Thermodynamic Properties of PGEAsS Phases

Abstract The platinum-group element (PGE) sulfarsenides, with a nominal stoichiometry of PGEAsS, comprise a group of platinum-group element minerals. The space groups and refined unit-cell parameters for the synthetic PGEAsS samples prepared in this work are as follows: RuAsS – P21/c, a = 5.9077(1), b = 5.9118(1), c = 5.9815(1) Å, β = 112.167(3)°; RhAsS – Pca21, a = 5.7806(1), b = 5.7808(1), c = 5.7763(1) Å; PdAsS – P213, a = 5.9496(1) Å; OsAsS – P21/c, a = 5.9338(3), b = 5.9187(3), c = 5.9951(3) Å, β = 111.80(1)°; IrAsS – Pa3, a = 5.7965(1) Å. Electron-probe microanalyses showed that the atomic ratios deviate slightly from the ideal 1:1:1 stoichiometry. In this work, we determined the thermodynamic properties of these phases using a combination of experimental syntheses, measurements of low-temperature heat capacity, and ab initio calculations. The calculated Gibbs energies of formation (at T = 298.15, relative to metals, As, and orthorhombic S, in kJ/mol) are as follows: RuAsS −197.22, RhAsS −204.05, PdAsS −96.49, OsAsS −126.25, IrAsS −179.19, PtAsS −111.82. The data assign particular stability to IrAsS, RuAsS, and RhAsS. This finding agrees well with the compositions of the minerals of this group, which are dominated by the hollingworthite, irarsite, and ruarsite components. Even when these minerals are included in isoferroplatinum (Pt3Fe), they contain essentially no Pt or Fe. The phase PtAsS appears to be unstable, and the mineral “platarsite” was previously discredited as a S-rich variety of sperrylite [Pt(As,S)2]. Calculations up to T = 1000 K show that the relative stability does not change and PtAsS is unlikely to form in magmatic systems. Calculations at T = 298.15 further show that IrAsS can co-exist in stable equilibrium with hematite or goethite in oxidation profiles of mineralized rocks.