Most frequently arsenic is nominally monovalent (As1–) in pyrite (FeS2) and substituted for S. Nominally trivalent arsenic (As3+) has been reported previously in hydrothermal Peruvian pyrite and was considered to be substituted for Fe based on the negative correlation between the concentrations of the two elements. Here, we provide the first observation of the incorporation of As3+ in goldfieldite [Cu12(As,Sb,Bi)2Te2S13] and As5+ in colusite [Cu26V2(As,Sb)4Sn2S32] inclusions in As1–-pyrite from high-sulfidation deposits in Peru. This information was obtained by combining spatially resolved electron probe (EPMA), synchrotron-based X‑ray fluorescence (SXRF), and absorption spectroscopy (micro-XANES and micro-EXAFS) with new high energy-resolution XANES spectroscopy (HR-XANES). The two Cu sulfide inclusions range from several to one hundred micrometers in size, and the As3+/As5+ concentration varies from a few parts per million (ppm) to a maximum of 17.33 wt% compared to a maximum of 50 ppm As1– in pyrite. They also contain variable amounts of Sn (18.47 wt% max), Te (15.91 wt% max), Sb (8.54 wt% max), Bi (5.53 wt% max), and V (3.25 wt% max). The occurrence of As3+/As5+-containing sulfosalts in As1–-containing pyrite grains indicates that oxidizing hydrothermal conditions prevailed during the late stage of the mineralization process in the ore deposits from Peru. From an environmental perspective, high concentrations of potentially toxic As, contained in what appear to be non-As-bearing pyrite, may pose a heretofore unrecognized threat to ecosystems in acid mine drainage settings. More generally, the combination of techniques used in this study offers a new perspective on the mineralogy and crystal chemistry of hazardous elements in pyrite, such as highly toxic and little studied thallium.
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 implications 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 form As–As pairs at 3.2 Å, whereas monovalent Se does not form Se–Se pairs at this distance for unknown reasons. 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 a S atom at 3.2–3.3 Å with 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 is 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 with the same EXAFS spectra being fit indifferently with a cluster and 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 %.
Pyrite (cubic FeS2) is the most abundant metal sulfide in nature and also the main host mineral of toxic mercury (Hg). Release of mercury in acid mine drainage resulting from the oxidative dissolution of pyrite in coal and ore and rock resulting from mining, processing, waste management, reclamation, and large construction activities is an ongoing environmental challenge. The fate of mercury depends on its chemical forms at the point source, which in turn depends on how it occurs in pyrite. Here, we show that pyrite in coal, sedimentary rocks, and hydrothermal ore deposits can host varying structural forms of Hg which can be identified with high energy-resolution XANES (HR-XANES) spectroscopy. Nominally divalent Hg is incorporated at the Fe site in pyrite from coal and at a marcasite-type Fe site in pyrite from sedimentary rocks. Distinction of the two Hg bonding environments offers a mean to detect microscopic marcasite inclusions (orthorhombic FeS2) in bulk pyrite. In epigenetic pyrite from Carlin-type Au deposit, up to 55 ± 6 at. % of the total Hg occurs as metacinnabar nanoparticles (β-HgSNP), with the remainder being substitutional at the Fe site. Pyritic mercury from Idrija-type Hg deposit (α-HgS ore) is partly divalent and substitutional and partly reduced into elemental form (liquid). Divalent mercury ions, mercury sulfide nanoparticles, and elemental mercury released by the oxidation of pyrite in acid mine drainage settings would have different environmental pathways. Our results could find important applications for designing control strategies of mercury released to land and water in mine-impacted watersheds.
