Platinum-group mineral formation: Evidence of an interchange process from the entropy of activation values |
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| Authors: | Athinoula L Petrou Maria Economou-Eliopoulos |
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| Institution: | aDepartment of Chemistry, University of Athens, Athens 15771, Greece;bDepartment of Geology and Geoenvironment, University of Athens, Athens 15784, Greece |
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| Abstract: | Small grains of Ru–Os–Ir sulfides or alloys that formed in the early stages of the crystallization of basaltic magmas occur as inclusions in chromitites from many ophiolite complexes, Ural/Alaskan-type and layered intrusions. The nature of platinum-group elements is debated, i.e., whether they originally occurred in a metallic state or bonded with sulfur or other ligands. The application of kinetics and suggestions regarding the mechanisms of PGE formation may contribute to a better understanding of the genetic processes for platinum-group mineral (PGM) formation. The aim of the present study is to investigate the type of mechanism (associative, dissociative or interchange) and the possibility of direct formation of PGM from the reaction of free metals with sulfur, defining also the structure of possible intermediate species.The literature on the grain sizes (r) of platinum-group minerals (PGM) and their formation temperatures (range between 700 and 1100 °C) reveals an Arrhenius temperature dependence. The activation energy was estimated to be 450 ± 45 kJ mol−1. Applying the Eyring equation to the same data, a linear relationship between 2.5ln(r) + ln(T) versus 1/T was obtained, leading to an estimation of the free energy of activation, ΔG≠ = 440 ± 43 kJ mol−1. The thermodynamic relation ΔG≠ = ΔH≠ − TΔS≠, used together with the relation ΔΗ≠ = Eact − RT leads to the calculation of ΔS≠ at various temperatures. The estimated entropy of activation at various temperatures being almost zero, we suggest an interchange mechanism for PGM formation.The possibility of PGM formation from the direct reaction of free metals with sulfur would be inconsistent with the estimated entropy of activation values (ΔS≠  0) at various temperatures. In the cases of PGMs of known r, the formation temperatures can be estimated, and then the free energy of activation obtained, applying 2.5ln(r) + ln(T) versus 1/T (straight line). From that value, the entropy of activation can be calculated ΔS≠ = −R + (Eact − ΔG≠)/T], and an interchange mechanism is proposed. Furthermore, applying the relation of 2.5ln(r) + ln(T) versus 1/T on PGM grains of known r and T, an interchange mechanism for their formation can be suggested if they fit the above line. |
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