The Mode of Incorporation of As(-I) and Se(-I) in Natural Pyrite Revisited

Abstract

Pyrite (FeS2) from coal, sedimentary rocks, and hydrothermal ore deposits generally contains hazardous selenium (Se) and arsenic (As) that are released in natural waters through oxidative dissolution of the host. Knowing how As and Se are structurally incorporated into pyrite has important implication in controlling or preventing their release because trace metal(loid) substitution accelerates the dissolution of pyrite. Previous extended X-ray absorption fine structure (EXAFS) studies have reported that nominally monovalent arsenic clusters at the sulfur site forming As-As pairs at 3.2 Å, whereas monovalent Se does not form Se-Se pairs at this distance for unknown reason. Here, we revisit this question using As and Se K-edge X-ray absorption near-edge structure (XANES) and EXAFS spectroscopy complemented with atomistic calculations. We find that neither As nor Se atoms can be differentiated from S atom at 3.2-3.3 Å, the cluster and dilute model-fits to As-and Se-EXAFS data yielding equivalent least-squares solutions. Thermodynamic calculations of Fe48As3S93 (3.8 wt.% As) and Fe48Se3S93 (4.0 wt.% Se) structures show that the formation of As-As pairs is energetically favorable and the formation of Se-Se pairs unfavorable. Thus, the equilibrium distribution of As and Se predicted by calculation agrees with published EXAFS data. However, this agreement is incidental because EXAFS fits are ambiguous, the same EXAFS spectra being fit indifferently with a cluster and 2 a dilute model. Regarding Se, the dilute model-fit is probably correct since Se-Se pairs are precluded thermodynamically. The situation is less clear for As. The lowest energy atomic arrangement of As in Fe48S93As3 is similar to the local structure of As in arsenopyrite (FeAsS), thus supporting the cluster model. However, the energy gain to total energy provided by the formation of As clusters decreases with decreasing As concentration, making them thermodynamically less favorable below 1.0 wt.%.

Fossil Bioapatites with Extremely High Concentrations of Rare Earth Elements and Yttrium from Deep-Sea Pelagic Sediments

“…Thanks to its enhanced sensitivity and resolution, Ce L 3 -edge HR-XANES appears to be the most straightforward and reliable way to assess the oxidation state of Ce and the identity of the Ce phase in earth materials. Besides Ce, this technique has been used previously for the distinction of the As­(-I)/As­(III)/As­(V), , Se­(-I)/Se­(IV)/Se­(VI), Eu­(II)/Eu­(III), Au(0)/Au­(I)/Au­(III), − Tl­(I)/Tl­(III), U­(IV)/U­(V)/U­(VI), , and Hg(0)/Hg­(II) , oxidation states.…”

Trace Element Geochemistry of Pyrite from Bitumen-Bearing Stratabound Cu–(Ag) Deposits, Northern Chile

“…Data plotted in the Au–As solubility diagram (Figure B) show that analyses with lower Au/As ratios are related to Au in solid solution, while those with a high Au/As ratio are probably related to the presence of Au- nano- to micro-inclusions. Arsenic concentration in pyrite varies from 4 ppm to ∼5 wt % (Figure ) and may occur in solid solution as As 1– within the structure, although incorporation as As 3+ ,, or even as FeAsS nanoclusters and amorphous As–Fe–S nanoscale inclusions, cannot be ruled out (Figure C).…”

Auriferous pyrite formed by episodic fluid inputs in the Akeshi and Kasuga high-sulfidation deposits, Southern Kyushu, Japan

“…Since we do not have clear evidence for As speciation in pyrite in the studied deposits, we consider it difficult to evaluate the role of As in Au uptake in the Akeshi and the Kasuga deposits. Future spectroscopic studies (e.g., synchrotron XANES and EXFAS) should provide further evidence on As oxidation state(s) and incorporation into pyrite (e.g., Simon et al 1999a;Savage et al 2000;Manceau et al 2020) and its influence on Au enrichment.…”