (9 vol.%) and platinum group element- (PGE-) sulfides and stannides (2 vol.%), associated predominantly with base-metal- and iron-sulfides. A Pt-Fe-alloy-dominated facies has been known for considerable time from potholes and discordant bodies and has been attributed to fluid activity with high fO2 and low fS2. Our petrographic results indicate that the normal thin reef has also undergone hydrothermal alteration. For the first time, the rare mineral troilite (stoichiometric FeS) was found as intergrowths with masses of Pt-Fe-alloy, together with Fe-rich pyrrhotite, secondary hydrous silicates, magnetite and calcite. The observed mineral assemblage and texture is interpreted as the product of partial desulfurization, caused by migrating S-undersaturated fluids, which led to the exsolution of Pt-Fe-alloy from pyrrhotite (Fex–1S) with the latter approaching a stoichiometric composition. Overall our new observations provide convincing support for the importance of metasomatism in the secondary modification of ore mineralogy and textures even in the undisturbed Merensky Reef.

The influence of LF refining slag composition on desulfurization and inclusionmodification was investigated through industrial experiments and thermodynamic analysis.The results show that the sulfide capacity decreases with the decrease in(%CaO)/(%SiO2)and (%CaO)/(%Al2O3) of the slag. The theoretical sulfur distributions agreewell with the analysis results when the sulfide capacity is calculated by Zhang’s model,but the theoretical sulfur distributions are higher than the analysis results when thesulfide capacity is calculated by the KTH model. During the LF refining process,inclusions are transformed from Al2O3 inclusions into CaO-MgO-Al2O3 system inclusions. Higher slagbasicity results in higher CaO and MgO and lower Al2O3 content inclusions in moltensteel at the end of the LF process.